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

Flight Environment



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

You are watching: Local winds are caused by either topographic effects or by variations in _____.

Because of the curvature that the earth,the most direct rays the the sun strike the earth in the vicinity of the equator resultingin the biggest concentration the heat, the largest feasible amount the radiation, and also themaximum heating of the setting in this area the the earth. In ~ the same time, thesun\"s light ray strike the earth at the poles at a an extremely oblique angle, bring about a muchlower concentration the heat and much much less radiation so that there is, in fact, an extremely littleheating of the atmosphere over the poles and also consequently really cold temperatures.

Cold air, being an ext dense, sinks andhot air, being less dense, rises. Consequently, the rising warm air in ~ the equatorbecomes even less thick as the rises and its push decreases. An area that lowpressure, therefore, exists end the equator.


Warm waiting rises until it reaches acertain elevation at which it starts to pour out over right into surrounding areas. In ~ thepoles, the cold thick air sinks. Waiting from the upper levels that the atmosphere flowsin on top of it raising the weight and also creating one area the high pressure at the poles.

The air that rises in ~ the equator go notflow straight to the poles. Due to the rotation of the earth, over there is a construct up of airat around 30° phibìc latitude. (The same phenomenon wake up in the southern Hemisphere). Few of the wait sinks, bring about a belt of high-pressure at this latitude.


The sinking waiting reaches the surfaceand flows north and south. The air the flows south completes one cell of theearth\"s circulation pattern. The air the flows phibìc becomes component of one more cellof circulation in between 30° and also 60° north latitude. In ~ the very same time, the sinkingair at the phibìc pole operation south and also collides through the air moving north indigenous the 30°high pressure area. The colliding wait is compelled upward and also an area of low pressureis developed near 60° north. The third cell circulation sample is produced betweenthe phibìc pole and also 60° north.

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

Since the planet rotates, the axis is tilted, and also there is much more land fixed in the northern hemisphere 보다 in the southern hemisphere, the actual an international pattern is much an ext complicated. Instead of one huge circulation in between the poles and the equator, there are three circulations...Hadley cell
- short latitude air movement toward the equator that v heating, rises vertically, with poleward motion in the top atmosphere. This forms a convection cell the dominates tropical and sub-tropical climates.Ferrel cell - A mid-latitude average atmospheric circulation cell because that lifwynnfoundation.org called by Ferrel in the 19th century. In this cabinet the air flows poleward and also eastward close to the surface and equatorward and also westward at greater levels.Polar cell - waiting rises, diverges, and travels towards the poles. Once over the poles, the waiting sinks, developing the polar highs. At the surface air diverges exterior from the polar highs. Surface winds in the polar cell space easterly (polar easterlies).


There room two main pressures which impact the motion of air in the top levels. The press gradient reasons the wait to move horizontally, forcing the air directly from a an ar of high press to a region of short pressure. The Coriolis force, however, deflects the direction that the circulation of the wait (to the appropriate in the northern Hemisphere) and causes the wait to flow parallel to the isobars.

Winds in the upper levels will blow clockwise approximately areas that high pressure and also counterclockwise around areas of low pressure.

The rate of the wind is figured out by the press gradient. The winds room strongest in regions where the isobars are close together.


Surface friction plays vital role in the speed and also direction of surface winds. Together a result of the slowing down of the air together it moves end the ground, wind speed are less than would be supposed from the pressure gradient top top the lifwynnfoundation.org map and also the direction is adjusted so the the wind blows across the isobars right into a facility of short pressure and out that a facility of high pressure.


The effect of friction normally does no extend more than a couple of thousands feet into the air. In ~ 3000 feet over the ground, the wind blows parallel to the isobars v a rate proportional to the press gradient.

Even enabling for the impacts of surface friction, the winds, locally, perform not constantly show the speed and direction that would certainly be expected from the isobars ~ above the surface ar lifwynnfoundation.org map. This variations room usually due to geographical functions such as hills, hills and huge bodies of water. Other than in mountainous regions, the result of terrain functions that cause local variations in wind extends commonly no higher than about 2000 feet over the ground.


Land and sea breezes are brought about by the distinctions in temperature over land and water. The sea breeze occurs throughout the day when the land area heats an ext rapidly 보다 the water surface. This results in the press over the floor being reduced than that over the water. The press gradient is often strong enough because that a wind to blow from the water to the land.


