Wednesday, September 5, 2012

Wind:flow of gases on a large scale

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar windis the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of lightchemical elements from a planet's atmosphere into space. Sony VAIO PCG-3B1M Battery
Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in our solar system occur on Neptune and Saturn.
In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Sony VAIO PCG-3C1T Battery
Short bursts of high speed wind are termed gusts. Strong winds of intermediate duration (around one minute) are termedsqualls. Long-duration winds have various names associated with their average strength, such as breeze, gale, storm,hurricane, and typhoon. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, Sony VAIO PCG-3D1M Battery
to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption ofsolar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Sony VAIO PCG-3G2M Battery
Within the tropics,thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds. Sony VAIO PCG-5R1M Battery
In human civilization, wind has inspired mythology, influenced the events of history, expanded the range of transport andwarfare, and provided a power source for mechanical work, electricity and recreation. Wind powers the voyages of sailing ships across Earth's oceans. Sony VAIO PCG-7141M Battery
Hot air balloons use the wind to take short trips, and powered flight uses it to increase lift and reduce fuel consumption. Areas of wind shear caused by various weather phenomena can lead to dangerous situations foraircraft. When winds become strong, trees and man-made structures are damaged or destroyed. Sony VAIO PCG-7143M Battery
Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and byerosion. Dust from large deserts can be moved great distances from its source region by the prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Sony VAIO PCG-7151M Battery
Wind affects the spread of wildfires. Winds disperse seeds from various plants, enabling the survival and dispersal of those plant species, as well as flying insect populations. When combined with cold temperatures, wind has a negative impact on livestock. Wind affects animals' food stores, as well as their hunting and defensive strategies. Sony VAIO PCG-7154M Battery
Wind is caused by differences in pressure. When a difference in pressure exists, the air is accelerated from higher to lower pressure. On a rotating planet, the air will be deflected by the Coriolis effect, except exactly on the equator. Globally, the two major driving factors of large-scale winds (the atmospheric circulation) Sony VAIO PCG-7162M Battery
are the differential heating between the equator and the poles (difference in absorption of solar energy leading to buoyancy forces) and the rotation of the planet. Outside the tropics and aloft from frictional effects of the surface, the large-scale winds tend to approach geostrophic balance. Sony VAIO PCG-7181M Battery
Near the Earth's surface, friction causes the wind to be slower than it would be otherwise. Surface friction also causes winds to blow more inward into low pressure areas.[1]
Winds defined by an equilibrium of physical forces are used in the decomposition and analysis of wind profiles. Sony VAIO PCG-41112M Battery
They are useful for simplifying the atmospheric equations of motion and for making qualitative arguments about the horizontal and vertical distribution of winds. The geostrophic wind component is the result of the balance between Coriolis force and pressure gradient force. Sony VAIO PCG-7153M Battery
It flows parallel to isobars and approximates the flow above the atmospheric boundary layer in the midlatitudes.[2] The thermal wind is the difference in the geostrophic wind between two levels in the atmosphere. It exists only in an atmosphere with horizontal temperature gradients. Sony VAIO PCG-71312M Battery
The ageostrophic wind component is the difference between actual and geostrophic wind, which is responsible for air "filling up" cyclones over time.[4] The gradient wind is similar to the geostrophic wind but also includes centrifugal force (or centripetal acceleration).
Wind direction is reported by the direction from which it originates. Sony VAIO PCG-7144M Battery
For example, a northerly wind blows from the north to the south.[6] Weather vanes pivot to indicate the direction of the wind.[7] At airports, windsocks are primarily used to indicate wind direction, but can also be used to estimate wind speed by its angle of hang.Wind speed is measured byanemometers, most commonly using rotating cups or propellers. Sony VAIO PCG-7191L Battery
When a high measurement frequency is needed (such as in research applications), wind can be measured by the propagation speed of ultrasound signals or by the effect of ventilation on the resistance of a heated wire.Another type of anemometer uses pitot tubes that take advantage of the pressure differential Sony VAIO PCG-3C1M Battery
between an inner tube and an outer tube that is exposed to the wind to determine the dynamic pressure, which is then used to compute the wind speed.[10]
Sustained wind speeds are reported globally at a 10 meters (33 ft) height and are averaged over a 10 minute time frame. Sony VAIO PCG-3F1M Battery
TheUnited States reports winds over a 1 minute average for tropical cyclones,[11] and a 2 minute average within weather observations.[12] India typically reports winds over a 3 minute average.[13] Knowing the wind sampling average is important, as the value of a one-minute sustained wind is typically 14% greater than a ten-minute sustained wind. Sony VAIO PCG-3H1M Battery
A short burst of high speed wind is termed a wind gust, one technical definition of a wind gust is: the maxima that exceed the lowest wind speed measured during a ten minute time interval by 10 knots (19 km/h). A squall is a doubling of the wind speed above a certain threshold, which lasts for a minute or more. Sony VAIO PCG-3J1M Battery
To determine winds aloft, rawinsondes determine wind speed by GPS, radio navigation, or radar tracking of the probe.[15]Alternatively, movement of the parent weather balloon position can be tracked from the ground visually using theodolites.[16]Remote sensing techniques for wind include SODAR, Sony VAIO PCG-8141M Battery
Doppler LIDARs and RADARs, which can measure the Doppler shift ofelectromagnetic radiation scattered or reflected off suspended aerosols or molecules, and radiometers and radars can be used to measure the surface roughness of the ocean from space or airplanes. Ocean roughness can be used to estimate wind velocity close to the sea surface over oceans. Sony VAIO PCG-8161M Battery
Geostationary satellite imagery can be used to estimate the winds throughout the atmosphere based upon how far clouds move from one image to the next. Wind Engineering describes the study of the effects of the wind on the built environment, including buildings, bridges and other man-made objects. Sony VAIO PCG-3C2M Battery
Historically, the Beaufort wind force scale provides an empirical description of wind speed based on observed sea conditions. Originally it was a 13-level scale, but during the 1940s, the scale was expanded to 17 levels.[17] There are general terms that differentiate winds of different average speeds such as a breeze, a gale, a storm, tornado, or a hurricane. Sony VAIO PCG-5N2M Battery
Within the Beaufort scale, gale-force winds lie between 28 knots (52 km/h) and 55 knots (102 km/h) with preceding adjectives such as moderate, fresh, strong, and whole used to differentiate the wind's strength within the gale category.[18] A storm has winds of 56 knots (104 km/h) to 63 knots (117 km/h).[19] Sony VAIO PCG-5P1M Battery
The terminology for tropical cyclones differs from one region to another globally. Most ocean basins use the average wind speed to determine the tropical cyclone's category.
Total wind power could be captured only if the wind velocity is reduced to zero. In a realistic wind turbine this is impossible, as the captured air must also leave the turbine. Sony VAIO PCG-5S1M Battery
A relation between the input and output wind velocity must be considered. Using the concept of stream tube, the maximal achievable extraction of wind power by a wind turbine is 59% of the total theoretical wind power[24] (see: Betz' law).
Easterly winds, on average, dominate the flow pattern across the poles, Sony VAIO PCG-9Z1M Battery
westerly winds blow across the mid-latitudes of the earth, to the north of the subtropical ridge, while easterlies again dominate the tropics.
