Page 36 - PDF_Flip_Book

Rod Machado’s Private/Commercial Pilot Handbook
12-34

Fig. 58
Fig. 57

When birds and other airplanes are flying in the
opposite direction, this should cause you concern.

troughs. Figure 57 depicts this process. Notice that the cold front lies along the border between two high pressure
areas. As the front approaches, the pressure falls. It increases after frontal passage, since a high pressure area lies
behind the front. But, why is the area along the front associated with a trough or area of low pressure?
Figure 58 shows the cold front air mass approaching a warmer air mass. Cold air is heavy, so it easily slips under
the warmer air. As it does, it lifts the warmer air (which usually contains a lot of moisture). Given sufficient lift and
lapse rate, it condenses and releases its latent heat. Heat released into the air enhances upward movement, intensify-
ing the low pressure along the front. Simply stated, the frontal zone itself helps intensify the trough’s low pressure.
(While there are other reasons for trough
formation, this gives you a basic under-
standing of how troughs form and what
they do). Fig. 59
Remember, the trough is a line where
the pressure is lower than on either side of
the line, and where the isobars form a
counterclockwise curvature but don’t form
a closed circulation. Along the cold front in
Figure 57, the isobars are elongated diago-
nally in a downward and upward direction.
Air flows in a counterclockwise circulation
but it’s not the typically smooth, circular
flow of a low pressure center. In other
words, the isobars look pinched and bent
along the trough axis.
Polar cold fronts tend to move toward
the low pressure trough, as shown in
Figure 59. Keep in mind that high and low
pressure centers move, so their associated