Dew Points

AVIATION WEATHER

CHAPTER THREE

flying through a front. The amount and rate of change partially indicates the front's intensity.

Strong and weak fronts are accompanied by abrupt and gradual changes in temperature

respectively.

Dew Points

The dew point temperatures reported from weather observing stations are helpful in locating the

position of a front. The dew point temperature and air temperature give an indication of the

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

All fronts are located in troughs of low pressure. The arrows in Figure 3-8 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 3-8 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 3-8 Pressure Changes Across a Front

Winds

Near the Earth's surface, the wind changes direction across a front. In the Northern Hemisphere,

as the front approaches and passes a station, the wind changes direction in a clockwise rotation.

When flying across a front, because of this wind shift you must adjust heading to the right to

Mechanics of Frontal Systems 3-9