Directly under the subtropical ridge are the doldrums, or horse latitudes, where winds are lighter. Sony VAIO PCG-7171M Battery
Many of the Earth's deserts lie near the average latitude of the subtropical ridge, where descent reduces the relative humidity of the air mass.[25] The strongest winds are in the mid-latitudes where cold Arctic air meets warm air from the tropics.
The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics towards the Earth's equator. Sony VAIO PCG-7186M Battery
The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.[27] The trade winds act as the steering flow for tropical cyclones that form over world's oceans.[28] Trade winds also steer African dust westward across the Atlantic Ocean into the Caribbean Sea, as well as portions of southeast North America.[29] Sony VAIO PCG-81112M Battery
A monsoon is a seasonal prevailing wind that lasts for several months within tropical regions. The term was first used in English in India, Bangladesh, Pakistan, and neighboring countries to refer to the big seasonal winds blowing from the Indian Ocean and Arabian Sea in the southwest bringing heavy rainfall to the area. Sony VAIO PCG-31311M Battery
Its poleward progression is accelerated by the development off a heat low over the Asian, African, and North American continents during May through July, and over Australia in December.
The Westerlies or the Prevailing Westerlies are the prevailing winds in the middle latitudes between 35 and 65 degreeslatitude.Sony VAIO PCG-8152M Battery
These prevailing winds blow from the west to the east to the north of the subtropical ridge,[34][35] and steer extratropical cyclones in this general manner. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. Sony VAIO PCG-31111M Battery
They are strongest in the winter when the pressure is lower over the poles, and weakest during the summer and when pressures are higher over the poles.
Together with the trade winds, the westerlies enabled a round-trip trade route for sailing ships crossing the Atlantic and Pacific Oceans, Sony VAIO PCG-61111M Battery
as the westerlies lead to the development of strong ocean currents on the western sides of oceans in both hemispheres through the process of western intensification.[37] These western ocean currents transport warm, sub tropical water polewards toward the polar regions. Sony VAIO PCG-51112M Battery
The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. The strongest westerly winds in the middle latitudes are within a band known as the Roaring Forties, between 40 and 50 degrees latitude south of the equator. Sony VAIO PCG-51211M Battery
The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents,[39][40] especially in the southern hemisphere because of its vast oceanic expanse.
The polar easterlies, also known as Polar Hadley cells, are dry, Sony VAIO PCG-51212M Battery
cold prevailing winds that blow from the high-pressure areas of the polar highs at the northand south poles towards the low-pressure areas within the Westerlies at high latitudes. Unlike the Westerlies, these prevailing winds blow from the east to thewest, and are often weak and irregular.[41] Sony VAIO PCG-41111M Battery
Because of the low sun angle, cold air builds up and subsides at the pole creating surface high-pressure areas, forcing an equatorward outflow of air;[42] that outflow is deflected eastward by the Coriolis effect.
In coastal regions, sea breezes and land breezes can be important factors in a location's prevailing winds. Sony VAIO PCG-41111V Battery
The sea is warmed by the sun more slowly because of water's greater specific heat compared to land.[43] As the temperature of the surface of the land rises, the land heats the air above it by conduction. The warm air is less dense than the surrounding environment and so it rises. This causes a pressure gradient of about 2 millibars from the ocean to the land. Sony VAIO PCG-61412V Battery
The cooler air above the sea, now with higher sea level pressure, flows inland into the lower pressure, creating a cooler breeze near the coast. When large-scale winds are calm, the strength of the sea breeze is directly proportional to the temperature difference between the land mass and the sea. Sony VAIO PCG-71112M Battery
If an offshore wind of 8 knots (15 km/h) exists, the sea breeze is not likely to develop.
At night, the land cools off more quickly than the ocean because of differences in their specific heat values. This temperature change causes the daytime sea breeze to dissipate. Sony VAIO PCG-81111V Battery
When the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land, establishing a land breeze, as long as an onshore wind is not strong enough to oppose it.
Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level,Sony VAIO PCG-81111V Battery
creating an associated thermal low over the terrain and enhancing any thermal lows that would have otherwise existed,[45][46] and changing the wind circulation of the region. In areas where there is rugged topography that significantly interrupts the environmental wind flow, Sony VAIO PCG-81212M Battery
the wind circulation between mountains and valleys is the most important contributor to the prevailing winds. Hills and valleys substantially distort the airflow by increasing friction between the atmosphere and landmass by acting as a physical block to the flow, deflecting the wind parallel to the range just upstream of the topography, which is known as a barrier jet. Sony VAIO PCG-81212V Battery
This barrier jet can increase the low level wind by 45%.[47] Wind direction also changes because of the contour of the land.[48]
If there is a pass in the mountain range, winds will rush through the pass with considerable speed because of the Bernoulli principle that describes an inverse relationship between speed and pressure. Sony VAIO PCG-51111M Battery
The airflow can remain turbulent and erratic for some distance downwind into the flatter countryside. These conditions are dangerous to ascending and descendingairplanes.[48] Cool winds accelerating through mountain gaps have been given regional names. In Central America, examples include the Papagayo wind, the Panama wind, and the Tehuano wind. Sony VAIO VPCS13X9E/B battery
In Europe, similar winds are known as the Bora, Tramontane, and Mistral. When these winds blow over open waters, they increase mixing of the upper layers of the ocean that elevates cool, nutrient rich waters to the surface, which leads to increased marine life.[49]
In mountainous areas, local distortion of the airflow becomes severe. Sony VAIO VPCS12V9E/B battery
Jagged terrain combines to produce unpredictable flow patterns and turbulence, such as rotors, which can be topped by lenticular clouds. Strong updrafts, downdrafts and eddiesdevelop as the air flows over hills and down valleys. Sony VAIO VPCS12V9E/B battery
Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, also known as upslope flow, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), Sony VAIO VPCF13M8E/B battery
a more moist climate usually prevails on the windward side of a mountain than on the leewardor downwind side. Moisture is removed by orographic lift, leaving drier air on the descending and generally warming, leeward side where a rain shadow is observed.[50] Winds that flow over mountains down into lower elevations are known as downslope winds. Sony VAIO VPCF13Z0E/B battery
These winds are warm and dry. In Europe downwind of the Alps, they are known as foehn. In Poland, an example is the halny wiatr. In Argentina, the local name for downsloped winds is zonda. In Java, the local name for such winds is koembang. In New Zealand, they are known as the Nor'west arch, and are accompanied by the cloud formation they are named after that has inspired artwork over the years.[51] Sony VAIO VPCM13M1E/L battery
 In the Great Plains of the United States, the winds are known as a chinook. In California, downsloped winds are funneled through mountain passes, which intensify their effect, and examples into Santa Ana and sundowner winds. Wind speeds during downslope wind effect can exceed 160 kilometers per hour (99 mph). Sony VAIO VPCM13M1E/W battery
As described earlier, prevailing and local winds are not spread evenly across the earth, which means that wind speeds also differ by region. In addition, the wind speed also increases with the altitude.
