a http-equiv="Content-Type" content="text/html; charset=iso-8859-1">Prevailing Winds

Flight Environment



Since the environment is resolved to the earthby gravity and rotates with the earth, there would be no circulation if some pressure did notupset the atmosphere"s equilibrium. The heating of the earth"s surface by the sun isthe pressure responsible for developing the circulation that does exist.

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Because that the curvature that the earth,the most straight rays of the sunlight strike the planet in the vicinity of the equator resultingin the biggest concentration of heat, the largest feasible amount the radiation, and themaximum heating of the atmosphere in this area the the earth. At the exact same time, thesun"s beam strike the earth at the poles in ~ a really oblique angle, leading to a muchlower concentration that heat and much less radiation so that there is, in fact, an extremely littleheating the the setting over the poles and also consequently an extremely cold temperatures.

Cold air, being more dense, sink andhot air, being much less dense, rises. Consequently, the rising warm air in ~ the equatorbecomes even less thick as that rises and also its pressure decreases. An area the lowpressure, therefore, exists over the equator.


Warm wait rises until it get acertain elevation at which that starts to spill over into surrounding areas. In ~ thepoles, the cold thick air sinks. Waiting from the upper levels that the atmosphere flowsin on top of it increasing the weight and creating one area that high pressure at the poles.

The air that rises in ~ the equator go notflow directly to the poles. Because of the rotation of the earth, there is a develop up that airat about 30° phibìc latitude. (The same phenomenon wake up in the southern Hemisphere). Some of the air sinks, causing a belt of high-pressure in ~ this latitude.


The sinking wait reaches the surfaceand operation north and south. The air the flows southern completes one cell of theearth"s circulation pattern. The air that flows phibìc becomes component of an additional cellof circulation between 30° and also 60° phibìc latitude. In ~ the very same time, the sinkingair in ~ the north pole operation south and also collides with the air moving north native the 30°high push area. The colliding wait is forced upward and also an area of low pressureis created near 60° north. The third cell circulation pattern is created betweenthe north pole and 60° north.

Because the the rotation of the earthand the coriolis force, wait is deflected come the right in the northern Hemisphere. Asa result, the movement of wait in the polar cell circulation produce the polar easterlies. In the circulation cell that exists between 60° and 30° north, the activity ofair produce the prevailing westerlies. In the tropic circulation cell, thenortheast trade winds are produced. These space the so-called long-term wind systemsof the each.

Since the planet rotates, the axis is tilted, and there is much more land fixed in the north hemisphere than in the southerly hemisphere, the actual worldwide pattern is much an ext complicated. Rather of one huge circulation in between the poles and also the equator, there space three circulations...Hadley cell
- low latitude air activity toward the equator that with heating, rises vertically, with poleward motion in the upper atmosphere. This develops a convection cell the dominates tropical and sub-tropical climates.Ferrel cell - A mid-latitude median atmospheric circulation cell because that thedailysplash.tv called by Ferrel in the 19th century. In this cabinet the air operation poleward and also eastward near the surface and equatorward and westward at greater levels.Polar cell - waiting rises, diverges, and also travels toward the poles. Once over the poles, the wait sinks, developing the polar highs. At the surface air diverges exterior from the polar highs. Surface winds in the polar cell room easterly (polar easterlies).


There room two main forces which influence the activity of air in the top levels. The pressure gradient causes the wait to relocate horizontally, forcing the air directly from a an ar of high push to a region of short pressure. The Coriolis force, however, deflects the direction of the circulation of the air (to the appropriate in the north Hemisphere) and also causes the waiting to circulation parallel come the isobars.

Winds in the top levels will blow clockwise approximately areas that high pressure and counterclockwise roughly areas of low pressure.

The rate of the wind is established by the push gradient. The winds are strongest in areas where the isobars space close together.


