Wind is a vital consideration in navigation. It can assist or hinder the
speed of an aircraft relative to the ground, and can blow an aircraft from
its intended course. The movement of air may become turbulent and under extreme
conditions, turbulence can pose a danger to aircraft. Weather charts use wind
arrows to depict the speed and direction of wind. The head of the arrow points
in the direction toward which the wind is blowing. (However, winds are described
by the direction from which they are blowing - a north wind is a wind
blowing from the north.) Wind speed is indicated by the feathers on the arrow.
Full feathers represent 10 knot increments; half feathers represent five knots.
blowing from the southwest (225 degrees) at 25 knots.
blowing from the east (90 degrees) at 10 knots.
Differences in atmospheric pressure from one area to another result in
wind. Atmospheric pressure can also affect aircraft performance; under low
pressure the air is less dense, and aircraft are not as efficient at taking-off
and climbing. Changes in pressure can herald changes in weather; a sudden
drop in pressure may indicate the approach of a storm. Atmospheric pressure
is typically expressed in millibars. Normal pressure at sea level is 1013.2
millibars. Weather charts usually depict atmospheric pressure using isobars
- lines connecting areas with equal pressure.
Clouds can have a dramatic effect on visibility. This is especially critical
for VFR (Visual Flight Rules) flights which depend on the ability to see and
be seen. Precipitation, which may be associated with clouds, can further impact
visibility. Clouds can pose additional dangers. For example, aircraft flying
into cumulonimbus clouds may encounter extreme turbulence and violent updrafts
and downdrafts that can literally tear a plane apart as well as lightning,
icing, and other hazards. A variety of weather charts depict a wide range
of information about cloud cover. On surface weather maps, the following symbols
are combined with the heads of the wind arrows to depict the amount of cloud
Overcast with Breaks
In the following exercise, interpret the information on the simplified surface weather map in order to match
the verbal weather descriptions with the stations shown on the map.
Wind 220 degrees at 20 knots. Pressure 996. Overcast.
Wind 45 degrees at 10 knots. Pressure 1017. Clear.
Wind 225 degrees at 25 knots. Pressure 1011. Scattered clouds.
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