Nowadays, a yardstick used to determine the best locations for wind energy development is referred to as wind power density (WPD). Sony VAIO VPCF22M1E battery
It is a calculation relating to the effective force of the wind at a particular location, frequently expressed in terms of the elevation above ground level over a period of time. It takes into account wind velocity and mass. Color coded maps are prepared for a particular area are described as, for example, "mean annual power density at 50 meters." Sony VAIO VPCF11M1E/H battery
The results of the above calculation are included in an index developed by the National Renewable Energy Lab and referred to as "NREL CLASS." The larger the WPD calculation, the higher it is rated by class.[53] At the end of 2008, worldwide nameplate capacity of wind-powered generators was 120.8 gigawatts.[54] Sony VAIO VPCF13M0E/B battery
Although wind produces only about 1.5% of worldwide electricity use,[54] it is growing rapidly, having doubled in the three years between 2005 and 2008. In several countries it has achieved relatively high levels of penetration, accounting for approximately 19% of electricity production inDenmark, Sony VAIO VPCYB2M1E battery
10% in Spain and Portugal, and 7% in Germany and the Republic of Ireland in 2008. One study indicates that an entirely renewable energy supply based on 70% wind is attainable at today's power prices by linking wind farms with an HVDC supergrid.
Wind shear, sometimes referred to as windshear or wind gradient, Sony VAIO VPCYB3V1E battery
is a difference in wind speed and direction over a relatively short distance in the Earth's atmosphere.[56] Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across weather fronts and near the coast,[57] and vertical shear typically near the surface, Sony VAIO VPCY11M1E battery
though also at higher levels in the atmosphere near upper level jets and frontal zones aloft.[59]
Wind shear itself is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. Sony VAIO VPCS12L9E/B battery
It is commonly observed nearmicrobursts and downbursts caused by thunderstorms,[60] weather fronts, areas of locally higher low level winds referred to as low level jets, near mountains,[61] radiation inversions that occur because of clear skies and calm winds, buildings,[62]wind turbines,[63] and sailboats.[64] Sony VAIO VPCF11S1E/B battery
Wind shear has a significant effect during take-off and landing of aircraft because of their effects on control of the aircraft,[65] and was a significant cause of aircraft accidents involving large loss of life within the United States.[60]
Sound movement through the atmosphere is affected by wind shear, Sony VAIO VPCYB3V1E/R Battery
which can bend the wave front, causing sounds to be heard where they normally would not, or vice versa.[66] Strong vertical wind shear within the troposphere also inhibits tropical cyclone development,[67] but helps to organize individual thunderstorms into living longer life cycles that can then producesevere weather.[ Sony VAIO VPCF23P1E Battery
The thermal wind concept explains how differences in wind speed with height are dependent on horizontal temperature differences, and explains the existence of the jet stream.
As a natural force, the wind was often personified as one or more wind gods or as an expression of the supernatural in many cultures. Sony VAIO VPCF23N1E Battery
Vayu is the Hindu God of Wind.[70][71] The Greek wind gods include Boreas, Notus, Eurus, and Zephyrus.[71] Aeolus, in varying interpretations the ruler or keeper of the four winds, has also been described as Astraeus, the god of dusk who fathered the four winds with Eos, goddess of dawn. Sony VAIO VPCY21S1E/L Battery
The Ancient Greeks also observed the seasonal change of the winds, as evidenced by the Tower of the Winds in Athens.[71] Venti are the Roman gods of the winds.[72] Fūjin, the Japanese wind god and is one of the eldest Shinto gods. Sony VAIO VPCY21S1E/G battery
According to legend, he was present at the creation of the world and first let the winds out of his bag to clear the world of mist.[73] In Norse mythology, Njord is the god of the wind.[71] There are also four dvärgar (Norse dwarves), named Norðri, Suðri, Austri and Vestri, and probably the four stags of Yggdrasil, personify the four winds, and parallel the four Greek wind gods.[74] Sony VAIO VPCF24M1E battery
Stribog is the name of the Slavic god of winds, sky and air. He is said to be the ancestor (grandfather) of the winds of the eight directions.[71]
Kamikaze (神風) is a Japanese word, usually translated as divine wind, believed to be a gift from the gods. HP Compaq HSTNN-105C Battery
The term is first known to have been used as the name of a pair or series of typhoons that are said to have saved Japan from two Mongol fleets under Kublai Khan that attacked Japan in 1274 and again in 1281.[75] Protestant Wind is a name for the storm that deterred the Spanish Armada from an invasion of England in 1588 where the wind played a pivotal role, HP Compaq HSTNN-C12C Battery
 or the favorable winds that enabled William of Orange to invade England in 1688.[77] During Napoleon's Egyptian Campaign, the French soldiers had a hard time with the khamsin wind: when the storm appeared "as a blood-stint in the distant sky", the natives went to take cover, HP Compaq HSTNN-C66C Battery
while the French "did not react until it was too late, then choked and fainted in the blinding, suffocating walls of dust."[78] During the North African Campaign of the World War II, "allied and German troops were several times forced to halt in mid-battle because of sandstorms caused by khamsin ... HP Compaq HSTNN-C66C-4 Battery
Grains of sand whirled by the wind blinded the soldiers and created electrical disturbances that rendered compasses useless."
There are many different forms of sailing ships, but they all have certain basic things in common. Except for rotor shipsusing the Magnus effect, HP Compaq HSTNN-C66C-5 Battery
every sailing ship has a hull, rigging and at least one mast to hold up the sails that use the wind to power the ship.[80] Ocean journeys by sailing ship can take many months,[81] and a common hazard is becoming becalmed because of lack of wind,[82] or being blown off course by severe storms or winds that do not allow progress in the desired direction. HP Compaq HSTNN-C67C Battery
A severe storm could lead to shipwreck, and the loss of all hands.[84] Sailing ships can only carry a certain quantity of supplies in their hold, so they have to plan long voyages carefully to include appropriate provisions, including fresh water.[85]
For aerodynamic aircraft which operate relative to the air, winds affect groundspeed, HP Compaq HSTNN-C67C-4 Battery
and in the case of lighter-than-air vehicles, wind may play a significant or solitary role in their movement and ground track.[87] The velocity of surface wind is generally the primary factor governing the direction of flight operations at an airport, and airfield runways are aligned to account for the common wind direction(s) of the local area. HP Compaq HSTNN-C67C-5 Battery
While taking off with a tailwind may be necessary under certain circumstances, a headwind is generally desirable. A tailwind increases takeoff distance required and decreases the climb gradient.
Historically, the ancient Sinhalese of Anuradhapura and in other cities around Sri Lanka used the monsoon winds to power furnaces as early as 300 BCE. HP Compaq HSTNN-DB05 Battery
The furnaces were constructed on the path of the monsoon winds to exploit the wind power, to bring the temperatures inside up to 1,200 °C (2,190 °F). An early historical reference to a rudimentary windmill was used to power an organ in the first century CE.[90] The first practical windmills were later built in Sistan, Afghanistan, from the 7th century CE. HP Compaq HSTNN-DB06 Battery
These were vertical-axle windmills, which had long vertical driveshafts with rectangle shaped blades.[91] Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind corn and draw up water, and were used in the gristmilling and sugarcane industries. HP Compaq HSTNN-DB0E Battery
Horizontal-axle windmills were later used extensively in Northwestern Europe to grind flour beginning in the 1180s, and many Dutch windmills still exist. High altitude wind power is the focus of over 30 companies worldwide using tethered technology rather than ground-hugging compressive-towers. HP Compaq HSTNN-DB11 Battery
Oil is being saved by using wind for powering cargo ships by use of the mechanical energy converted from the wind's kinetic energy using very large kites.
