The cause of atmospheric circulation
The factor that upsets the normal equilibrium is the
uneven heating of the Earth. At the Equator, the Earth receives more heat
than in areas to the north and south. This heat is transferred to the
atmosphere, warming the air and causing it to expand and become less
dense. Colder air to the north and south, being more dense, moves toward
the Equator forcing the less dense air upward. This air in turn becomes
warmer and less dense and is forced upward, thus establishing a constant
circulation that might consist of two circular paths; the air rising at
the Equator, traveling aloft toward the poles, and returning along the
Earth’s surface to the Equator, as shown in figure 5-5. |
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Figure 5-5.—Heat at the Equator would cause the air to circulate
uniformly, as shown, if the Earth did not
rotate. |
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This theoretical pattern, however, is greatly modified by many forces,
a very important one being the rotation of the Earth. In the Northern
Hemisphere, this rotation causes air to deflect to the right of its normal
path. In the Southern Hemisphere, air is deflected to the left of its
normal path. For simplicity, this discussion will be confined to the
motion of air in the Northern Hemisphere. [Figure 5-6]
As the air rises and moves northward from the Equator, it is deflected
toward the east, and by the time it has traveled about a third of the
distance to the pole, it is no longer moving northward, but eastward. This
causes the air to |
Figure 5-6.—Principal air currents in the Northern
Hemisphere. |
| accumulate in a belt at about latitude 30°,
creating an area of high pressure. Some of this air is then forced down to the
Earth’s surface, where part flows southwestward, returning to the Equator, and
part flows northeastward along the surface.
A portion of the air aloft continues its journey northward, being
cooled en route, and finally settles down near the pole, where it begins a
return trip toward the Equator. Before it has progressed very far southward, it
comes into conflict with the warmer surface air flowing northward from latitude
30°. The warmer air moves up over a wedge of the colder air, and continues
northward, producing an accumulation of air in the upper latitudes.
Further complications in the general circulation of the air are brought
about by the irregular distribution of oceans and continents, the relative
effectiveness of different surfaces in transferring heat to the atmosphere, the
daily variation in temperature, the seasonal changes, and many other factors.
Regions of low pressure, called “lows,” develop where air lies over
land or water surfaces that are warmer than the surrounding areas. In India, for
example, a low forms over the hot land during the summer months, but moves out
over the warmer ocean when the land cools in winter. Lows of this type are
semipermanent, however, and are less significant to the pilot than the
“migratory cyclones” or “cyclonic depressions” that form when unlike air masses
meet. These lows will be discussed later in this chapter.
Wind Patterns
This is a discussion of wind patterns associated with areas of high and
low pressure. As previously stated, air flows from an area of high pressure to
an area of low pressure. In the Northern Hemisphere, during this flow, the air
is deflected to the right. Therefore, as the air leaves the high pressure area,
it is deflected to produce a clockwise circulation. As the air flows toward the
low pressure area, it is deflected to produce a counterclockwise flow around the
low pressure area.
Another important aspect is that air moving out of a high pressure
area depletes the quantity of air. Therefore, highs are areas of
descending air. Descending air favors dissipation of cloudiness; hence the
association, high pressure—good weather. By similar reasoning, when air
converges into a low pressure area; it cannot go outward against the
pressure gradient, nor can it go downward into the ground; it must go
upward. Rising air is conducive to cloudiness and precipitation; thus the
general association low pressure—bad weather.
A knowledge of these patterns frequently enables a pilot to plan a
course to take advantage of favorable winds, |
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Figure 5-7.—Circulation of wind within a
“low.” | particularly during long flights.
In flying from east to west, for example, the pilot would find favorable winds
to the south of a high, or to the north of a low. It also gives the pilot a
general idea of the type of weather to expect relative to the “highs” and
“lows.” [Figures 5-7 and 5-8]
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The theory of general circulation in the atmosphere, and the wind
patterns formed within areas of high pressure and low pressure have been
discussed. These concepts account for the large scale movements of the
wind, but do not take into consideration the effects of local conditions
that frequently cause drastic modifications in wind direction and speed
near the Earth’s surface.
Convection Currents
Certain kinds of surfaces are more effective than others in
heating the air directly above them. Plowed ground, sand, rocks, and
barren land give off a great deal of heat, whereas water and vegetation
tend to absorb and retain heat. The |
Figure 5-8.—Use of favorable winds in flight |
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uneven heating of the air causes small local circulations
called “convection currents,” which are similar to the general circulation
just described.
