JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
relative humidity of the air. Cold air masses will usually have lower dew point temperatures than
warm air masses. Higher dew points indicate a greater amount of moisture available to produce
clouds, fog, or precipitation.
Pressures
Figure 3-5 — Pressure Changes Across a Front
All fronts are located in troughs of low pressure. The arrows in Figure 3-5 indicate the trough
(where low pressure extends outward from the center of the low), as well as the direction of
movement of the low-pressure system. Therefore, when a front approaches a station, or a pilot
flies toward a front, the pressure decreases. Pressure then rises immediately following frontal
passage. Figure 3-5 illustrates this pressure fall and rise with the time-sequence of the weather at
station NSE. The earliest time is pictured in the upper right, when the pressure is 1011 mb, and
the last point in time is at the lower left, with a pressure of 1007 mb. Because of this pressure
change, it is extremely important to obtain a new altimeter setting when flying in the vicinity of a
front.
Figure 3-6 — Wind Shift Across a Cold Front
Version 3.2/Dec 08 3-7