Wind figures prominently in several popular sports, including recreational hang gliding, hot air ballooning, kite flying,snowkiting,HP Compaq HSTNN-DB16 Battery
 kite landboarding, kite surfing, paragliding, sailing, and windsurfing. In gliding, wind gradients just above the surface affect the takeoff and landing phases of flight of a glider. Wind gradient can have a noticeable effect on ground launches, also known as winch launches or wire launches. HP Compaq HSTNN-DB28 Battery
If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, the indicated airspeed will increase, possibly exceeding the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the gradient.[95] When landing, wind shear is also a hazard, particularly when the winds are strong. HP Compaq HSTNN-DB29 Battery
As the glider descends through the wind gradient on final approach to landing, airspeed decreases while sink rate increases, and there is insufficient time to accelerate prior to ground contact. The pilot must anticipate the wind gradient and use a higher approach speed to compensate for it. HP Compaq HSTNN-DB67 Battery
In arid climates, the main source of erosion is wind.[97] The general wind circulation moves small particulates such as dust across wide oceans thousands of kilometers downwind of their point of origin, which is known as deflation. Westerly winds in the mid-latitudes of the planet drive the movement of ocean currents from west to east across the world's oceans. HP Compaq HSTNN-FB05 Battery
Wind has a very important role in aiding plants and other immobile organisms in dispersal of seeds, spores, pollen, etc. Although wind is not the primary form of seed dispersal in plants, it provides dispersal for a large percentage of the biomass of land plants.
Erosion can be the result of material movement by the wind. HP Compaq HSTNN-FB18 Battery
There are two main effects. First, wind causes small particles to be lifted and therefore moved to another region. This is called deflation. Second, these suspended particles may impact on solid objects causing erosion by abrasion (ecological succession). Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation. HP Compaq HSTNN-FB51 Battery
An example is the formation of sanddunes, on a beach or in a desert.[99] Loess is a homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (Aeolian) sediment.[100] It generally occurs as a widespread blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. HP Compaq HSTNN-FB52 Battery
Loess often stands in either steep or vertical faces.[101] Loess tends to develop into highly rich soils. Under appropriate climatic conditions, areas with loess are among the most agriculturally productive in the world.[102] Loess deposits are geologically unstable by nature, and will erode very readily. HP Compaq HSTNN-I04C Battery
Therefore, windbreaks (such as big trees and bushes) are often planted by farmers to reduce the wind erosion of loess.
During mid-summer (July), the westward-moving trade winds south of the northward-moving subtropical ridge expand northwestward from the Caribbean Sea into southeastern North America. HP Compaq HSTNN-I12C Battery
When dust from the Sahara moving around the southern periphery of the ridge within the belt of trade winds moves over land, rainfall is suppressed and the sky changes from a blue to a white appearance, which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates. HP Compaq HSTNN-I39C Battery
Over 50% of the African dust that reaches the United States affects Florida.[104] Since 1970, dust outbreaks have worsened because of periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year. HP Compaq HSTNN-I40C Battery
Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s.[106] Similar dust plumes originate in the Gobi desert, which combined with pollutants, spread large distances downwind, or eastward, into North America.[98] HP Compaq HSTNN-I44C Battery
There are local names for winds associated with sand and dust storms. The Calima carries dust on southeast winds into the Canary islands.The Harmattancarries dust during the winter into the Gulf of Guinea. The Sirocco brings dust from north Africa into southern Europe because of the movement of extratropical cyclones through the Mediterranean Sea. HP Compaq HSTNN-I44C-A Battery
Spring storm systems moving across the eastern Mediterranean Sea cause dust to carry across Egypt and the Arabian peninsula, which are locally known as Khamsin.[110] The Shamal is caused by cold fronts lifting dust into the atmosphere for days at a time across the Persian Gulf states. HP Compaq HSTNN-I44C-B Battery
Wind dispersal of seeds, or anemochory, is one of the more primitive means of dispersal. Wind dispersal can take on one of two primary forms: seeds can float on the breeze or alternatively, they can flutter to the ground.[112] The classic examples of these dispersal mechanisms include dandelions (Taraxacum spp., Asteraceae), HP Compaq HSTNN-I45C Battery
which have a feathery pappus attached to their seeds and can be dispersed long distances, and maples (Acer (genus) spp., Sapindaceae), which have winged seeds and flutter to the ground. An important constraint on wind dispersal is the need for abundant seed production to maximize the likelihood of a seed landing in a site suitable for germination. HP Compaq HSTNN-I45C-A Battery
There are also strong evolutionary constraints on this dispersal mechanism. For instance, species in the Asteraceae on islands tended to have reduced dispersal capabilities (i.e., larger seed mass and smaller pappus) relative to the same species on the mainland.[113] Reliance upon wind dispersal is common among many weedy or ruderal species. HP Compaq HSTNN-I45C-B Battery
Unusual mechanisms of wind dispersal include tumbleweeds. A related process to anemochory isanemophily, which is the process where pollen is distributed by wind. Large families of plants are pollinated in this manner, which is favored when individuals of the dominant plant species are spaced closely together.[114] HP Compaq HSTNN-I48C-A Battery
Wind also limits tree growth. On coasts and isolated mountains, the tree line is often much lower than in corresponding altitudes inland and in larger, more complex mountain systems, because strong winds reduce tree growth. High winds scour away thin soils through erosion,[115] as well as damage limbs and twigs. HP Compaq HSTNN-I48C-B Battery
When high winds knock down or uproot trees, the process is known as windthrow. This is most likely on windward slopes of mountains, with severe cases generally occurring to tree stands that are 75 years or older.[116] Plant varieties near the coast, such as the Sitka spruce and sea grape,[117] are pruned back by wind and salt spray near the coastline. HP Compaq HSTNN-I49C Battery
Wind can also cause plants damage through sand abrasion. Strong winds will pick up loose sand and topsoil and hurl it through the air at speeds ranging from 25–40 miles per hour. Such windblown sand causes extensive damage to plant seedlings because it ruptures plant cells, making them vulnerable to evaporation and drought. HP Compaq HSTNN-I50C-B Battery
Using a mechanical sandblaster in a laboratory setting, scientists affiliated with the Agricultural Research Service studied the effects of windblown sand abrasion on cotton seedlings. The study showed that the seedlings responded to the damage created by the windblown sand abrasion by shifting energy from stem and root growth to the growth and repair of the damaged stems.[ HP Compaq HSTNN-I54C Battery
After a period of four weeks the growth of the seedling once again became uniform throughout the plant, as it was before the windblown sand abrasion occurred.