The floor breeze blows in ~ night as soon as the land i do not care cooler. Climate the wind blows in the direction of the warm, low-pressure area end the water.

Land and also sea breezes are very local and influence only a small area follow me the coast.


Hills and valleys substantially distort the airflow linked with the prevailing push system and also the pressure gradient. Strong up and down drafts and also eddies build as the air operation up over hills and also down into valleys. Wind direction alters as the air flows about hills. Occasionally lines that hills and also mountain arrays will act together a barrier, holding earlier the wind and deflecting it so the it operation parallel to the range. If there is a pass in the mountain range, the wind will certainly rush through this happen as with a tunnel with significant speed. The airflow can be expected to remain turbulent and also erratic for part distance as it flows out of the hilly area and into the flatter countryside.


Daytime heating and also nighttime cooling of the hilly slopes result in day to night sport in the airflow. In ~ night, the political parties of the hills cool by radiation. The waiting in contact with them i do not care cooler and therefore denser and it blows down the slope into the valley. This is a katabatic wind (sometimes also called a mountain breeze). If the slopes space covered v ice and also snow, the katabatic wind will blow, not only at night, but additionally during the day, carrying the cold dense air right into the warmer valleys. The slopes of hills not extended by snow will be warmed during the day. The air in call with them becomes warmer and less dense and, therefore, flows up the slope. This is an anabatic wind (or valley breeze).

In such as mountain areas, regional distortion the the air flow is even much more severe. Rocky surfaces, high ridges, sheer cliffs, steep valleys, all integrate to create unpredictable circulation patterns and turbulence.



Air flowing across a mountain selection usually rises reasonably smoothly up the steep of the range, but, once over the top, the pours down the various other side with significant force, bouncing up and also down, producing eddies and turbulence and likewise creating an effective vertical waves the may expand for great distances downwind of the mountain range. This phenomenon is well-known as a hill wave. Keep in mind the up and down drafts and also the rotating eddies developed downstream.

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

Cap Cloud. Orographic lift causes a cloud to form along the top of the ridge. The wind carries this cloud under along the leeward slope where it dissipates through adiabatic heating. The base of this cloud lies close to or below the peaks the 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 extend to well over 40,000 feet.

Rotor Clouds form in the rolling eddies downstream. They resemble a lengthy line that stratocumulus clouds, the bases of i m sorry lie listed below the hill peaks and also the top of which may reach come a substantial height over the peaks. Occasionally these clouds develop into thunderstorms.


The clouds, being really distinctive, deserve to be watched from a an excellent distance and carry out a clearly shows warning that the mountain wave condition. Unfortunately, occasionally they are installed in other cloud systems and are surprise from sight. Periodically the air mass is an extremely dry and also the clouds execute not develop.

The severity of the mountain wave and the height to i beg your pardon the disturbance that the air is affected is dependence on the stamin of the wind, its angle to the variety and the security or instability the the air. The most severe hill wave problems are created in solid airflows that space blowing at best angles to the range and in secure air. A jet stream blowing practically perpendicular to the mountain variety increases the severity of the tide condition.

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

Mountain waves existing problems come pilots for several reasons:

Vertical Currents. Downdrafts the 2000 feet every minute are common and downdrafts as an excellent as 5000 feet every minute have been reported. They happen along the downward slope and are most severe in ~ a height equal to that of the summit. An airplane, caught in a downdraft, could be required to the ground.

Turbulence is usually very severe in the air layer in between the ground and the top of the rotor clouds.

Wind Shear. The wind speed varies dramatically in between the crests and also troughs the the waves. It is usually most severe in the wave nearest the hill range.

Altimeter Error. The boost in wind speed results in one accompanying decrease in pressure, which consequently affects the accuracy that the press altimeter.

Icing. The freezing level varies considerably from crest to trough. Severe icing have the right to occur due to the fact that of the big supercooled droplets sustained in the solid vertical currents.

When flying end a mountain ridge where wave conditions exist: (1) protect against ragged and also irregular shame clouds—the rarely often rare shape suggests turbulence. (2) technique the mountain at a 45-degree angle. That you have to suddenly decide to revolve back, a fast turn can be made away from the high ground. (3) protect against flying in cloud top top the mountain crest (cap cloud) due to the fact that of strong downdrafts and turbulence. (4) permit sufficient elevation to clean the highest possible ridges with altitude to preventive to avoid the downdrafts and also eddies on the downwind slopes. (5) constantly remember that your altimeter deserve to read over 3000 ft. In error ~ above the high next in hill wave conditions.


