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

Flight Environment



Since the setting is fixed to the earthby gravity and also rotates through the earth, there would be no circulation if some pressure did notupset the atmosphere"s equilibrium. The heater of the earth"s surface ar by the sun isthe pressure responsible for creating the circulation that does exist.

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

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


Warm wait rises until it get acertain height at which it starts to pour out over right into surrounding areas. At thepoles, the cold thick air sinks. Waiting from the top levels of the atmosphere flowsin on top of it raising the weight and creating an area of high push at the poles.

The air that rises at the equator walk notflow directly to the poles. As result of the rotation the the earth, over there is a construct up of airat around 30° north latitude. (The very same phenomenon occurs in the southern Hemisphere). Some of the wait sinks, resulting in a belt of high-pressure in ~ this latitude.


The sinking wait reaches the surfaceand operation north and south. The air that flows south completes one cell of theearth"s circulation pattern. The air that flows phibìc becomes component of another cellof circulation in between 30° and 60° phibìc latitude. At the exact same time, the sinkingair at the north pole operation south and also collides v the air relocating north from the 30°high press area. The colliding waiting is forced upward and an area of short pressureis produced near 60° north. The third cell circulation pattern is developed betweenthe north pole and also 60° north.

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

Since the earth rotates, the axis is tilted, and also there is more land fixed in the north hemisphere than in the southern hemisphere, the actual an international pattern is much much more complicated. Instead of one large circulation between the poles and the equator, there space three circulations...Hadley cell
- short latitude air motion toward the equator that v heating, rises vertically, v poleward motion in the upper atmosphere. This forms a convection cell the dominates tropical and sub-tropical climates.Ferrel cell - A mid-latitude median atmospheric circulation cell for rewildtv.com called by Ferrel in the 19th century. In this cell the air operation poleward and eastward near the surface and also equatorward and also westward at higher levels.Polar cell - wait rises, diverges, and travels toward the poles. Once over the poles, the wait sinks, creating the polar highs. At the surface air diverges outward from the polar highs. Surface winds in the polar cell space easterly (polar easterlies).


There are two main pressures which influence the movement of air in the top levels. The push gradient causes the wait to move horizontally, forcing the air straight from a region of high push to a an ar of low pressure. The Coriolis force, however, deflects the direction of the circulation of the wait (to the appropriate in the northern Hemisphere) and also causes the waiting to circulation parallel come the isobars.

Winds in the upper levels will certainly blow clockwise approximately areas that high pressure and counterclockwise approximately areas of short pressure.

The rate of the wind is determined by the pressure gradient. The winds are strongest in areas where the isobars room close together.


Surface friction plays an important role in the speed and direction of surface ar winds. As a an outcome of the slowing under of the air together it moves over the ground, wind speeds are much less than would be intended from the pressure gradient on the rewildtv.com map and also the direction is changed so that the wind blows across the isobars into a center of short pressure and out of a center of high pressure.


The impact of friction normally does no extend an ext than a couple of thousands feet into the air. In ~ 3000 feet above the ground, the wind blows parallel to the isobars with a rate proportional come the push gradient.

Even enabling for the results of surface ar friction, the winds, locally, carry out not constantly show the speed and also direction that would certainly be expected from the isobars top top the surface rewildtv.com map. These variations room usually due to geographical attributes such together hills, hills and large bodies that water. Other than in such as mountain regions, the effect of terrain features that reason local variations in wind extends normally no greater than about 2000 feet over the ground.


Land and also sea breezes are led to by the distinctions in temperature end land and water. The sea breeze occurs during the day once the floor area heats an ext rapidly 보다 the water surface. This results in the pressure over the soil being lower than that over the water. The press gradient is often strong enough because that a wind to punch from the water come the land.