Cattle and sheep are prone to wind chill caused by a combination of wind and cold temperatures, HP Compaq HSTNN-I64C-5 Battery
when winds exceed 40 kilometers per hour (25 mph), rendering their hair and wool coverings ineffective.[121] Although penguins use both a layer of fat and feathers to help guard against coldness in both water and air, their flippers and feet are less immune to the cold. HP Compaq HSTNN-I65C-5 Battery
In the coldest climates such as Antarctica, emperor penguins use huddling behavior to survive the wind and cold, continuously alternating the members on the outside of the assembled group, which reduces heat loss by 50%.[122] Flying insects, a subset of arthropods, are swept along by the prevailing winds, HP Compaq HSTNN-IB05 Battery
while birds follow their own course taking advantage of wind conditions, in order to either fly or glide.[124] As such, fine line patterns within weather radar imagery, associated with converging winds, are dominated by insect returns.[125] Bird migration, which tends to occur overnight within the lowest 7,000 feet (2,100 m) of the Earth's atmosphere, HP Compaq HSTNN-IB08 Battery
contaminates wind profiles gathered by weather radar, particularly the WSR-88D, by increasing the environmental wind returns by 15 knots (28 km/h) to 30 knots (56 km/h).[126]
Pikas use a wall of pebbles to store dry plants and grasses for the winter in order to protect the food from being blown away. HP Compaq HSTNNIB12 Battery
Cockroaches use slight winds that precede the attacks of potential predators, such as toads, to survive their encounters. Their cerci are very sensitive to the wind, and help them survive half of their attacks. Elk have a keen sense of smell that can detect potential upwind predators at a distance of 0.5 miles (800 m). HP Compaq HSTNN-IB16 Battery
Increases in wind above 15 kilometers per hour (9.3 mph) signals glaucous gulls to increase their foraging and aerial attacks on thick-billed murres.
High winds are known to cause damage, depending upon their strength. Infrequent wind gusts can cause poorly designedsuspension bridges to sway. HP Compaq HSTNN-IB18 Battery
When wind gusts are at a similar frequency to the swaying of the bridge, the bridge can be destroyed more easily, such as what occurred with the Tacoma Narrows Bridge in 1940.[131] Wind speeds as low as 23 knots (43 km/h) can lead to power outages due to tree branches disrupting the flow of energy through power lines. HP Compaq HSTNN-IB28 Battery
While no species of tree is guaranteed to stand up to hurricane-force winds, those with shallow roots are more prone to uproot, and brittle trees such as eucalyptus, sea hibiscus, and avocado are more prone to damage.[133] Hurricane-force winds cause substantial damage to mobile homes, and begin to structurally damage homes with foundations. HP Compaq HSTNN-IB51 Battery
Winds of this strength due to downsloped winds off terrain have been known to shatter windows and sandblast paint from cars.[52] Once winds exceed 135 knots (250 km/h), homes completely collapse, and significant damage is done to larger buildings. Total destruction to man-made structures occurs when winds reach 175 knots (324 km/h). HP Compaq HSTNN-IB52 Battery
The Saffir-Simpson scale and Enhanced Fujita scale were designed to help estimate wind speed from the damage caused by high winds related to tropical cyclones and tornadoes, and vice versa.[134][135]
Australia's Barrow Island holds the record for the strongest wind gust, HP Compaq HSTNN-IB55 Battery
reaching 408 km/h (253 mph) during tropical cyclone Olivia on 10 April 1996, surpassing the previous record of 372 km/h (231 mph) set on Mount Washington (New Hampshire) on the afternoon of 12 April 1934.[136] The most powerful gusts of wind on Earth were created by nuclear detonations. HP Compaq HSTNN-IB62 Battery
The blast wave is similar to a strong wind gust over the ground. The largest nuclear explosion (50–58 megatons at an altitude of about 13,000 ft) generated a 20 bar blast pressure at ground zero, which is similar to a wind gust of 3,100 miles per hour.
Wildfire intensity increases during daytime hours. HP Compaq HSTNN-LB05 Battery
For example, burn rates of smoldering logs are up to five times greater during the day because of lower humidity, increased temperatures, and increased wind speeds.[137] Sunlight warms the ground during the day and causes air currents to travel uphill, and downhill during the night as the land cools. HP Compaq HSTNN-LB08 Battery
Wildfires are fanned by these winds and often follow the air currents over hills and through valleys.[138] United States wildfire operations revolve around a 24-hour fire day that begins at 10:00 a.m. because of the predictable increase in intensity resulting from the daytime warmth. HP Compaq HSTNN-LB0E Battery
The solar wind is quite different from a terrestrial wind, in that its origin is the sun, and it is composed of charged particles that have escaped the sun's atmosphere. Similar to the solar wind, the planetary wind is composed of light gases that escape planetary atmospheres. HP Compaq HSTNN-LB11 Battery
Over long periods of time, the planetary wind can radically change the composition of planetary atmospheres.
The hydrodynamic wind within the upper portion of a planet's atmosphere allows light chemical elements such as hydrogen to move up to the exobase, HP Compaq HSTNN-LB51 Battery
the lower limit of the exosphere, where the gases can then reach escape velocity, entering outer space without impacting other particles of gas. This type of gas loss from a planet into space is known as planetary wind.[140] Such a process over geologic time causes water-rich planets such as the Earth to evolve into planets likeVenus. HP Compaq HSTNN-LB52 Battery
Additionally, planets with hotter lower atmospheres could accelerate the loss rate of hydrogen.
Rather than air, the solar wind is a stream of charged particles—a plasma—ejected from the upper atmosphere of the sun at a rate of 400 kilometers per second (890,000 mph). HP Compaq HSTNN-MB05 Battery
It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time. These particles are able to escape the sun's gravity, in part because of the high temperature of the corona,but also because of high kinetic energy that particles gain through a process that is not well-understood. HP Compaq HSTNN-OB06 Battery
The solar wind creates the Heliosphere, a vast bubble in theinterstellar medium surrounding the solar system.[144] Planets require large magnetic fields in order to reduce the ionization of their upper atmosphere by the solar wind.[142] Other phenomena include geomagnetic storms that can knock out power grids on Earth, HP Compaq HSTNN-OB52 Battery
 the aurorae such as the Northern Lights,[146] and the plasma tails of comets that always point away from the sun.
Strong 300 kilometers per hour (190 mph) winds at Venus's cloud tops circle the planet every four to five earth days. HP Compaq HSTNN-OB62 Battery
When the poles of Mars are exposed to sunlight after their winter, the frozen CO2 sublimes, creating significant winds that sweep off the poles as fast as 400 kilometers per hour (250 mph), which subsequently transports large amounts of dust and water vapor over its landscape. HP Compaq HSTNN-UB05 Battery
 Other Martian winds have resulted in cleaning events and dust devils.[150][151] On Jupiter, wind speeds of 100 meters per second (220 mph) are common in zonal jet streams.[152] Saturn's winds are among the solar system's fastest. Cassini–Huygens data indicated peak easterly winds of 375 meters per second (840 mph). HP Compaq HSTNN-UB11 Battery
On Uranus, northern hemisphere wind speeds reach as high as 240 meters per second (540 mph) near 50 degrees north latitude. At the cloud tops of Neptune, prevailing winds range in speed from 400 meters per second (890 mph) along the equator to 250 meters per second (560 mph) at the poles. HP Compaq HSTNN-UB18 Battery
At 70° S latitude on Neptune, a high-speed jet stream travels at a speed of 300 meters per second (670 mph).
north wind is a wind that originates in the north and blows south. The north wind has had historical and literal significance, since it often signals cold weather and seasonal change in the Northern hemisphere. HP Compaq HSTNN-UB68 Battery
In the bible, Song of Solomon 4:16, the bride is calling for the North (and South) Wind to blow on her garden.
One of Aesop's most famous fables is called The North Wind and the Sun.