Surface friction plays critical role in the speed and direction of surface ar winds. As a result of the slowing under of the air as it moves end the ground, wind speed are much less than would be intended from the push gradient top top the thedailysplash.tv map and also the direction is readjusted so that the wind blows across the isobars right into a center of low pressure and also out the a facility of high pressure.


The effect of friction usually does no extend more than a pair of thousands feet right into the air. In ~ 3000 feet above the ground, the wind blows parallel come the isobars through a rate proportional come the push gradient.

Even allowing for the results of surface ar friction, the winds, locally, perform not always show the speed and direction that would certainly be meant from the isobars ~ above the surface thedailysplash.tv map. These variations room usually because of geographical functions such together hills, mountains and big bodies of water. Except in such as mountain regions, the impact of terrain attributes that reason local sports in wind extends usually no higher than around 2000 feet above the ground.


Land and also sea breezes are brought about by the differences in temperature end land and also water. The sea breeze occurs during the day once the soil area heats an ext rapidly than the water surface. This outcomes in the press over the land being reduced than that over the water. The push gradient is often solid enough for a wind to blow from the water to the land.


The floor breeze blows at night as soon as the land becomes cooler. Then the wind blows towards the warm, low-pressure area over the water.

Land and sea breezes are very local and affect only a narrow area follow me the coast.


Hills and valleys dramatically distort the airflow linked with the prevailing push system and the push gradient. Strong up and also down drafts and also eddies build as the air operation up over hills and down right into valleys. Wind direction alters as the waiting flows roughly hills. Occasionally lines the hills and mountain varieties will act as a barrier, holding ago the wind and also deflecting the so the it flows parallel come the range. If over there is a pass in the mountain range, the wind will certainly rush v this pass as through a tunnel with significant speed. The airflow can be supposed to continue to be turbulent and also erratic for some distance together it flows out of the hilly area and into the flatter countryside.


Daytime heating and nighttime cooling that the hilly slopes cause day come night sports in the airflow. In ~ night, the sides of the hills cool by radiation. The waiting in call with them i do not care cooler and also therefore denser and it blows under the slope into the valley. This is a katabatic wind (sometimes likewise called a mountain breeze). If the slopes space covered with ice and snow, the katabatic wind will certainly blow, not just at night, but also during the day, transporting the cold dense air right into the warmer valleys. The slopes of hills not covered by snow will be warmed throughout the day. The air in contact with them becomes warmer and less dense and, therefore, flows up the slope. This is an anabatic wind (or sink breeze).

In like the mountain areas, local distortion of the airflow is even more severe. Rocky surfaces, high ridges, thin cliffs, steep valleys, all combine to produce unpredictable flow patterns and also turbulence.



Air flowing throughout a mountain selection usually rises reasonably smoothly up the slope of the range, but, as soon as over the top, the pours down the other side with considerable force, bouncing up and down, producing eddies and also turbulence and additionally creating powerful vertical waves the may extend for good distances downwind of the hill range. This phenomenon is well-known as a mountain wave. Note the up and also down drafts and also the rotating eddies developed downstream.

If the wait mass has a high humidity content, clouds of very distinctive appearance will certainly develop.

Cap Cloud. Orographic lift causes a cloud to kind along the optimal of the ridge. The wind dead this cloud under along the leeward slope wherein it dissipates with adiabatic heating. The base of this cloud lies near or listed below the peaks of the ridge; the top might reach a couple of thousand feet above the peaks.

Lenticular (Lens Shaped) Clouds form in the wave crests aloft and lie in bands the may expand to well above 40,000 feet.

Rotor Clouds form in the rolling eddies downstream. They resemble a long line the stratocumulus clouds, the bases of i m sorry lie listed below the mountain peaks and also the tops of which might reach to a considerable height over the peaks. Sometimes these clouds build into thunderstorms.


The clouds, being very distinctive, can be checked out from a great distance and administer a clearly shows warning the the mountain wave condition. Unfortunately, sometimes they are installed in other cloud systems and are surprise from sight. Periodically the waiting mass is really dry and also the clouds carry out not develop.