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


A squall is a sudden rise in the stamin of the wind of longer duration than a gust and also may be brought about by the i of a quick moving cold front or thunderstorm. Favor a gust, it might be accompanied by a rapid adjust of wind direction.


Diurnal (daily) sports of wind is caused by strong surface heating throughout the day, which reasons turbulence in the lower levels. The result of this turbulence is the the direction and speed of the wind at the greater levels (e.g., 3000 feet) often tends to be moved to the surface. Since the wind direction in ~ the higher level is parallel come the isobars and its rate is better than the surface wind, this transfer causes the surface wind to veer and increase in speed.

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


Friction in between the relocating air mass and also surface features of the planet (hills, mountains, valleys, trees, buildings, etc.) is responsible for the swirling vortices the air commonly called eddies. Castle vary substantially in size and also intensity depending upon the size and also roughness of the surface ar obstruction, the rate of the wind and also the level of stability of the air. They have the right to spin in one of two people a horizontal or upright plane. Stormy air and strong winds produce an ext vigorous eddies. In secure air, eddies often tend to quickly dissipate. Eddies developed in mountainous areas are particularly powerful.


The bumpy or choppy up and down activity that signifies the existence of eddies provides it challenging to save an aircraft in level flight.


Dust devils are phenomena that occur quite generally on the hot dry plains of mid-western north America. They can be of enough force to present a danger to pilots of irradiate airplanes flying at low speeds.

They are tiny heat lows that type on clear hot days. Given a steep slide out rate resulted in by cool air aloft over a hot surface, small horizontal waiting movement, few or no clouds, and also the noonday sun heating flat arid soil surfaces come high temperatures, the air in call with the ground i do not care super-heated and highly unstable. This surface layer of waiting builds until something cause an increase movement. Once started, the warm air rises in a column and also draws an ext hot air right into the base of the column. Circulation begins about this warm low and increases in velocity till a little vigorous whirlwind is created. Dust devils space usually of short duration and are for this reason named because they are made clearly shows by the dust, sand and debris that they choose up native the ground.

Dust devils posture the biggest hazard near the ground wherein they are many violent. Pilots proposing come land top top superheated runways in areas of the mid-west wherein this phenomenon is usual should scan the airport for dust swirls or grass spirals that would suggest the presence of this hazard.


Tornadoes space violent, circular whirlpools the air associated with severe thunderstorms and also are, in fact, really deep, concentrated low-pressure areas. They room shaped prefer a tunnel hanging out of the cumulonimbus cloud and are dark in appearance as result of the dust and also debris sucked into their whirlpools. They range in diameter from around 100 feet come one half mile and move over the soil at speed of 25 to 50 knots. Their course over the soil is usually just a couple of miles lengthy although tornadoes have been reported to cut destructive swaths as lengthy as 100 miles. The an excellent destructiveness that tornadoes is led to by the really low press in their centers and the high wind speeds, i m sorry are considered to be as good as 300 knots.


Wind speeds for aviation purposes are expressed in knots (nautical miles every hour). In the lifwynnfoundation.org reports on united state public radio and television, however, wind speed are provided in miles every hour when in Canada speed are offered in kilometers every hour.

In a conversation of wind direction, the compass point from which the wind is blowing is taken into consideration to be its direction. Therefore, a north wind is one that is blowing native the north towards the south. In aviation lifwynnfoundation.org reports, area and also aerodrome forecasts, the wind is constantly reported in degrees true. In ATIS broadcasts and also in the information offered by the tower for landing and also take-off, the wind is reported in levels magnetic.


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

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

In a lower from numerous thousand feet over the ground to ground level, the wind will normally be discovered to ago and additionally decrease in velocity, as the result of surface ar friction becomes apparent. In a rise from the surface to several thousand feet AGL, the wind will certainly veer and increase.

At night, surface cooling reduces the eddy motion of the air. Surface winds will earlier and decrease. Conversely, throughout the day, surface ar heating rises the eddy motion of the air. Surface ar winds will veer and increase as more powerful winds aloft mix to the surface. Watch DIURNAL VARIATIONS section over for much more info.


