Understanding weather systems and fronts is an important quality in a safe pilot. Even though we could never possibly understand nature and what it has in store for us next, we can at least scratch the surface. With a basic understanding of how fronts and pressure systems behave, we can make a better forecast when trying to make that 'Go' or 'No-Go' decision.
The Term Front came from Norwegian meteorologists after World War I, from the term battle front. This weather phenomenon is in a sense a battle in which one air mass attempts to take over another. As one moves in, the other one is forced out of the way. Where these air masses collide is called a "Front." As a part of larger weather systems, fronts usually exist in areas of low atmospheric pressure.
Through uneven heating of the earth's surface by the sun, our entire weather system takes shape. Generally, in the northern hemisphere warm air from the equator moves northward, as cool dense air from the poles push southward. Because cooler air is denser, gravity will pull cold air under warm air under normal circumstances. This is where a front forms, at the point in which the cold and warm air mass meet, and attempt to overtake one another. The type of front will be determined by the overtaking air mass.
Symbolic lines are used on weather maps to depict where the boundary between the two masses touch the ground. Using many external variables such as pressure readings, meteorologists can make forecasts as to where the weather may make its next move. Different fronts behave in different ways, especially based on other variables as mentioned. The following, is a brief description of each type of front and pressure system:
Cold Front: Depicted with a blue line, embedded with triangles, pointing in the direction the front is moving. Cold, dense air moves underneath warmer air. This forces the warm air upward higher and higher as the cold mass continues in. Clouds form upward, often creating cumulus or cumulonimbus clouds, resulting in thunderstorms. This front usually billows up as it plunges southward from the North Pole, giving it a fairly steep slope. Often times, a cold front will cause a wind shift from southwest to northwest.
Warm Front: Depicted by a red line with embedded red semi-circles, pointing in the direction the front is moving. Warm air is replacing cooler air, sliding slowly up over the cold air. Usually having a shallow slope, the warm front contains stable air. Cirrus and high altitude stratus clouds are a sign that warm air is moving in at just a few hundred miles away. A warm front usually causes a wind shift from southeast to southwest.
Stationary Front: Depicted by a blue line with blue triangles and red semi-circles on opposing sides, this front is characterized by its lack of movement. Neither the cold air nor the warm air is advancing in this type of front. It is common to see widespread clouds on both sides of the boundaries.
Occluded Fronts: Depicted by a blue line with blue triangles and red semi-circles pointing in the direction that the front is moving. Three air masses are involved with this type of front, where warm air is being forced upward by two cooler fronts below. In a cold occlusion, the colder front is overtaking the cool or warmer front. In a warm occlusion, the cool or warmer air is overtaking the cold front. Any type of cloud can from in these fronts.
Ridge: A high-pressure warm air front, usually resulting in good weather. Pressure increases here as upper air converges. The stratosphere acts like a lid here, causing the air to move downward. As pressure builds, the air has no place to go, and is forced outward. This type of storm flows in a clockwise direction due to gravity.
Trough: A low-pressure cold air front, usually resulting in bad weather. Pressure decreases as air diverges. As air from below moves upward to fill in the space, the pressure decreases around the surface. As the air flows upward and away from gravity it begins to turn counterclockwise. The lower pressure around the ground creates surface winds flowing to the center of the storm and becomes the engine in moving the associated warm and cold fronts. The worst types of weather from hurricanes to thunderstorms and heavy icing are seen in a trough or low-pressure cold front.
Weather fronts are a complex phenomenon and rarely behave exactly as a book would describe them to. Understanding at least what types of weather exist within these systems will give any pilot a greater understanding of what is to be expected in flight. For an easy to understand, excellent book in understanding weather, pick up a copy of "The Weather Book" by Jack Williams. Its probably the best book we have seen about weather, and is the source for all information contained herein.
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