The North wind plays a part in the Norwegian fable, "East of the sun and west of the moon", as the only wind capable helping a woman fly to the titular location to find her husband. HP Compaq HSTNN-UB69 Battery
The wind plays an important role in another Norwegian folktale, "The Lad who went to the North Wind", giving the lad a tablecloth that produces food, a donkey that produces gold, and a stick that beats a person on command. HP Compaq HSTNN-W42C Battery
In George MacDonald's children's novel At the Back of the North Wind (serialized in the children's magazine Good Words for the Young beginning in 1868 and was published in book form in 1871), the title character, in the form of a beautiful woman, appears to a boy named Diamond and takes him on a series of nightly journeys. HP Compaq HSTNN-W42C-A Battery
Oscar Wilde's fairy tale The Selfish Giant (1888) personifies the North Wind as a man who "was wrapped in furs, and he roared all day about the garden, and blew the chimney-pots down".
In the second generation of the Pokémon games HP Compaq HSTNN-W42C-B Battery
 (first released in Japan in 1999 and to Australia and North America in 2000 and Europe in 2001), Suicune is said to be the incarnation of the north winds.
The wind is also a character in Bill Willingham's comic series, Fables (2002-). He is the father of Bigby Wolf and adept at shapeshifting. HP Compaq HSTNN-XB0E Battery
Winter's Child (2009), Cameron Dokey's novel adaptation of the Hans Christian Andersen fairytale, "The Snow Queen", cites the North Wind as the main factor in the Snow Queen's transformation from a mortal to the titular Winter Child.
south wind is a wind that originates in the south and blows north. HP Compaq HSTNN-XB11 Battery
Words used in English to describe the south wind are auster, buster (a violent south gale), föhn/foehn (alps), gibli (Libya with various spellings), friagem (a cold south wind blowing into Brazil from the Antarctic), khamsin (a hot spring wind in Egypt, with various spellings), HP Compaq HSTNN-XB18 Battery
kona (stormy southwest wind in Hawaii), notus (see mythology below for origin) and sirocco (North Africa).
In Greek mythology, Notus was the god of the south wind and bringer of the storms of late summer and autumn. HP Compaq HSTNN-XB21 Battery
west wind is a wind that blows from the west, in an eastward direction. In Western tradition, it has usually been considered the mildest and most favorable of the directional winds.
In Greek mythology, Zephyrus was the personification of the west wind and the bringer of light spring and early summer breezes;HP Compaq HSTNN-XB24 Battery
his Roman equivalent was Favonius. In the myth of Cupid and Psyche, Zephyrus was the attendant of Cupid, who brought Psyche to his master's palace.
Geoffrey Chaucer wrote of the "swete breth" of Zephryus, and a soft, gentle breeze may be referred to as a zephyr, as in Shakespeare's Cymbeline (IV, ii): "They are as gentle / As zephyrs blowing below the violet, / Not wagging his sweet head." HP Compaq HSTNN-XB28 Battery
An east wind is a wind that originates in the east and blows west. In Greek mythology, Eurus, the east wind, was the only wind not associated with any of the three Greek seasons, and is the only one of these four Anemoi not mentioned in Hesiod's Theogony or in the Orphic Hymns. HP Compaq HSTNN-XB51 Battery
Some 17 references to East Wind exist in the Authorized King James Version of the English Old Testament. In chapter 41 of Genesis, the pharaoh's dream, that is interpreted by Joseph, describes seven ears of corn blasted by the east wind. In chapters 10 and 14 of Exodus, HP Compaq HSTNN-XB52 Battery
the east wind is summoned by Moses to bring the locusts that plague Egypt and to part the Red Sea so that the Children of Israel can escape pharaoh's armies. Several other references exist, most associating the east wind with destruction. Often this destruction is of the wicked by God. HP Compaq HSTNN-XB59 Battery
In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surfaceof oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. HP Compaq HSTNN-XB61 Battery
Waves in the oceans can travel thousands of miles before reaching land. Wind waves range in size from small ripples to huge waves over 30 meters high.[1]
When directly being generated and affected by the local winds, a wind wave system is called a wind sea. After the wind ceases to blow, wind waves are called swellHP Compaq HSTNN-XB62 Battery
Or, more generally, a swell consists of wind generated waves that are not—or are hardly—affected by the local wind at that time. They have been generated elsewhere, or some time ago.[2] Wind waves in the ocean are called ocean surface waves.
Wind waves have a certain amount of randomness: subsequent waves differ in height, duration and shape, with a limited predictability. HP Compaq HSTNN-XB68 Battery
They can be described as a stochastic process, in combination with the physics governing their generation, growth, propagation and decay—as well as governing the interdependence between flow quantities such as: the water surfacemovements, flow velocities and water pressure. HP Compaq HSTNN-XB69 Battery
The key statistics of wind waves (both seas and swells) in evolving sea statescan be predicted with wind wave models.
Tsunamis are a specific type of wave not caused by wind but by geological effects. In deep water, tsunamis are not visible because they are small in height and very long in wavelength. HP Compaq HSTNN-XB85 Battery
They may grow to devastating proportions at the coast due to reduced water depth.
Waves in a given area typically have a range of heights. For weather reporting and for scientific analysis of wind wave statistics, their characteristic height over a period of time is usually expressed as significant wave heightHP Compaq 360482-001 Battery
This figure represents an average height of the highest one-third of the waves in a given time period (usually chosen somewhere in the range from 20 minutes to twelve hours), or in a specific wave or storm system. The significant wave height is also the value a "trained observer" (e.g. from a ship's crew) would estimate from visual observation of a sea state. HP Compaq 360483-001 Battery
Given the variability of wave height, the largest individual waves are likely to be somewhat less than twice the reported significant wave height for a particular day or storm.
Ripples appear on smooth water when the wind blows, but will die quickly if the wind stops. HP Compaq 360483-003 Battery
The restoring force that allows them to propagate is surface tension. Seas are the larger-scale, often irregular motions that form under sustained winds. These waves tend to last much longer, even after the wind has died, and the restoring force that allows them to propagate is gravity. HP Compaq 360483-004 Battery
As seas propagate away from their area of origin, they naturally separate into groups of common direction and wavelength. The sets of waves formed in this way are known as swells.
Individual "rogue waves" (also called "freak waves", "monster waves", "killer waves", and "king waves") much higher than the other waves in the sea state can occur. HP Compaq 360484-001 Battery
In the case of the Draupner wave, its 25 m (82 ft) height was 2.2 times the significant wave height. Such waves are distinct from tides, caused by the Moon and Sun's gravitational pull, tsunamis that are caused by underwater earthquakes or landslides, and waves generated by underwater explosions or the fall of meteorites—all having far longer wavelengthsthan wind waves. HP Compaq 361909-001 Battery
Yet, the largest ever recorded wind waves are common—not rogue—waves in extreme sea states. For example: 29.1 m (95 ft) high waves have been recorded on the RRS Discovery in a sea with 18.5 m (61 ft) significant wave height, so the highest wave is only 1.6 times the significant wave height.[6] HP Compaq 361909-002 Battery
 The biggest recorded by a buoy (as of 2011) was 32.3 m (106 ft) high during the 2007 typhoon Krosa near Taiwan.