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

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


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


Daytime heating and also nighttime cooling that the hilly slopes lead to day come night sport in the airflow. At night, the sides of the hills cool through radiation. The waiting in call with them becomes cooler and therefore denser and also it blows down the slope right into the valley. This is a katabatic wind (sometimes likewise called a hill breeze). If the slopes room covered through ice and snow, the katabatic wind will blow, not just at night, but also during the day, moving the cold thick air into the warmer valleys. The slopes the hills not extended by snow will certainly be warmed during the day. The wait in contact with them becomes warmer and also less dense and, therefore, operation up the slope. This is one anabatic wind (or sink breeze).

In such as mountain areas, neighborhood distortion that the airflow is even more severe. Rocky surfaces, high ridges, slim cliffs, steep valleys, all combine to create unpredictable circulation patterns and also turbulence.



Air flowing throughout a mountain variety usually rises relatively smoothly increase the steep of the range, but, as soon as over the top, that pours under the various other side with considerable force, bouncing up and down, developing eddies and also turbulence and also creating an effective vertical waves that may prolong for great distances downwind that the hill range. This phenomenon is recognized as a mountain wave. Note the up and also down drafts and the rotating eddies developed downstream.

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

Cap Cloud. Orographic lift reasons a cloud to kind along the height of the ridge. The wind carries this cloud under along the leeward slope wherein it dissipates with adiabatic heating. The base of this cloud lies close to or listed below the peaks that the ridge; the top may reach a few thousand feet above the peaks.

Lenticular (Lens Shaped) Clouds form in the tide crests aloft and also lie in bands that may prolong to well over 40,000 feet.

Rotor Clouds form in the roll eddies downstream. Lock resemble a long line of stratocumulus clouds, the bases of which lie listed below the hill peaks and the tops of which may reach to a considerable height over the peaks. Periodically these clouds construct into thunderstorms.


The clouds, being really distinctive, can be viewed from a good distance and provide a clearly shows warning that the mountain wave condition. Unfortunately, sometimes they are installed in various other cloud systems and also are hidden from sight. Occasionally the wait mass is an extremely dry and the clouds perform not develop.

The severity of the mountain wave and also the elevation to which the disturbance that the waiting is affected is dependency on the strength of the wind, its edge to the selection and the security or instability that the air. The many severe mountain wave problems are produced in solid airflows that room blowing at appropriate angles come the range and in stable air. A jet stream blowing nearly perpendicular come the mountain selection increases the severity that the wave condition.

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

Mountain waves existing problems to pilots for number of reasons:

Vertical Currents. Downdrafts the 2000 feet per minute space common and downdrafts as good as 5000 feet per minute have been reported. They happen along the bottom slope and also are many severe at a elevation equal to the of the summit. One airplane, caught in a downdraft, might be compelled to the ground.

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

Wind Shear. The wind speed varies dramatically in between the crests and also troughs that the waves. The is usually many severe in the tide nearest the hill range.

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

Icing. The freezing level varies considerably from crest to trough. Serious icing can occur due to the fact that of the huge supercooled droplets continual in the solid vertical currents.

When flying end a mountain ridge where wave problems exist: (1) protect against ragged and irregular shame clouds—the rarely often, rarely shape suggests turbulence. (2) method the mountain at a 45-degree angle. It you must suddenly decision to turn back, a quick turn deserve to be made far from the high ground. (3) protect against flying in cloud on the mountain crest (cap cloud) since of solid downdrafts and also turbulence. (4) enable sufficient height to clean the highest ridges through altitude to spare to avoid the downdrafts and also eddies on the downwind slopes. (5) always remember that your altimeter can read end 3000 ft. In error top top the high side in mountain wave conditions.


A gust is a rapid and irregular fluctuation of varying intensity in the upward and also downward activity of waiting currents. It might be connected with a rapid change in wind direction. Gusts are led to by mechanical turbulence that outcomes from friction between the air and 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 toughness of the wind of much longer duration 보다 a gust and also may be led to by the passage of a fast moving cold former or thunderstorm. Choose a gust, it might be add by a rapid adjust 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 that the direction and speed that the wind in ~ the greater levels (e.g., 3000 feet) tends to be transferred to the surface. Since the wind direction at the higher level is parallel come the isobars and also its rate is greater than the surface wind, this transfer causes 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 common direction and also speed. It backs and also decreases. Check out VEERING and BACKING section below for more info.