The severity of the hill wave and also the height to which the disturbance of the wait is influenced is dependency on the stamin of the wind, its edge to the range and the stability or instability that the air. The many severe hill wave conditions are produced in solid airflows that space blowing at best angles to the variety and in secure air. A jet stream blowing practically perpendicular to the mountain selection increases the severity of the wave condition.

The hill wave phenomenon is not minimal only to high hill ranges, such together the Rockies, but is likewise present to a lesser degree in smaller mountain systems and even in present of little hills.

Mountain waves current problems come pilots for numerous reasons:

Vertical Currents. Downdrafts that 2000 feet every minute space common and downdrafts as good as 5000 feet per minute have actually been reported. They occur along the bottom slope and are most severe in ~ a height equal to the of the summit. One airplane, caught in a downdraft, might be compelled to the ground.

Turbulence is usually extremely severe in the waiting layer in between the ground and the tops of the rotor clouds.

Wind Shear. The wind rate varies dramatically in between the crests and also troughs the the waves. That is usually most severe in the tide nearest the mountain range.

Altimeter Error. The rise in wind speed outcomes in an accompanying to decrease in pressure, which in turn affects the accuracy of the press altimeter.

Icing. The freeze level varies considerably from crest to trough. Severe icing can occur since of the large supercooled droplets sustained in the strong vertical currents.

When flying over a mountain ridge wherein wave conditions exist: (1) avoid ragged and also irregular shaped clouds—the rarely often, rarely shape shows turbulence. (2) technique the hill at a 45-degree angle. The you must suddenly decide to revolve back, a fast turn have the right to be made away from the high ground. (3) stop flying in cloud ~ above the hill crest (cap cloud) since of solid downdrafts and turbulence. (4) allow sufficient elevation to clean the highest ridges v altitude to spare to stop the downdrafts and eddies on the downwind slopes. (5) constantly remember that your altimeter have the right to read end 3000 ft. In error ~ above the high side in mountain wave conditions.


A gust is a rapid and irregular fluctuation of differing intensity in the upward and downward motion of air currents. It may be connected with a rapid readjust in wind direction. Gusts are resulted in by mechanical disturbance that results from friction in between the air and also the ground and by the unequal heating of the earth"s surface, specifically on warm summer afternoons.


A squall is a sudden rise in the strength of the wind of longer duration than a gust and may be caused by the i of a fast moving cold former or thunderstorm. Like a gust, it may be attach by a rapid readjust of wind direction.


Diurnal (daily) variation of wind is resulted in by strong surface heating throughout the day, which causes turbulence in the lower levels. The result of this turbulence is the the direction and speed that the wind at the greater levels (e.g., 3000 feet) often tends to be transferred to the surface. Because the wind direction at the greater level is parallel come the isobars and its speed is better than the surface ar wind, this transfer reasons the surface wind to veer and increase in speed.

At night, over there is no surface ar heating and also therefore much less turbulence and also the surface ar wind has tendency to resume its regular direction and speed. The backs and also decreases. Watch VEERING and also BACKING section below for more info.


Friction in between the relocating air mass and also surface functions of the earth (hills, mountains, valleys, trees, buildings, etc.) is responsible because that the swirling vortices the air commonly called eddies. They vary considerably in size and intensity relying on the size and also roughness of the surface ar obstruction, the speed of the wind and also the degree of security of the air. They can spin in one of two people a horizontal or upright plane. Turbulent air and solid winds produce much more vigorous eddies. In steady air, eddies often tend to easily dissipate. Eddies developed in mountainous areas are especially powerful.


The bumpy or choppy up and down motion that signifies the existence of eddies makes it complicated to store an aircraft in level flight.


Dust devils are phenomena that take place quite commonly on the warm dry levels of mid-western phibìc America. They deserve to be of adequate force to present a hazard to pilots of irradiate airplanes paris at short speeds.

