During much of the year it's hard to look at a weather map of the United States for more than a day or so without seeing at least one front. That familiar weather term comes from World War I headlines about the battles along the war's various fronts, including the Western Front across France.
The terms cold front, warm front, and even occluded front are so common now that it's hard to believe people ever talked about the weather without using them.
If you look at a weather map of the United States from the 1920s you'll see the lines marking equal barometric pressures � isobars - with centers of high and low pressure marked. You'll see symbols showing which way the wind was blowing at each weather station, and indicating temperature, humidity, and other observations. But you won't see any of the symbols for fronts that are such a big part of today's weather maps.
During World War I, with many of its European sources of food cut off, Norway established a meteorological institute in Bergen to improve weather forecasts for the country's farmers and fishermen. Its close observations led its researchers to conclude that masses of air with different characteristics were the major players in the weather, with the most violent weather occurring where the differing air masses come into conflict.
Air masses are created when air stays over a region with generally uniform characteristics long enough for the air to take on these characteristics. For instance, air that's been over frigid land for several days becomes cold and dry. Air from over a warm ocean grows warm and humid while air from over a desert is warm and dry. The differing characteristics give the air masses different densities.
Cold, dry air is denser than warm, humid air (humidity makes the air less dense than dry air).
When an air mass - say, cold, dry air from the polar regions - begins moving south, it comes in conflict with warmer, maybe more humid air. The differing densities mean that the air masses don't merge to become a new air mass with some characteristics of both. Instead, the heavier air tends to slide under the lighter, warm air. If the warm air is on the move, it tends to ride over the cold air.
In both cases, the warm, more humid air rises. As the air rises, it cools and the moisture in it begins condensing to form clouds and maybe precipitation. In other words, fronts are often a weather war zone because they push air up, which leads to clouds and precipitation.
You visualize air masses as huge heaps of air - maybe the size of several Midwestern states - with high atmospheric pressure near the middle, where the most air is piled up. The area where two air masses meet, a front, is an area of lower air pressure. In fact, fronts are in the elongated areas of low pressure called troughs, or "trofs" as they are sometimes spelled on weather charts.
One consequence this has for pilots is that if you are flying across a front, the air pressure drops, which means that your altimeter tells you that you are higher than you really are. This is because an altimeter measures only the air pressure outside your airplane but indicates it as an altitude. A lower air pressure is read as a higher altitude.
In addition to a lower atmospheric pressure, you will find that the wind shifts direction as you fly across a front. This happens because air flows into an area of low atmospheric pressure at the surface. Since the air is coming from within the air mass on its side of the front, it's blowing toward the trough line - the front - from different directions. In fact, the change in wind direction is the best indication that a front has moved across a weather station and the wind shift prompts weather observers to record "frontal passage."
You might think that since fronts divide air masses with different characteristics, the change from warm to cold or cold to warm would be a sign that the front has passed.
This doesn't work for a couple of reasons.
First, even though fronts drawn as lines, they are really "zones" where the two air masses often do merge to some degree.
Also, differences in air masses sometimes aren't all that great. This is especially true in the summer. As a mass of air that's been cooling off in northern Canada in July heads south it is cooler than the air over the Midwest. On its way south, however, the cool air mass is passing over ground that's being heated by the sun, warming the ground and the air right above the ground.
By the time the air mass arrives over the United States, it's probably just about as warm as the air that's already there - if this air isn't too hot. It is likely to be drier since the air mass hasn't been over any large bodies of water that would add humidity to it by evaporation. This is why you sometimes hear television meteorologists talk about "dry fronts."
One kind of true "dry front" is common on the western Plains where hot and very dry air from the Southwest deserts pushes east into warm, humid air that's moved up from the Gulf of Mexico. When one of these dry lines passes, the temperature might even go up but the humidity goes way down.
Dry lines are often very violent weather war zones, triggering large thunderstorms and sometimes tornadoes.
A front's name depends on which air mass happens to be winning the battle. In other words, a cold front is a warm-cold battle that the cold air is winning. When the warm air is advancing and the cool or cold air is retreating we call the zone between them a warm front.
Cold fronts are usually the most violent since cold air that's charging across the countryside can give warm, humid air a vigorous shove upward. If all of the conditions are right, this can lead to strong thunderstorms along the front. Normally the clouds and precipitation with a cold front will be confined to the frontal zone, but you can't always count on this.
While the cold, or cool, air mass that moves in behind the front often brings clear skies, this isn't always the case.
On weather maps cold fronts are show as a line with triangles on one side, pointing in the direction of the front's movement. If the map is in color the front should be shown in blue. In the depth of winter, as you're beginning to wonder whether you'll ever be able to dig your airplane out of the snow, news that a warm front is on the way might make you want to celebrate. Don't.
Warm fronts, especially in the winter, can bring some of the year's worst weather for those on the ground and in the air. As warm air advances it rides over the cold air since it's less dense than the cold air. Warm air is replacing the cold air at the ground where you see a warm-front symbol on a weather map - a red line with red half circles indicating the direction of movement. The half circles are on the side of the front where it's moving into the cold air.
Ahead of the front at the surface, the warm air is riding over the cold air. As the air rises its humidity condenses into clouds and precipitation. If it's a winter warm front, the precipitation can include snow, sleet, freezing rain, and ordinary rain. The clouds and precipitation can stretch 300 or more miles into the cold air from the surface front. The bad weather can last for a couple of days since warm fronts move slower than cold fronts.
Stationary fronts can trick you. In some cases the differences between the air masses are fading away and in a day or so the front is no longer there. At other times, new areas of low air pressure can form along a stationary front and grow into a new storm. Or waves of low pressure can ride along the front, bringing on-and-off clouds and precipitation.
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