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In the Northern Hemisphere, a general cycle of highs and lows moves through the temperate zones from west to east. The movement of the pressure systems is more rapid in the winter season when the low pressure systems are most intense and the high pressure systems extend farthest to the south. [Figure 4-4]
A trough is an elongated area of low pressure, with the lowest pressure along the trough line. The weather in a trough is commonly violent. Also, troughs can be slow moving.
A ridge is an elongated area of high pressure with highest pressure along the ridge line. The weather in a ridge is generally favorable for flying.
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H
ill of airHill of air101610081004100010129961008100410001012996Depression or valley of air2,000 FT1,500 FT1,000 FT500 FT
Figure 4-4. A cross-section of the pressure systems depicted in Figure 4-3.Fronts
Fronts are the boundaries between two air masses and are classified as to which type of air mass (cold or warm) is replacing the other. For example, a cold front demarcates the leading edge of a cold air mass displacing a warmer air mass. A warm front is the leading edge of a warmer air mass replacing a colder air mass. Fronts are also transition zones (boundaries) between air masses that have different densities. The density of air is controlled primarily by the temperature of the air. Therefore, fronts in temperate zones usually form between tropical and polar air masses, but they may also form between arctic and polar air masses. A typical surface weather map shows air mass boundary zones at ground level. Designs on the boundary lines indicate the type of front and its direction of movement. On weather maps in local weather stations, fronts may also be indicated by colored lines. A working knowledge of fronts and their accompanying weather hazards is important to pilots.
Types of Fronts
The four major frontal types are:
• Cold,
• Warm,
• Stationary, and
• Occluded.
A front type is determined from the movement of the air masses involved.Cold Front
A cold front is the leading edge of an advancing mass of cold air. A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air. Cold fronts move more rapidly than warm fronts, generally progressing at a rate of 25 to 30 miles per hour (mph). However, extreme cold fronts have been recorded moving at speeds of up to 60 mph. A typical cold front moves in a manner opposite that of a warm front. Because it is so dense, it stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing the less dense air aloft. [Figure 4-5] The rapidly ascending air causes the temperature to decrease suddenly, forcing the creation of clouds. The type of clouds that form depends on the stability of the warmer air mass. A cold front in the Northern Hemisphere is normally oriented in a northeast to southwest manner and can extend for several hundred miles, encompassing a large area of land. Prior to the passage of a typical cold front, cirriform or towering cumulus clouds are present, and cumulonimbus clouds are possible. Rain showers and haze are possible due to the rapid development of clouds. The wind from the south-southwest helps to replace the warm temperatures with the relative colder air. A high dew point and falling barometric pressure are indicative of imminent cold front passage.
As the cold front passes, towering cumulus or cumulonimbus clouds continue to dominate the sky. [Figure 4-6] Depending on the intensity of the cold front, heavy rain showers form and might be accompanied by lightning, thunder, and/or hail. More severe cold fronts can also produce tornadoes. During cold front passage, the visibility is poor, with winds variable and gusty, and the temperature and dew point drop rapidly. A quickly falling barometric pressure bottoms out during frontal passage, then begins a gradual increase. After frontal passage, the towering cumulus and cumulonimbus clouds begin to dissipate to cumulus clouds, with a corresponding decrease in the precipitation. Good visibility eventually prevails with the winds from the west-northwest. Temperatures remain cooler and the barometric pressure continues to rise.
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Figure 4-6. A cold front underrunning warm, moist, unstable air. Clouds are cumuliform with possible showers or thunderstorms near the surface position of the front. Convective clouds often develop in the warm air ahead of the front. The warm, wet ground behind the front generates low-level convection and fair-weather cumulus in the cold air.
St. Louis
Indianapolis200 milesColumbus400 milesPittsburgh600 milesCOLD AIRWARM AIRNIMBOSTRATUSALTOSTRATUSCIRROSTRATUSCIRRUS
Figure 4-5. A cold front underrunning warm, moist, stable air. Clouds are stratified and precipitation continuous. Precipitation induces stratus in the cold air.
　
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