This may be particularly noticeable over land adjacent to a body of
water. During the day, air over land becomes heated and less dense; colder
air over water moves in to replace it forcing the warm air aloft and
causing an on-shore wind. At night, the land cools, and the water is
relatively warmer. The cool air over the land, being heavier, then moves
toward the water as an off-shore wind, lifting the warmer air and
reversing the circulation. [Figures 5-9 and 5-10]
Convection currents cause the bumpiness experienced by pilots flying at
low altitudes in |
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Figure 5-9.—Convection currents form on-shore winds in the
daytime. | warmer weather. On a low flight over
varying surfaces, the pilot will encounter updrafts over pavement or barren
places and downdraft over vegetation and water. Ordinarily, this can be avoided
by flight at higher altitudes. When the larger convection currents form cumulus
clouds, the pilot will invariably find smooth air above the cloud level. [Figure
5-11]
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Figure 5-10.—Convection currents form off-shore winds at
night. |
Figure 5-11.—Avoiding turbulence caused by convection currents by
flying above the cloud level. |
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Convection currents also cause difficulty in making landings, since
they affect the rate of descent. For example, a pilot flying a normal
glide frequently tends to land short of or overshoot the intended landing
spot, depending upon the presence and severity of convection currents.
[Figures 5-12 and 5-13] |
Figure 5-12.— Varying surfaces affect the normal glidepath. Some
surfaces create rising currents which tend to cause the pilot to overshoot
the field. |
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The effects of local convection, however, are less
dangerous than the turbulence caused when wind is forced to flow around or
over obstructions. The only way for the pilot to avoid this invisible
hazard is to be forewarned, and to know where to expect unusual
conditions. |
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Figure 5-13.—Descending currents prevail above some surfaces and
tend to cause the pilot to land short of the
field. |
Effect of Obstructions on Wind
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When the wind flows around an obstruction, it breaks into eddies—gusts
with sudden changes in speed and direction—which may be carried along some
distance from the obstruction. A pilot flying through such turbulence
should anticipate the bumpy and unsteady flight that may be encountered.
This turbulence—the intensity of which depends upon the size of the
obstacle and the velocity of the wind—can present a serious hazard during
takeoffs and landings. For example, during landings, it can cause an
aircraft to “drop in;” during takeoffs, it could cause the aircraft to
fail to gain enough altitude to clear low objects in its path. Any
landings or takeoffs attempted under gusty conditions should be made at
higher speeds, to maintain adequate control during such conditions.
[Figure 5-14] |
Figure 5-14.—Turbulence caused by obstructions. |
| This same condition is more noticeable where
larger obstructions such as bluffs or mountains are involved. As shown in figure
5-15, the wind blowing up the slope on the windward side is relatively smooth,
and its upward current helps to carry the aircraft over the peak. The wind on
the leeward side, following the terrain contour, flows definitely downward with
considerable turbulence and would tend to force an aircraft into the mountain
side.
The stronger the wind, the greater the downward pressure
and the accompanying turbulence. Consequently, in approaching a hill or
mountain from the leeward side, a pilot should gain enough altitude well
in advance. Because of these downdrafts, it is recommended that mountain
ridges and peaks be cleared by at least 2,000 feet. If there is any doubt
about having adequate clearance, the pilot should turn away at once and
gain more altitude. Between hills or mountains, where there is a canyon or
narrow valley, the wind will generally veer from its normal course and
flow through the passage with increased velocity and turbulence. A pilot
flying over such terrain needs to be alert for wind shifts, and
particularly cautious if making a landing. |
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Figure 5-15.—Airplanes approaching hills or mountains from windward
are helped by rising currents. Those approaching from leeward encounter
descending currents. | Low-Level Wind
Shear
Wind shear is best described as a change in wind direction and/or speed
within a very short distance in the atmosphere. Under certain conditions, the
atmosphere is capable of producing some dramatic shears very close to the
ground; for example, wind direction changes of 180° and speed changes of 50
knots or more within 200 feet of the ground have been observed. This, however,
is not something encountered every day. In fact, it is unusual, which makes it
more of a problem. It has been thought that wind cannot affect an aircraft once
it is flying except for drift and groundspeed. This is true with steady winds or
winds that change gradually. It isn’t true, however, if the wind changes faster
than the aircraft mass can be accelerated or decelerated. The most
prominent meteorological phenomena that cause significant low-level wind shear
problems are thunderstorms and certain frontal systems at or near an airport.