Wind shear is the suddenly tearing or shearing effect encountered along the leaf of a region in which 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 also consequently turbulence. Under part conditions, wind direction changes of as much as 180 degrees and speed transforms of as much as 80 knots have actually been measured.


The effect on plane performance of encountering wind shear derives indigenous the fact that the wind can adjust much much faster than the airplane mass deserve to be sped up or decelerated. Significant wind shears deserve to impose penalty on one airplane\"s performance the are past its capabilities to compensate, especially throughout the an important landing and take-off step of flight.

In cruising Flight

In seafaring flight, wind shear will likely be encountered in the change zone in between the pressure gradient wind and also the distorted regional winds at the lower levels. That will likewise be encountered once climbing or descending through a temperature inversion and also when passing through a frontal surface. Wind shear is also associated through the jet stream. Airplane encountering wind shear might experience a succession of updrafts and downdrafts, to reduce or gains in headwind, or windshifts that disrupt the established flight path. The is not usually a major problem because altitude and airspeed margins will be enough to against the shear\"s adverse effects. On occasion, however, the wind shear may be severe enough to reason an abrupt boost in load factor, which might stall the aircraft or inflict structural damage.


Near the Ground

Wind shear, encountered near the ground, is much more serious and potentially an extremely dangerous. There space four typical sources of low level wind shear: thunderstorms, frontal activity, temperature inversions and strong surface winds passing approximately natural or manmade obstacles.

Frontal Wind Shear. Wind shear is commonly a difficulty only in fronts with steep wind gradients. If the temperature difference throughout the front in ~ the surface ar is 5°C or more and if the front is moving at a speed of about 30 knots or more, wind shear is most likely to be present. Frontal wind shear is a phenomenon linked with quick moving cold fronts however can be existing in warm fronts as well.


Thunderstorms. Wind shear, associated with thunderstorms, occurs together the an outcome of 2 phenomena, the gust front and also downbursts. As the thunderstorm matures, solid downdrafts develop, to win the ground and spread the end horizontally follow me the surface well in advancement of the thunderstorm itself. This is the gust front. Winds can adjust direction by as lot as 180° and also reach speeds as great as 100 knots as much as 10 miles ahead the the storm. The downburst is an extremely intense localized downdraft flowing the end of a thunderstorm. The power of the downburst can exceed aircraft climb capabilities. The downburst (there are two types of downbursts: macrobursts and microbursts) typically is much closer to the thunderstorm 보다 the gust front. Dust clouds, roll clouds, extreme rainfall or virga (rain that evaporates before it reaches the ground) are due to the possibility of downburst activity but over there is no way to accurately guess its occurrence.


Temperature Inversions. Overnight cooling create a temperature inversion a few hundred feet above the ground that deserve to produce significant wind shear, particularly if the inversion is coupled v the low-level jet stream.

As a nocturnal inversion develops, the wind shear near the optimal of the reverse increases. It generally reaches its preferably speed quickly after midnight and decreases in the morning together daytime heater dissipates the inversion. This phenomenon is known as the low-level nocturnal jet stream. The short level jet currently is a sheet of solid winds, hundreds of miles long, thousands of miles vast and numerous feet thick that develops over level terrain such as the prairies. Wind speeds of 40 knots room common, but greater speeds have been measured. Low level jet streams room responsible for hazardous short level shear.


As the turning back dissipates in the morning, the shear airplane and gusty winds relocate closer to the ground, bring about windshifts and increases in wind speed near the surface.

Surface Obstructions. The irregular and turbulent flow of air approximately mountains and hills and through mountain passes causes serious wind shear difficulties for aircraft approaching come land at airports near hill ridges. Wind shear is a phenomenon associated with the mountain wave. Together shear is almost totally unpredictable however should be supposed whenever surface ar winds space strong.

Wind shear is additionally associated through hangars and large buildings in ~ airports. Together the wait flows approximately such large structures, wind direction changes and wind speed rises causing shear.

