Weather Front

Although the glassy beauty of the atmosphere in pictures from space might lead one to believe that it is actually a single, uniform layer of air, it is actually divided into many smaller, ever-shifting regions of air. These regions of air are called air masses, and they circulate around the globe, jostling against each other, pushing and shoving, sometimes mixing together or dissipating while others form in the 'source regions' that generate air masses, like the tropics, the arctic regions, and the seas.

Air masses tend to be fairly uniform internally, with a general temperature (cool, warm, hot, cold) and a level of humidity (dry or moist) depending on where they originated. Where they meet, however, the difference between them generate weather effects. The boundary where one air mass is pushing against another is known as a weather front. The exact weather to be found at the weather front -- as well as the 'depth' of the weather front itself -- is determined by which air mass is advancing and which is retreating, and what the nature of these air masses is.

When a colder air mass is pushing out a warmer air mass, a cold front marks the boundary. Such a front usually moves from west to east, although it can come from the northwest or the southwest depending on how the front is developing and what forces are acting on the air mass. Warm fronts occur where tropical air is pushing out arctic or cooler air, and move towards the poles, which means that they advance from the south in the northern hemisphere and from the north in the southern hemisphere.

Occluded fronts and stationary fronts are other types of weather front, and develop from more complex interactions between air masses and frontal boundaries. Whatever the exact nature of a front, it represents the impact of one air mass against another, and is almost always the scene of storms, rain, or unsettled weather. With the exception of people in the tropics and the desert zones, humans everywhere experience scores of weather fronts each year. With their winds, rain, and shifts in conditions from wet to dry, cool to hot, and so on, these phenomena are one of the most crucial parts of the Earth's grand weather cycle.

Cold fronts

Cold fronts occur when a colder, drier air mass -- often born in the arctic -- pushes out a warmer, wetter one -- typically from the tropics. Since cold air is heavier than warm air, the advancing wedge of cold hugs the ground, lifting the warmer air while simultaneously pushing strongly against it. Cold fronts tend to be very vigorous and move twice as fast as warm fronts. This, combined with the fact that they act to lift the warm air powerfully, creating instability and huge, moist updrafts, means that cold fronts often have a fairly narrow band of violent weather along them. In the summer, they are usually marked by lines of thunderstorms and even derechos.

Warm fronts

Warm fronts occur when a warmer air mass pushes out cooler, drier air. It is more slow-moving, typically, and rides up over the cooler air, 'squeezing' it out of a region rather than wedging it out from below. Rainfall along the frontal boundary typically is far gentler than that of the cold front, but can also last much longer -- several days, on occasion. Fog often occurs at the frontal boundary itself. Rather than the clear, crisp air that follows a cold front, a warm front usually ushers in hazy days.

Occluded fronts

Occluded fronts occur when a cyclone, or rotating storm system, involves both a cold and warm front rotating around a common point. If the cold front catches up with the warm front, the two will mix and combine, and the two air masses (cold and dry, warm and moist) also combine to some extent. The end result is a front that can produce any kind of weather from nothing more than a wind shift, to violent thunderstorms, or anything in between. An occluded front usually brings tepid cooling (less often, mild warming) and drier air after it passes.

Stationary fronts

Stationary fronts crop up where two air masses meet, but neither one is strong enough to push out the other. The stationary front usually begins as a warm or cold front, which eventually meets an air mass it isn't strong enough to shift, and thus 'stalls.' The result is generally a long period of rain along the frontal boundary, sometimes with sequences of thunderstorms as well. Eventually, either conditions in the atmosphere will change, allowing one or the other air mass to advance as a new front, or the temperature differences between the air masses will disappear as one cools and the other warms, and they will combine.