Basically, there are two potentially hazardous shear situations. First, a
tailwind may shear to either a calm or headwind component. In this instance,
initially the airspeed increases, the aircraft tends to pitch up, and the
altitude may increase. Second, a headwind may shear to a calm or tailwind
component. In this situation, initially the airspeed decreases, the aircraft
pitches down, and the altitude decreases. Aircraft speed, aerodynamic
characteristics, power/weight ratio, powerplant response time, and pilot
reactions along with other factors have a bearing on wind shear effects. It is
important, however, to remember that shear can cause problems for any aircraft
and any pilot.
There are two atmospheric conditions that cause the type of low-level wind
shear discussed herein. They are thunderstorms and fronts. The winds
around a thunderstorm are complex. Wind shear can be found on all sides of a
cell. The wind shift line or gust front associated with thunderstorms can
precede the actual storm by up to 15 nautical miles. Consequently, if a
thunderstorm is near an airport of intended landing or takeoff, low-level wind
shear hazards may exist. At some large airports a low-level wind shear alert
system (LLWAS) has been installed which aids in detecting wind shear.
While the direction of the winds above and below a front can be
accurately determined, existing procedures do not provide precise and current
measurements of the height of the front above an airport. The following is a
method of determining the approximate height of the front, with the
consideration that wind shear is most critical when it occurs close to the
ground.
• A cold front wind shear occurs just after the front passes the airport and
for a short period thereafter. If the front is moving 30 knots or more, the
frontal surface will usually be 5,000 feet above the airport about 3 hours after
the frontal passage.
• With a warm front, the most critical period is before the front passes the
airport. Warm front windshear may exist below 5,000 feet for approximately 6
hours; the problem ceases to exist after the front passes the airport. Data
compiled on wind shear indicate that the amount of shear in warm fronts is much
greater than that found in cold fronts.
• Turbulence may or may not exist in wind shear conditions. If the surface
wind under the front is strong and gusty, there will be some turbulence
associated with wind shear.
The pilot should be alert to the possibilities of low-level wind shear
at any time the conditions stated are present.
Wind and Pressure Representation on Surface Weather Maps
The excerpted portion of a surface weather map provides information
about winds at the surface. The wind direction at each station is shown by an
arrow. The arrowhead is represented by the station circle, and points in the
direction toward which the wind is blowing. Winds are given the name of the
direction from which they blow; a northwest wind is a “wind blowing from the
northwest.” [Figure 5-16]
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Windspeed is shown by “barbs” and/or “pennants” placed on the end of
the arrow. The speed is indicated by the number of half barbs, full barbs,
or pennants. Each half barb represents approximately 5 knots, each full
barb indicates approximately 10 knots, and each pennant 50 knots. Thus two
and one-half barbs indicate a windspeed of approximately 25 knots; a
pennant and two and one-half barbs indicate a windspeed of approximately
75 knots, etc. |
Figure 5-16.—Speed and direction of wind are shown on a weather map
by wind arrows and isobars. |
| The pilot can thus tell at a glance, the wind
conditions prevailing at map time at any weather station. Pilots can obtain this
information and forecasts of expected winds from all weather reporting
stations. The pressure at each station is recorded on the weather map, and
lines (isobars) are drawn to connect points of equal pressure. Many of the lines
make complete circles to surround pressure areas marked “H” (high) or “L” (low).
Isobars are quite similar to the contour lines appearing on aeronautical charts.
However, instead of indicating altitude of terrain and
steepness of slopes, isobars indicate the amount of pressure and steepness
of pressure gradients. If the gradient (slope) is steep, the isobars will
be close together, and the wind will be strong. If the gradient is
gradual, the isobars will be far apart, and the wind gentle. [Figure 5-17]
Isobars furnish valuable information about winds in the first few
thousand feet above the surface. Close to the Earth, wind direction is
modified by the contours over which it passes, and windspeed is reduced by
friction with the surface. At levels 2,000 or 3,000 feet above the
surface, however, the speed is greater and the direction is usually
parallel to the isobars. Thus, while wind arrows on the weather map
excerpt indicate wind near the surface, isobars indicate winds at slightly
higher levels. [Figure 5-16]
In the absence of specific information on upper wind conditions, the
pilot can often make a fairly reasonable estimate of the wind conditions
in the lower few thousand feet on the basis of the observed surface wind.
Generally, it will be found that the wind at an altitude of 2,000 feet
above the surface will veer about 20° to 40° to the right and almost
double in speed. The veering will be greatest over rough terrain and least
over flat surfaces. Thus, a north wind of 20 knots at the airport would be
likely to change to a northeast wind of 40 knots at 2,000 feet. This
subject will be reviewed later in this chapter. |
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Figure 5-17.—Above: Flow of air around a “high.” Below: Isobars on
a weather map indicate various degrees of pressure within a
high. |
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