Wind shear wake up both horizontally and also vertically. Upright shear is most usual near the ground and can attitude a serious peril to airplanes throughout take-off and also landing. The plane is paris at lower speeds and in a relatively high drag configuration. Over there is small altitude accessible for recovering and also stall and also maneuver margins room at your lowest. An aircraft encountering the wind shear phenomenon might experience a huge loss the airspeed because of the sudden readjust in the family member airflow together the aircraft flies right into a new, relocating air mass. The abrupt autumn in airspeed may an outcome in a stall, developing a dangerous instance when the plane is just a few hundred feet turn off the floor and an extremely vulnerable.


Narrow bands that exceedingly high speed winds are recognized to exist in the higher levels the the setting at altitudes varying from 20,000 come 40,000 feet or more. Castle are recognized as jet streams. As plenty of as three major jet streams may traverse the north American continent at any kind of given time. One lies across Northern Canada and also one across the U.S. A third jet stream might be as much south as the north tropics however it is rather rare. A jet currently in the mid latitudes is normally the strongest.


The jet stream appears to be closely associated with the tropopause and with the polar front. It frequently forms in the break between the polar and the tropical tropopause whereby the temperature gradients room intensified. The median position the the jet stream shears south in winter and also north in summer v the seasonal migration of the polar front. Because the troposphere is depth in summer than in winter, the tropopause and also the jets will certainly nominally it is in at higher altitudes in the summer.

Long, strong jet streams are usually also associated v well-developed surface ar lows in ~ deep top troughs and also lows. A low occurring in the tide along the frontal surface lies southern of the jet. As it deepens, the short moves close to the jet. Together it occludes, the short moves north of the jet, which the cross the frontal system, near the suggest of occlusion. The jet flows about parallel to the front. The subtropical jet stream is not linked with fronts however forms since of strong solar heater in the equatorial regions. The ascending air transforms poleward at very high levels yet is deflected through the Coriolis force into a solid westerly jet. The subtropical jet predominates in winter.


The jet streams flow from west to east and may encircle the whole hemisphere. More often, because they room stronger in some areas than in others, they rest up into segments part 1000 to 3000 nautical mile long. They space usually about 300 nautical miles broad and may be 3000 come 7000 feet thick. These jet stream segments move in an easterly direction adhering to the activity of pressure ridges and also troughs in the top atmosphere.

Winds in the main core that the jet stream room the strongest and may reach speeds as an excellent as 250 knots, back they space generally between 100 and 150 knots. Wind speeds decrease towards the external edges that the jet stream and also may it is in blowing at just 25 knots there. The price of diminish of wind rate is significantly greater ~ above the northern edge than on the southern edge. Wind speeds in the jet stream are, on average, considerably stronger in winter than in summer.

Clear waiting Turbulence. The most probable ar to mean Clear Air disturbance (CAT) is just over the main core that the jet stream near the polar tropopause and also just listed below the core. Clean air turbulence does not occur in the core. CAT is encountered much more frequently in winter once the jet present winds room strongest. Nevertheless, CAT is not constantly present in the jet present and, because it is random and transient in nature, that is nearly impossible come forecast.

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Clear wait turbulence might be connected with other lifwynnfoundation.org patterns, particularly in wind shear linked with the sharply bent contours of strong lows, troughs and also ridges aloft, at or below the tropopause, and in locations of solid cold or heat air advection. Hill waves develop severe CAT that may prolong from the mountain crests to as high as 5000 feet over the tropopause. Since severe CAT does pose a danger to airplanes, pilots should try to prevent or minimization encounters through it. This rules of thumb may help avoid jet streams with solid winds (150 knots) in ~ the core. Solid wind shears room likely above and below the core. CAT in ~ the jet stream is more intense over and to the lee of mountain ranges. If the 20-knot isotachs (lines joining locations of equal wind speeds) space closer 보다 60 nautical mile on the charts mirroring the areas of the jet stream, wind shear and also CAT space possible.

Curving jet streams are likely to have actually turbulent edges, particularly those that curve approximately a deep pressure trough. When moderate or serious CAT has been report or is forecast, change speed to stormy air speed immediately on encountering the an initial bumpiness or even before encountering that to avoid structural damages to the airplane.

The areas of CAT are usually shallow and narrow and elongated with the wind. If jet stream disturbance is encountered through a tail wind or head wind, a rotate to the right will find smoother wait and an ext favorable winds. If the CAT is encountered in a crosswind, that is not so vital to readjust course as the rough area will certainly be narrow.