Friction between the relocating air mass and surface attributes of the earth (hills, mountains, valleys, trees, buildings, etc.) is responsible for the swirling vortices of air typically called eddies. Lock vary considerably in size and intensity depending upon 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 have the right to spin in one of two people a horizontal or upright plane. Turbulent air and strong winds produce more vigorous eddies. In secure air, eddies often tend to easily dissipate. Eddies developed in mountainous areas are especially powerful.


The bumpy or choppy up and also down motion that signifies the presence of eddies makes it an overwhelming to keep an airplane in level flight.


Dust devils are phenomena that take place quite generally on the warm dry levels of mid-western north America. They deserve to be of enough force to present a hazard to pilots of irradiate airplanes flying at low speeds.

They are small heat short that form on clear hot days. Given a steep slide away rate caused by cool waiting aloft end a hot surface, small horizontal wait movement, couple of or no clouds, and also the noonday sun heating flat arid soil surfaces to high temperatures, the wait in contact with the ground i do not care super-heated and also highly unstable. This surface layer of air builds till something triggers an upward movement. When started, the warm air rises in a column and draws an ext hot air right into the base of the column. Circulation begins approximately this heat low and also increases in velocity till a little vigorous whirlwind is created. Dust devils space usually of quick duration and also are therefore named because they space made visible by the dust, sand and also debris the they pick up indigenous the ground.

Dust devils posture the biggest hazard close to the ground whereby they are most violent. Pilots proposing to land on superheated runways in areas of the mid-west wherein this phenomenon is usual should scan the airport for dust swirls or grass spirals the would indicate the existence of this hazard.


Tornadoes are violent, circular whirlpools the air linked with severe thunderstorms and also are, in fact, really deep, concentrated low-pressure areas. They room shaped favor a tunnel hanging the end of the cumulonimbus cloud and also are dark in appearance due to the dust and also debris sucked into their whirlpools. They selection in diameter from around 100 feet come one half mile and also move end the soil at speeds of 25 to 50 knots. Their course over the floor is usually only a few miles lengthy although tornadoes have been reported to reduced destructive swaths as lengthy as 100 miles. The great destructiveness of tornadoes is brought about by the very low press in their centers and the high wind speeds, which are reputed to be as good as 300 knots.


Wind speeds because that aviation objectives are expressed in knots (nautical miles every hour). In the rewildtv.com reports on united state public radio and television, however, wind speed are offered in miles every hour when in Canada speed are offered in kilometers every hour.

In a discussion of wind direction, the compass point from which the wind is blowing is considered to it is in its direction. Therefore, a north wind is one that is blowing from the north towards the south. In aviation rewildtv.com reports, area and aerodrome forecasts, the wind is always reported in degrees true. In ATIS broadcasts and in the information provided by the tower because that landing and also take-off, the wind is report in levels magnetic.


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

The wind backs as soon as it alters direction anti-clockwise. Example: The wind direction at 2000 feet is 090° and also at 3000 feet is 085°. The is changing in a left-hand, or anti-clockwise, direction.

In a lower from numerous thousand feet over the ground to ground level, the wind will generally be uncovered to ago and also decrease in velocity, together the effect of surface ar friction i do not care apparent. In a rise from the surface ar to several thousand feet AGL, the wind will veer and also increase.

At night, surface ar cooling to reduce the eddy movement of the air. Surface winds will ago and decrease. Conversely, during the day, surface ar heating boosts the eddy motion of the air. Surface ar winds will certainly veer and increase as more powerful winds aloft mix come the surface. Check out DIURNAL VARIATIONS section over for more info.


Wind shear is the suddenly tearing or shearing result encountered follow me the sheet of a zone in which there is a violent adjust in wind speed or direction. It can exist in a horizontal or upright direction and also produces churning motions and also consequently turbulence. Under some conditions, wind direction alters of as lot as 180 degrees and speed alters of as much as 80 knots have been measured.


