AIR MASSES AND FRONTS
Large, high pressure systems frequently stagnate over large areas of land or water with relatively uniform surface conditions. They take on characteristics of these “source regions”—the coldness of polar regions, the heat of the tropics, the moisture of oceans, or the dryness of continents. As they move away from their source regions and pass over land or sea, the air masses are constantly being modified through heating or cooling from below, lifting or subsiding, absorbing or losing moisture. Actual temperature of the air mass is less important than its temperature in relation to the land or water surface over which it is passing. For example, an air mass moving from polar regions usually is colder than the land and sea surfaces over which it passes. On the other hand, an air mass moving from the Gulf of Mexico in winter usually is warmer than the territory over which it passes. If the air is colder than the surface, it will be warmed from below and convection currents will be set up, causing turbulence. Dust, smoke, and atmospheric pollution near the ground will be carried upward by these currents and dissipated at higher levels, improving surface visibility. Such air is called “unstable.” Conversely, if the air is warmer than the surface, there is no tendency for convection currents to form, and the air is smooth. Smoke, dust, etc., are concentrated in lower levels, with resulting poor visibility. Such air is called “stable.” From the combination of the source characteristics and the temperature relationship just described, air masses can be associated with certain types of weather. The following are general characteristics of certain air masses but
they may vary considerably. • Type of clouds—cumulus and cumulonimbus. When two air masses meet, they will not mix readily unless their temperatures, pressures, and relative humidities are very similar. Instead, they set up boundaries called frontal zones, or “fronts,” the colder air mass projecting under the warmer air mass in the form of a wedge. This condition is termed a “stationary front,” if the boundary is not moving. Usually, however, the boundary moves along the Earth’s surface, and as one air mass withdraws from a given area, it is replaced by another air mass. This action creates a moving front. If warmer air is replacing colder air, the front is called “warm;” if colder air is replacing warmer air, the front is called “cold.” Warm Front When a warm front moves forward, the warm air slides up over the wedge
of colder air lying ahead of it.
Flight Toward an Approaching Warm Front Although no two fronts are exactly alike, a clearer understanding of
the general pattern may be gained if the atmospheric conditions which might
exist when a warm front is moving eastward from St. Louis, MO, is considered.
[Figure 5-22] At Indianapolis, IN, 200 miles in advance of the warm front, the sky would be overcast with nimbostratus clouds, and continuous rain. At Columbus, OH, 400 miles in advance, the sky would be overcast with stratus and altostratus clouds predominating. The beginning of a steady rain would be probable. At Pittsburgh, PA, 600 miles ahead of the front, there would probably
be high cirrus and cirrostratus clouds. If a return flight to Pittsburgh was made, it would be recommended to wait until the front had passed beyond Pittsburgh, which might require three or four days. Warm fronts generally move at the rate of 10 to 25 MPH. On the trip from Pittsburgh to Indianapolis, a gradual increase in temperature would have been noticed, and a much faster increase in dewpoint until the two coincided. Also the atmospheric pressure would gradually lessen because the warmer air aloft would have less weight than the colder air it was replacing. This condition illustrates the general principle that a falling barometer indicates the approach of stormy weather. Cold Front When a cold front moves forward, it acts like a snow plow, sliding under the warmer air and forcing it aloft. This causes the warm air to cool suddenly and form cloud types that depend on the stability of the warm air. Fast-Moving Cold Fronts In fast-moving cold fronts, friction retards the front near the ground, which brings about a steeper frontal surface. This steep frontal surface results in a narrower band of weather concentrated along the forward edge of the front. If the warm air is stable, an overcast sky may occur for some distance ahead of the front, accompanied by general rain. If the warm air is conditionally unstable, scattered thunderstorms and showers may form in the warm air. At times an almost continuous line of thunderstorms may form along the front or ahead of it. These lines of thunderstorms (squall lines) contain some of the most turbulent weather experienced by pilots. Behind the fast-moving cold front, there is usually rapid clearing, with gusty and turbulent surface winds, and colder temperatures. Flight Toward an Approaching Cold Front If a flight was made from Pittsburgh toward St. Louis when a cold front
was approaching from St. Louis, weather conditions quite different from those
associated with a warm front would be experienced. The sky in Pittsburgh would
probably be somewhat overcast with stratocumulus clouds typical of a warm air
mass, the air smooth, and the ceilings and visibilities relatively low,
and a dark layer of nimbostratus lying low on the horizon, with perhaps cumulonimbus in the background would be noted. Two courses would now be open: (1) either turn around and outdistance the storm, or (2) make an immediate landing which might be extremely dangerous because of gusty wind and sudden wind shifts. If flight was continued, entrapment in a line of squalls and cumulonimbus clouds could occur. It may be unsafe to fly beneath these clouds; impossible, in a small plane, to fly above them. At low altitudes, there are no safe passages through them. Usually there is no possibility of flying around them because they often extend in a line for 300 to 500 miles. Comparison of Cold Fronts with Warm Fronts The slope of a cold front is much steeper than that of a warm front and the
progress is generally more rapid—usually from 20 to 35 MPH—although in extreme
cases, cold fronts have been known to move at 60 MPH. Weather activity is more
violent and usually takes place directly at the front instead of in advance of
the front. In late afternoon during the warm season, however, squall lines
frequently develop as much as 50 to 200 miles in advance of the actual cold
front. Whereas warm front dangers are low ceilings and visibilities, and cold
front dangers are chiefly sudden storms, high and gusty winds, and turbulence.
Occluded Front One other form of front with which the pilot should become familiar is the “exclusion” or “occluded front.” This is a condition in which a warm air mass is trapped between two colder air masses and forced aloft to higher and higher levels until it finally spreads out and loses its identity.
Figure 5-27 shows the development of the occluded front in greater detail.
A frontal line on the weather map represents the position of the frontal surface on the Earth’s surface. A pilot flying west at an altitude of 6,500 feet would pass through the frontal boundary about 100 miles in advance of the point where the warm front is shown, or about 25 to 50 miles to the rear of the line on the map representing the cold front. Figure 5-29 is a section of a surface weather map as transmitted on facsimile. It shows a low pressure center with warm, cold, and occluded fronts. The preceding discussion categorizes weather with types of fronts. However, weather with a front depends more on the characteristics of the conflicting air masses than on the type of front. A pilot should not attempt to determine expected weather from fronts and pressure centers on the surface chart alone. The pilot must rely heavily on other charts, reports, and forecasts which are discussed in the next section of this chapter.
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