As waves travel from deep to shallow water, their shape alters (wave height increases, speed decreases, and length decreases as wave orbits become asymmetrical). HP Compaq 364602-001 Battery
This process is called shoaling.Wave refraction is the process by which wave crests realign themselves as a result of decreasing water depths. Varying depths along a wave crest cause the crest to travel at different phase speeds, with those parts of the wave in deeper water moving faster than those in shallow water. HP Compaq 365750-001 Battery
This process continues until the crests become (nearly) parallel to the depth contours. Rays—lines normal to wave crests between which a fixed amount of energy flux is contained—converge on local shallows and shoals. Therefore, the wave energy between rays is concentrated as they converge, with a resulting increase in wave height. HP Compaq 365750-003 Battery
Because these effects are related to a spatial variation in the phase speed, and because the phase speed also changes with the ambient current – due to theDoppler shift – the same effects of refraction and altering wave height also occur due to current variations. In the case of meeting an adverse current the wave steepensHP Compaq 365750-004 Battery
i.e. its wave height increases while the wave length decreases, similar to the shoaling when the water depth decreases.
Some waves undergo a phenomenon called "breaking". A breaking wave is one whose base can no longer support its top, causing it to collapse. HP Compaq 367457-001 Battery
A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces. When the slope, or steepness ratio, of a wave is too great, breaking is inevitable.
Individual waves in deep water break when the wave steepness—the ratio of the wave height H to the wavelength λ—exceeds about 0.17, so for H > 0.17 λHP Compaq 372771-001 Battery
In shallow water, with the water depth small compared to the wavelength, the individual waves break when their wave height H is larger than 0.8 times the water depth h, that is H > 0.8 h.[9] Waves can also break if the wind grows strong enough to blow the crest off the base of the wave. HP Compaq 372772-001 Battery
Three main types of breaking waves are identified by surfers or surf lifesavers. Their varying characteristics make them more or less suitable for surfing, and present different dangers.
Spilling, or rolling: these are the safest waves on which to surf. They can be found in most areas with relatively flat shorelines. They are the most common type of shorebreak.HP Compaq 381373-001 Battery
Plunging, or dumping: these break suddenly and can "dump" swimmers—pushing them to the bottom with great force. These are the preferred waves for experienced surfers. Strong offshore winds and long wave periods can cause dumpers. They are often found where there is a sudden rise in the sea floor, such as a reef or sandbar. HP Compaq 381374-001 Battery
Surging: these may never actually break as they approach the water's edge, as the water below them is very deep. They tend to form on steep shorelines. These waves can knock swimmers over and drag them back into deeper water.
Wind waves are mechanical waves that propagate along the interface between water and air; HP Compaq 382553-001 Battery
the restoring force is provided by gravity, and so they are often referred to as surface gravity waves. As the wind blows, pressure and friction forces perturb the equilibrium of the water surface. These forces transfer energy from the air to the water, forming waves. HP Compaq 383220-001 Battery
The initial formation of waves by the wind is described in the theory of Phillips from 1957, and the subsequent growth of the small waves has been modeled by Miles, also in 1957.
In the case of monochromatic linear plane waves in deep water, particles near the surface move in circular paths, HP Compaq 383510-001 Battery
making wind waves a combination of longitudinal (back and forth) and transverse (up and down) wave motions. When waves propagate in shallow water, (where the depth is less than half the wavelength) the particle trajectories are compressed into ellipses.[13][14]
As the wave amplitude (height) increases, the particle paths no longer form closed orbits; HP Compaq 385843-001 Battery
rather, after the passage of each crest, particles are displaced slightly from their previous positions, a phenomenon known asStokes drift.[15][16]
As the depth below the free surface increases, the radius of the circular motion decreases. HP Compaq 385895-001 Battery
At a depth equal to half the wavelength λ, the orbital movement has decayed to less than 5% of its value at the surface. The phase speed (also called the celerity) of a surface gravity wave is – for pure periodic wave motion of small-amplitudewaves – well approximated by
Where c = phase speed;λ = wavelength;d = water depth;g = acceleration due to gravity at the Earth's surface. HP Compaq 393549-001 Battery
In deep water, where  , so   and the hyperbolic tangent approaches  , the speed   approximates
In SI units, with   in m/s,  , when   is measured in metres. This expression tells us that waves of different wavelengths travel at different speeds. HP Compaq 393652-001 Battery
The fastest waves in a storm are the ones with the longest wavelength. As a result, after a storm, the first waves to arrive on the coast are the long-wavelength swells.
For intermediate and shallow water, the Boussinesq equations are applicable, combining frequency dispersion and nonlinear effects. And in very shallow water, the shallow water equations can be used. HP Compaq 395790-001 Battery
If the wavelength is very long compared to the water depth, the phase speed (by taking the limit of c when the wavelength approaches infinity) can be approximated by
On the other hand, for very short wavelengths, surface tension plays an important role and the phase speed of these gravity-capillary waves can (in deep water) be approximated byHP Compaq 395790-003 Battery
W here S = surface tension of the air-water interface; = density of the water.[17]
When several wave trains are present, as is always the case in nature, the waves form groups. In deep water the groups travel at a group velocity which is half of the phase speed. HP Compaq 395790-132 Battery
Following a single wave in a group one can see the wave appearing at the back of the group, growing and finally disappearing at the front of the group.
As the water depth   decreases towards the coast, this will have an effect: wave height changes due to wave shoaling and refraction. HP Compaq 395790-163 Battery
As the wave height increases, the wave may become unstable when the crest of the wave moves faster than the trough. This causes surf, a breaking of the waves.
The movement of wind waves can be captured by wave energy devices. The energy density (per unit area) of regular sinusoidal waves depends on the water density  , HP Compaq 395791-001 Battery
gravity acceleration   and the wave height   (which, for regular waves, is equal to twice the amplitude,  ):
The velocity of propagation of this energy is the group velocity.
Surfers are very interested in the wave forecasts. There are many websites that provide predictions of the surf quality for the upcoming days and weeks. HP Compaq 395791-002 Battery
Wind wave models are driven by more general weather models that predict the winds and pressures over the oceans, seas and lakes.
Wind wave models are also an important part of examining the impact of shore protection and beach nourishment proposals. HP Compaq 395791-003 Battery
For many beach areas there is only patchy information about the wave climate, therefore estimating the effect of wind waves is important for managing littoral environments.
Ocean water waves generate land seismic waves that propagate hundreds of kilometers into the land. HP Compaq 395791-132 Battery
These seismic signals usually have the period of 6 ± 2 seconds. Such recordings were first reported and understood in about 1900.