The effect on plane performance the encountering wind shear derives indigenous the truth that the wind can readjust much much faster than the aircraft mass can be increased or decelerated. Severe wind shears deserve to impose penalties on an airplane"s performance that are past its capabilities to compensate, especially throughout the an essential landing and also take-off phase of flight.

In seafaring Flight

In setravel flight, wind shear will most likely be encountered in the change zone between the push gradient wind and the distorted regional winds in ~ the lower levels. The will also be encountered as soon as climbing or descending through a temperature inversion and also when passing v a frontal surface. Wind shear is also associated v the jet stream. Airplane encountering wind shear may experience a sequence of updrafts and also downdrafts, reductions or benefit in headwind, or windshifts the disrupt the created flight path. The is not typically a major problem because altitude and also airspeed margins will be adequate to against the shear"s disadvantage effects. On occasion, however, the wind shear might be severe enough to cause an abrupt rise in load factor, which might stall the airplane or inflict structure damage.


Near the Ground

Wind shear, encountered near the ground, is much more serious and also potentially very dangerous. There space four common sources of short level wind shear: thunderstorms, frontal activity, temperature inversions and strong surface winds passing around natural or manmade obstacles.

Frontal Wind Shear. Wind shear is normally a trouble only in fronts v steep wind gradients. If the temperature difference throughout the front in ~ the surface ar is 5°C or an ext and if the front is moving at a rate of around 30 knots or more, wind shear is likely to be present. Frontal wind shear is a phenomenon linked with rapid moving cold fronts yet can be existing in warmth fronts as well.


Thunderstorms. Wind shear, linked with thunderstorms, occurs together the result of two phenomena, the gust front and also downbursts. As the thunderstorm matures, strong downdrafts develop, win the ground and spread the end horizontally follow me the surface ar well in development of the thunderstorm itself. This is the gust front. Winds can adjust direction by as much as 180° and also reach speed as great as 100 knots as far as 10 mile ahead that the storm. The downburst is very intense localized downdraft flowing out of a thunderstorm. The power of the downburst can exceed aircraft climb capabilities. The downburst (there space two varieties of downbursts: macrobursts and also microbursts) normally is lot closer come the thunderstorm than the gust front. Dust clouds, roll clouds, extreme rainfall or virga (rain that evaporates prior to it reaches the ground) are as result of the possibility of downburst activity but over there is no means to accurately predict its occurrence.


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

As a nocturnal inversion develops, the wind shear near the height of the reverse increases. It typically reaches its maximum speed soon after midnight and decreases in the morning together daytime heating dissipates the inversion. This phenomenon is recognized as the low-level nocturnal jet stream. The short level jet currently is a paper of solid winds, hundreds of miles long, numerous miles vast and hundreds of feet thick that creates over flat terrain such together the prairies. Wind speed of 40 knots room common, yet greater speeds have actually been measured. Short level jet streams space responsible for hazardous short level shear.


As the reverse dissipates in the morning, the shear plane and gusty winds move closer come the ground, resulting in windshifts and increases in wind speed close to the surface.

Surface Obstructions. The irregular and also turbulent flow of air around mountains and hills and also through mountain passes causes serious wind shear troubles for plane approaching to land at airports near mountain ridges. Wind shear is a phenomenon connected with the hill wave. Together shear is almost totally unpredictable however should be supposed whenever surface winds room strong.

Wind shear is likewise associated through hangars and large buildings at airports. As the air flows around such large structures, wind direction changes and wind speed increases causing shear.

Wind shear wake up both horizontally and also vertically. Vertical shear is most common near the ground and can pose a serious hazard to airplanes during take-off and landing. The plane is flying at reduced speeds and also in a reasonably high drag configuration. There is tiny altitude accessible for recovering and stall and maneuver margins space at your lowest. An aircraft encountering the wind shear phenomenon may experience a big loss of airspeed since of the sudden adjust in the relative airflow together the aircraft flies into a new, relocating air mass. The abrupt drop in airspeed may an outcome in a stall, producing a dangerous situation when the airplane is just a couple of hundred feet turn off the soil and an extremely vulnerable.