They are little heat lows that type on clear warm days. Provided a steep slide out rate resulted in by cool wait aloft end a hot surface, small horizontal wait movement, few or no clouds, and also the noonday sunlight heating level arid soil surfaces to high temperatures, the air in contact with the ground i do not care super-heated and also highly unstable. This surface ar layer of air builds until something triggers an upward movement. As soon as started, the hot air rises in a column and also draws an ext hot air right into the base of the column. Circulation begins around this heat low and increases in velocity until a little vigorous whirlwind is created. Dust devils are usually of quick duration and also are for this reason named due to the fact that they space made clearly shows by the dust, sand and also debris that they choose up native the ground.

Dust devils posture the best hazard close to the ground wherein they are most violent. Pilots proposing come land on superheated runways in areas of the mid-west whereby this phenomenon is common should scan the airport because that dust swirls or grass spirals that would show the presence of this hazard.


Tornadoes are violent, circular whirlpools the air associated with significant thunderstorms and also are, in fact, an extremely deep, concentrated low-pressure areas. They room shaped like a tunnel hanging the end of the cumulonimbus cloud and also are dark in appearance due to the dust and debris sucked into their whirlpools. They variety in diameter from about 100 feet come one half mile and move end the floor at speeds of 25 to 50 knots. Their path over the soil is usually just a few miles long although tornadoes have been report to reduced destructive swaths as lengthy as 100 miles. The an excellent destructiveness that tornadoes is caused by the an extremely low press in your centers and the high wind speeds, i beg your pardon are deemed to be as an excellent as 300 knots.


Wind speeds because that aviation purposes are express in knots (nautical miles per hour). In the thedailysplash.tv reports on us public radio and also television, however, wind speed are offered in miles every hour while in Canada speeds are offered in kilometers per hour.

In a conversation of wind direction, the compass point from i m sorry the wind is punch is considered to it is in its direction. Therefore, a north wind is one the is blowing from the north towards the south. In aviation thedailysplash.tv reports, area and aerodrome forecasts, the wind is always reported in degrees true. In ATIS broadcasts and in the information offered by the tower for landing and take-off, the wind is reported in levels magnetic.


The wind veers as soon as it alters direction clockwise. Example: The surface ar wind is blowing indigenous 270°. At 2000 feet that is blowing from 280°. That has adjusted in a right-hand, or clockwise, direction.

The wind backs once it alters direction anti-clockwise. Example: The wind direction in ~ 2000 feet is 090° and at 3000 feet is 085°. The is an altering in a left-hand, or anti-clockwise, direction.

In a lower from number of thousand feet over the ground come ground level, the wind will usually be discovered to ago and also decrease in velocity, together the effect of surface friction becomes apparent. In a climb from the surface ar to several thousand feet AGL, the wind will veer and also increase.

At night, surface cooling reduces the eddy motion of the air. Surface ar winds will back and decrease. Conversely, throughout the day, surface heating boosts the eddy movement of the air. Surface winds will veer and also increase as more powerful winds aloft mix to the surface. See DIURNAL VARIATIONS section above for an ext info.


Wind shear is the sudden tearing or shearing impact encountered follow me the leaf of a zone in which over there is a violent adjust in wind rate or direction. It deserve to exist in a horizontal or vertical direction and produces churning motions and consequently turbulence. Under part conditions, wind direction transforms of as much as 180 degrees and also speed alters of as much as 80 knots have actually been measured.


The impact on aircraft performance of encountering wind shear derives indigenous the truth that the wind can readjust much faster than the aircraft mass deserve to be accelerated or decelerated. Severe wind shears deserve to impose penalty on one airplane"s performance the are beyond its capabilities come compensate, especially during the an important landing and also take-off phase of flight.

