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BY GEORGE: On the Front

There's a lot of information that the surface daily weather map contains, from high pressure areas and low pressure areas, to all types of precipitation, and of course, "fronts." Here, I'd like to take a closer look at what all these fronts mean, and what kinds there are.

Starting with the basic cold front, as depicted on a daily weather map by a blue line with triangles along it, the cold front marks the leading edge of a change of airmass. Now, it gets a little complicated. The temperature behind a cold front is not necessarily colder than the air ahead of the cold front. Sometimes, temperatures may actually rise behind a cold front. How can this be? Well, the truest definition of a front really deals with a measure of air density behind and ahead of the front. The air behind a cold front is always denser than the air that it is replacing. Most often this is true in the case of colder air replacing warmer air, as most of us have become accustomed to when hearing about a cold front coming in. Cold air is denser than warm air, so this makes sense. However, sometimes an airmass that is very dry replaces one that is very moist. A couple of things to note here: One, moist air is less dense, i.e., lighter than dry air. And the other thing to remember is that dry air heats up and cools down more quickly than moist air. An example of a "cold front" bringing warmer air would be a situation where very dry and dense air replaces moist and "lighter" air. The dry air behind the cold front heats up more readily than the moist air it's replacing. Presto! Behind the cold front you have drier air, but the temperature actually rises.

The bottom line is that temperature changes are not always the most reliable way to determine a cold frontal system and movement. What is true, however, is that behind the cold front the air is always denser, and usually drier.

A warm front marks the leading area of warmer and moister air replacing drier, denser, and cooler air. Because a warm front involves less dense air overtaking more dense air, the warm air moves more slowly in trying to displace the denser air. Cold fronts usually move about 50% or higher more quickly than warm fronts.

Since warm air is less dense, the warm air actually rides over the top of the denser air. This means that the warmer and less dense air arrives at any given location in the upper atmosphere before it reaches that same point at the surface. In the case of denser air masses (and usually colder and denser), the air rushes in at the surface much more readily, since the dense air easily displaces the less dense (and usually warmer) air.

The warm front on a weather map is drawn as red, and has half circles painted on it, facing the direction the warm air is moving.

Because a cold front can move much more quickly than a warm front, the cold front can catch up to the warm front circulating around a low pressure, or storm system. If this happens, we call the newly merged front an occluded front. In essence, the denser air has caught the less dense air and pinched it up into the atmosphere, i.e., an occlusion has occurred.

The occluded front on a weather map is usually drawn as purple with an alternating triangle and half circle on the same side of the line where the front is.

The stationary front marks the boundary of opposing air masses, such as tropical, moist and less dense with continental, drier and denser air. Both air masses in this case don't have enough push behind them to allow one to move and take over the other. This will eventually be alleviated by a strong push of one type of airmass or the other. In some cases, the stationary front will simply dissolve as the air masses become more like one another over the course of time.

Another note about fronts in general is the fact that in addition to the obvious temperature changes that usually occur, and the density changes we spoke about, wind shifts and air pressure changes occur along fronts as well. And, of course, whenever you displace one type of airmass with another, the potential for precipitation exists.

By: George Elliott