Narrow bands the exceedingly high rate winds are well-known to exist in the greater levels the the atmosphere at altitudes varying from 20,000 come 40,000 feet or more. Castle are recognized as jet streams. As countless as three significant jet streams may traverse the phibìc American continent at any kind of given time. One lies across Northern Canada and one across the U.S. A 3rd jet stream might be as far south as the northern tropics but it is somewhat rare. A jet present in the mid latitudes is typically the strongest.


The jet stream shows up to be closely connected with the tropopause and with the polar front. It frequently forms in the break in between the polar and also the tropic tropopause where the temperature gradients room intensified. The average position of the jet currently shears south in winter and also north in summer through the seasonal migration of the polar front. Because the troposphere is deeper in summer than in winter, the tropopause and the jets will nominally it is in at greater altitudes in the summer.

Long, strong jet streams room usually additionally associated with well-developed surface ar lows in ~ deep upper troughs and also lows. A low developing in the wave along the frontal surface lies south of the jet. As it deepens, the low moves close to the jet. As it occludes, the low moves north of the jet, which crosses the frontal system, near the suggest of occlusion. The jet flows roughly parallel to the front. The subtropical jet present is not associated with fronts yet forms since of solid solar heater in the equatorial regions. The ascending air transforms poleward at really high levels but is deflected through the Coriolis pressure into a solid westerly jet. The subtropical jet predominates in winter.


The jet streams circulation from west come east and may encircle the entire hemisphere. More often, since they room stronger in some places than in others, they rest up into segments some 1000 come 3000 nautical mile long. They are usually around 300 nautical miles wide and may be 3000 to 7000 feet thick. These jet present segments move in one easterly direction complying with the motion of pressure ridges and troughs in the top atmosphere.

Winds in the main core of the jet stream are the strongest and also may with speeds as great as 250 knots, return they are generally in between 100 and also 150 knots. Wind speed decrease toward the external edges that the jet stream and may it is in blowing at just 25 knots there. The price of decrease of wind speed is substantially greater ~ above the northern edge than on the southern edge. Wind speeds in the jet currently are, ~ above average, considerably stronger in winter 보다 in summer.

Clear waiting Turbulence. The many probable location to intend Clear Air disturbance (CAT) is just over the main core the the jet stream near the polar tropopause and also just below the core. Clean air disturbance does not occur in the core. CAT is encountered much more frequently in winter when the jet stream winds room strongest. Nevertheless, CAT is not constantly present in the jet currently and, because it is random and also transient in nature, it is practically impossible come forecast.

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Clear waiting turbulence may be associated with various other rewildtv.com patterns, particularly in wind shear associated with the sharply curved contours of strong lows, troughs and ridges aloft, in ~ or listed below the tropopause, and in areas of strong cold or warm air advection. Hill waves produce severe CAT the may extend from the hill crests to together high as 5000 feet above the tropopause. Since severe CAT does attitude a risk to airplanes, pilots should try to stop or minimization encounters through it. These rules of ignorance may help avoid jet streams with solid winds (150 knots) in ~ the core. Solid wind shears room likely over and below the core. CAT in ~ the jet present is much more intense above and come the lee of mountain ranges. If the 20-knot isotachs (lines joining locations of same wind speeds) room closer than 60 nautical miles on the charts reflecting the places of the jet stream, wind shear and CAT room possible.

Curving jet streams are likely to have turbulent edges, particularly those that curve around a deep push trough. Once moderate or serious CAT has been report or is forecast, readjust speed to rough air speed instantly on encountering the first bumpiness or even prior to encountering the to protect against structural damages to the airplane.

The locations of CAT room usually shallow and narrow and also elongated through the wind. If jet stream disturbance is encountered through a tail wind or head wind, a revolve to the right will uncover smoother air and an ext favorable winds. If the CAT is encountered in a crosswind, it is no so essential to change course as the unstable area will be narrow.