In seafaring Flight

In seafaring flight, wind shear will most likely be encountered in the shift zone between the push gradient wind and the distorted local winds at the reduced levels. The will additionally be encountered when climbing or descending through a temperature inversion and when passing through a frontal surface. Wind shear is additionally associated v the jet stream. Airplane encountering wind shear might experience a succession of updrafts and also downdrafts, to reduce or profit in headwind, or windshifts that disrupt the created flight path. That is not usually a major problem since altitude and airspeed margins will certainly be enough to counteract the shear"s adverse effects. On occasion, however, the wind shear might be severe enough to cause an abrupt increase in fill factor, which could stall the airplane or inflict structure damage.


Near the Ground

Wind shear, encountered close to the ground, is an ext serious and potentially an extremely dangerous. There room four usual sources of short level wind shear: thunderstorms, frontal activity, temperature inversions and solid surface winds passing approximately natural or manmade obstacles.

Frontal Wind Shear. Wind shear is typically a problem only in fronts v steep wind gradients. If the temperature difference across the front in ~ the surface ar is 5°C or much more and if the former is relocating at a speed of around 30 knots or more, wind shear is likely to be present. Frontal wind shear is a phenomenon associated with fast moving cold fronts yet can be current in warmth fronts together well.


Thunderstorms. Wind shear, linked with thunderstorms, occurs together the result of 2 phenomena, the gust front and also downbursts. As the thunderstorm matures, solid downdrafts develop, strike the ground and spread the end horizontally follow me the surface ar well in advancement of the thunderstorm itself. This is the gust front. Winds can readjust direction by as much as 180° and reach speed as great as 100 knots as far as 10 mile ahead that the storm. The downburst is really intense localized downdraft flowing the end of a thunderstorm. The power of the downburst have the right to exceed plane climb capabilities. The downburst (there room two varieties of downbursts: macrobursts and microbursts) typically is much closer to the thunderstorm than the gust front. Dust clouds, role clouds, intense rainfall or virga (rain the evaporates before it reaches the ground) are due to the opportunity of downburst task but over there is no means to accurately predict its occurrence.


Temperature Inversions. Overnight cooling create a temperature turning back a few hundred feet over the soil that have the right to produce far-ranging wind shear, especially if the reverse is coupled v the low-level jet stream.

As a nocturnal turning back develops, the wind shear near the peak of the reverse increases. It commonly reaches its preferably speed shortly after midnight and also decreases in the morning together daytime heater dissipates the inversion. This phenomenon is well-known as the low-level nocturnal jet stream. The low level jet stream is a sheet of strong winds, thousands of miles long, hundreds of miles broad and hundreds of feet thick that develops over level terrain such as the prairies. Wind speeds of 40 knots room common, however greater speeds have been measured. Low level jet streams are responsible because that hazardous low level shear.


As the inversion dissipates in the morning, the shear aircraft and gusty winds relocate closer come the ground, resulting in windshifts and also increases in wind speed near the surface.

Surface Obstructions. The irregular and also turbulent flow of air around mountains and hills and also through hill passes reasons serious wind shear problems for plane approaching to land at airports near mountain ridges. Wind shear is a phenomenon linked with the mountain wave. Such shear is almost totally unpredictable yet should be supposed whenever surface ar winds are strong.

Wind shear is also associated v hangars and large buildings at airports. As the air flows around such big structures, wind direction changes and also wind speed rises causing shear.

Wind shear wake up both horizontally and vertically. Upright shear is most typical near the ground and also can pose a serious peril to airplanes during take-off and landing. The airplane is flying at lower speeds and in a relatively high traction configuration. There is tiny altitude easily accessible for recovering and also stall and maneuver margins are at your lowest. An plane encountering the wind shear phenomenon might experience a big loss of airspeed because of the sudden change in the loved one airflow as the aircraft flies right into a new, moving air mass. The abrupt drop in airspeed may an outcome in a stall, creating a dangerous situation when the airplane is only a few hundred feet turn off the ground and really vulnerable.