There are two types of seismic "ocean waves". The primary waves are generated in shallow waters by direct water wave-land interaction and have the same period as the water waves (10 to 16 seconds). HP Compaq 395791-142 Battery
The more powerful secondary waves are generated by the superposition of ocean waves of equal period traveling in opposite directions, thus generating standing gravity waves of half the period – as discovered by P. Bernard in 1941 and explained by Longuet-Higgins in 1950. HP Compaq 395791-251 Battery
Prevailing winds are winds that blow predominantly from a single general direction over a particular point on Earth's surface. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface. HP Compaq 395791-261 Battery
A region's prevailing and dominant winds are often affected by global patterns of movement in the Earth's atmosphere.[1] In general, easterly flow exists at low and high latitudes globally. In the mid-latitudes, westerly winds are the rule and their strength is at the mercy of the polar cyclone. HP Compaq 395791-661 Battery
In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important to the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern. Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow. HP Compaq 395791-741 Battery
Wind roses are tools used to determine the direction of the prevailing wind. Knowledge of the prevailing wind allows the development of prevention strategies for soil erosion of agricultural land, such as across the Great Plains. Sand dunes can orient themselves along, or perpendicular, to the prevailing wind regime within coastal and desert locations. HP Compaq 395794-001 Battery
Insects drift along with the prevailing wind, while birds are able to fly more independently of the prevailing wind. Prevailing winds in mountainous locations can lead to significant rainfall gradients within the topography, ranging from wet across windward-facing slopes to desert-like conditions along their lee slopes. HP Compaq 395794-002 Battery
A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location. Presented in a polar coordinate grid, the wind rose shows the frequency of winds blowing from particular directions. HP Compaq 395794-261 Battery
The length of each spoke around the circle is related to the frequency that the wind blows from a particular direction per unit time. Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles. HP Compaq 396751-001 Battery
A wind rose plot may contain additional information, in that each spoke is broken down into color-coded bands that show wind speed ranges. Wind roses typically use 8 or 16 cardinal directions, such as north (N), NNE, NE, etc.,[2] although they may be subdivided into as many as 32 directions. HP Compaq 397809-001 Battery
The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics near the Earth'sequator,[4] equatorward of the subtropical ridge. These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.[5] HP Compaq 397809-003 Battery
The trade winds act as the steering flow for tropical cyclones that form over world's oceans, guiding their path westward.[6] Trade winds also steer African dust westward across the Atlantic ocean into theCaribbean sea, as well as portions of southeast North America. HP Compaq 397809-242 Battery
The westerlies or the prevailing westerlies are the prevailing winds in the middle latitudes between 35 and 65 degrees latitudewhich blow from west to east poleward of the high pressure area known as the subtropical ridge in the horse latitudes.[8][9]These prevailing winds blow from the west to the east,[10] HP Compaq 398650-001 Battery
 and steer extratropical cyclones in this general manner. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.[5] They are strongest in the winter when the pressure is lower over the poles, HP Compaq 398680-001 Battery
such as when the polar cyclone is strongest, and weakest during the summer when the polar cyclone is weakest and when pressures are higher over the poles.[11]
Together with the trade winds, the westerlies enabled a round-trip trade route for sailing ships crossing the Atlantic and Pacific oceans, HP Compaq 398854-001 Battery
as the westerlies lead to the development of strong ocean currents in both hemispheres. The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. THP Compaq 398874-001 Battery
he strongest westerly winds in the middle latitudes are called the Roaring Forties, between 40 and 50 degrees south latitude, within the Southern Hemisphere.[12] The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents especially in the southern hemisphere because of its vast oceanic expanse. HP Compaq 398875-001 Battery
The polar easterlies (also known as Polar Hadley cells) are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the north and South poles towards the low-pressure areas within the westerlies at high latitudes. Like trade winds and unlike the westerlies, HP Compaq 398876-001 Battery
these prevailing winds blow from the east to the west, and are often weak and irregular.[15] Due to the low sun angle, cold air builds up and subsides at the pole creating surface high-pressure areas, forcing an equatorward outflow of air;[16] that outflow is deflected westward by the Coriolis effect. HP Compaq 408545-001 Battery
In areas where the wind flow is light, sea breezes and land breezes are important factors in a location's prevailing winds. The sea is warmed by the sun to a greater depth than the land due to its greater specific heat.[17] The sea therefore has a greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than the land's surface. HP Compaq 408545-141 Battery
As the temperature of the surface of the land rises, the land heats the air above it. The warm air is less dense and so it rises. This rising air over the land lowers the sea level pressure by about 0.2%. The cooler air above the sea, now with higher sea level pressure, flows towards the land into the lower pressure, creating a cooler breeze near the coast. HP Compaq 408545-142 Battery
The strength of the sea breeze is directly proportional to the temperature difference between the land mass and the sea. If an offshore wind of 8 knots (15 km/h) exists, the sea breeze is not likely to develop. At night, the land cools off more quickly than the ocean due to differences in their specific heat values, which forces the daytime sea breeze to dissipate. HP Compaq 408545-241 Battery
If the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land, establishing a land breeze, as long as an onshore wind is not strong enough to oppose it.
Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, HP Compaq 408545-261 Battery
creating an associated thermal low over the terrain and enhancing any lows which would have otherwise existed,[19][20] and changing the wind circulation of the region. In areas where there is rugged topography that significantly interrupts the environmental wind flow, the wind can change direction and accelerate parallel to the wind obstruction. HP Compaq 408545-262 Battery
This barrier jet can increase the low level wind by 45 percent.[21] In mountainous areas, local distortion of the airflow is more severe. Jagged terrain combines to produce unpredictable flow patterns and turbulence, such as rotors. Strong updrafts, downdrafts andeddies develop as the air flows over hills and down valleys. HP Compaq 408545-621 Battery
Wind direction changes due to the contour of the land. If there is a pass in the mountain range, winds will rush through the pass with considerable speed due to the Bernoulli principle that describes an inverse relationship between speed and pressure. The airflow can remain turbulent and erratic for some distance downwind into the flatter countryside. HP Compaq 408545-721 Battery
These conditions are dangerous to ascending and descending airplanes.[22]
Daytime heating and nighttime cooling of the hilly slopes lead to day to night variations in the airflow, similar to the relationship between sea breeze and land breeze. At night, the sides of the hills cool through radiation of the heat. HP Compaq 408545-741 Battery
The air along the hills becomes cooler and denser, blowing down into the valley, drawn by gravity. This is known a katabatic wind or mountain breeze. If the slopes are covered with ice and snow, the katabatic wind will blow during the day, carrying the cold dense air into the warmer, barren valleys. HP Compaq 408545-761 Battery
The slopes of hills not covered by snow will be warmed during the day. The air that comes in contact with the warmed slopes becomes warmer and less dense and flows uphill. This is known as an anabatic wind or valley breeze. HP Compaq 409357-001 Battery
Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to consistent winds (for example, the trade winds), HP Compaq 409357-002 Battery
a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (seekatabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed.[24] HP Compaq 412779-001 Battery
In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina.[25] The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts. HP Compaq 415306-001 Battery
Insects are swept along by the prevailing winds, while birds follow their own course.[28] As such, fine line patterns withinweather radar imagery, associated with converging winds, are dominated by insect returns.[29] In the Great Plains, soil erosionof agricultural land is a significant problem, and is mainly driven by the prevailing wind. HP Compaq 418867-001 Battery
Because of this, wind barrier strips have been developed to minimize this type of erosion. The strips can be in the form of soil ridges, crop strips, crops rows, or trees which act as wind breaks. They are oriented at a right angle to the wind in order to be most effective.[30] HP Compaq 418871-001 Battery
In regions with minimal vegetation, such as coastal and desert areas, transverse sand dunes orient themselves perpendicular to the prevailing wind direction, while longitudinal dunes orient themselves parallel to the prevailing winds. HP Compaq 441675-001 Battery,HP Compaq 443884-001 Battery,HP Compaq 443885-001 Battery

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