Air masses generate their own types of weather, but their interactions on a front can become thunderous and squally, or at least foggy and drizzly.
Along a front, whenever there are temperature contrasts, the atmosphere has a lot of energy just waiting to be released. That's called potential energy. All could be quiet, but as soon as there is some motion toward less-dense or more-dense air, the weather goes over the edge. An area where a temperature contrast exists is said to have available potential energy. Such areas are the flashpoints for big-time weather. The technical name is a baroclinic zone. (You might hear that term on the Weather Channel, but probably not on your local TV forecast.)
The next figure summarizes the weather that precedes and follows cold and warm fronts. Again, the nature of the weather is really just related to the degree of rising motions.
Cold- and warm-front weather.
There's a Storm A-Brewin'
Cold fronts bring the most violent conditions, which isn't surprising. After all, rising motions write the weather script, and lifting mechanisms are sharpest along a cold front. There cold, heavy air is lifting warmer, lighter air, almost like a snowplow pushing aside a pile of fluffy, powdery snow. The warm air goes flying and, in the process, forms tall clouds. The air is already unstable in the warmer air mass. That added push is enough to get the storm rolling.
Potential energy is the potential to do work or the potential to move. A deck of cards sitting on the edge of a table has the potential to fall, but its energy will only be released if it is pushed over the edge. Temperature contrasts represent potential energy in the atmosphere. When warm air masses mix with colder air masses, that potential energy becomes available for storm development and is called available potential energy. Baroclinic zones are areas of temperature contrasts and available potential energy.
Thunderstorms and even tornadoes will form along cold fronts, generally in the warm air 50 to 100 miles ahead of the front. As a cold front approaches, the pressure falls rapidly because of all the lifting that's going on. The atmosphere is often hazy, steamy, and tropical. Winds freshen from the south and southwest, and then, through the haze, towering cumulus and cumulonimbus clouds form. Showers and thunderstorms develop, and if the lifting is really strong, tornadoes will form. The stormy weather is sharp, violent, but not necessarily long-lasting. Because the front is moving rapidly, the storm threat might only last for two or three hours.
Then, as the cold front passes, the barometer rises rapidly, the wind shifts into the northwest, the dew point lowers, and usually, the temperature drops. Typically, skies clear quickly because behind a cold front the air is subsiding, not rising. High pressure builds in from the west, and typically, the weather remains quiet for a few days.
It Looks Like Rain
In a warm front, the sequence of weather is different because the air is being lifted gently rather than sharply. Because the tropical air mass is less dense than the polar air it's replacing, the air rides up and over the colder air with a gradual slope. High, thin cirrus clouds appear in the colder air as much as 1,000 miles ahead of the warm front. The clouds form as the warm air comes into contact with the colder air mass and releases its moisture. The contact at 1,000 miles is at the very highest elevation within the atmosphere, so those cirrus clouds show up first. Those clouds will lower and thicken to cirrostratus, cirrocumulus, altostratus, altocumulus, and eventually, within 500 miles of the front, to nimbostratus. A steady, long-lasting rain develops. In the winter, the precipitation could begin as light snow.
As the warm front approaches and the depth of the warm air increases, snow will often change to sleet, freezing rain, and then to rain. Sometimes the lifting is so gentle that only drizzle develops along the front. Fog often develops as the warmer air moves over a colder surface. In general, the precipitation is on the light side near a warm front, but, as usual, there are exceptions. If the warming near the surface is very rapid, the atmosphere could turn unstable, and thunderstorms could develop, but that class of weather is usually reserved for cold fronts.
As a warm front approaches, the barometer falls slowly but steadily. The wind can be from the north, but usually ranges from northeast to southeast. The temperature could be biting cold, but more typically it will just be on the cool side. A maritime polar air mass usually precedes a warm front. A maritime tropical air mass usually follows. After the passage of the front, the pressure rises slowly or remains steady, the weather turns hazy, and the dew point climbs sharply. The rain ends, but as the tropical air takes hold, enough instability could occur to deliver isolated air-mass thunderstorms.
Stalled in Its Tracks
Stationary fronts deliver weather that is very much like warm fronts. Light precipitation falls in the maritime polar air, and some clearing occurs in the tropical air mass.
Occluded fronts are similar to cold fronts, except the weather isn't as violent. The surface temperature contrast isn't as great with an occluded front. This makes the weather less active. Typically, the occluded front divides maritime polar air from continental polar air, so the density difference is less than what is found between continental polar and maritime tropical air masses.
Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.