Narrow bands of exceedingly high rate winds are known to exist in the greater levels the the setting at altitudes varying from 20,000 come 40,000 feet or more. Lock are recognized as jet streams. As many as three significant jet streams may traverse the phibìc American continent at any type of given time. One lies across Northern Canada and also one across the U.S. A third jet stream might be as far south together the north tropics but it is rather rare. A jet stream in the mid latitudes is normally the strongest.


The jet stream shows up to it is in closely linked with the tropopause and with the polar front. It typically forms in the break in between the polar and also the tropic tropopause whereby the temperature gradients space intensified. The typical position of the jet currently shears south in winter and north in summer with the seasonal hike of the polar front. Due to the fact that the troposphere is depths in summer 보다 in winter, the tropopause and also the jets will certainly nominally it is in at greater altitudes in the summer.

Long, strong jet streams are usually additionally associated with well-developed surface ar lows in ~ deep upper troughs and also lows. A low emerging in the wave along the frontal surface lies south of the jet. As it deepens, the low moves near the jet. As it occludes, the low moves north of the jet, which the cross the frontal system, near the point of occlusion. The jet flows roughly parallel come the front. The subtropical jet currently is not connected with fronts but forms since of strong solar heating in the equatorial regions. The ascending air transforms poleward at really high levels however is deflected by the Coriolis pressure into a solid westerly jet. The subtropical jet predominates in winter.


The jet streams circulation from west to east and may encircle the entire hemisphere. Much more often, due to the fact that they space stronger in some locations than in others, they rest up right into segments some 1000 to 3000 nautical mile long. They space usually about 300 nautical miles broad and may be 3000 come 7000 feet thick. These jet currently segments move in one easterly direction complying with the motion of pressure ridges and also troughs in the top atmosphere.

Winds in the central core that the jet stream are the strongest and may reach speeds as an excellent as 250 knots, back they room generally in between 100 and 150 knots. Wind speed decrease towards the outer edges the the jet stream and may it is in blowing at just 25 knots there. The price of decrease of wind rate is substantially greater ~ above the north edge 보다 on the southern edge. Wind speeds in the jet currently are, ~ above average, substantially stronger in winter than in summer.

Clear wait Turbulence. The many probable ar to expect Clear Air turbulence (CAT) is just above the central core that the jet stream close to the polar tropopause and just below the core. Clean air turbulence does not happen in the core. CAT is encountered much more frequently in winter when the jet currently winds are strongest. Nevertheless, CAT is not always present in the jet present and, since it is random and transient in nature, that is nearly impossible come forecast.

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Clear waiting turbulence might be linked with other thedailysplash.tv patterns, particularly in wind shear linked with the sharply curved contours of solid lows, troughs and ridges aloft, at or listed below the tropopause, and in locations of solid cold or heat air advection. Hill waves develop severe CAT the may expand from the hill crests to as high together 5000 feet above the tropopause. Due to the fact that severe CAT does pose a danger to airplanes, pilots should shot to avoid or minimization encounters with it. These rules of ignorance may help avoid jet streams with strong winds (150 knots) at the core. Solid wind shears are likely over and below the core. CAT within the jet present is more intense over and come the lee of mountain ranges. If the 20-knot isotachs (lines joining locations of equal wind speeds) room closer 보다 60 nautical mile on the charts showing the areas of the jet stream, wind shear and also CAT space possible.

Curving jet streams are most likely to have actually turbulent edges, specifically those that curve approximately a deep push trough. As soon as moderate or serious CAT has actually been report or is forecast, readjust speed to turbulent air speed instantly on encountering the an initial bumpiness or even before encountering that to prevent structural damage to the airplane.

The areas of CAT are usually shallow and narrow and also elongated through the wind. If jet stream disturbance is encountered v a tail wind or head wind, a rotate to the right will find smoother air and an ext favorable winds. If the CAT is encountered in a crosswind, the is no so necessary to change course together the turbulent area will be narrow.