Cold fronts can travel twice as fast as warm ones, and hit harder
Where two air masses meet, weather gets interesting. Dense cold air both shoves and lifts the warm air ahead of it, so cold fronts can move up to twice as fast as warm fronts and bring sudden lines of thunderstorms, while warm fronts arrive slowly behind sheets of layered cloud and drizzle.
A front is a boundary between bodies of air that differ in temperature, humidity, density and wind. Unsettled weather tends to gather along it, though some fronts bring hardly any cloud or rain. The one constant is a shift in wind direction as the boundary passes. In summer, subtle moisture contrasts called dry lines can also spark severe storms.
Cold fronts usually travel west to east and warm fronts towards the poles, although any direction is possible. On a weather map, a cold front is a blue line with triangles pointing where the cold air is heading; a warm front is a red line with semicircles. Ahead of a warm front, cloud thickens gradually, rain builds steadily and fog may form, with rapid clearing and warming once it passes. The popular picture of cold air sliding under warm air like a wedge is too simple: the rising motion is really part of how the spinning Earth maintains balance as a front forms.
When a speedy cold front catches a warm front, the two merge into an occluded front, typically around mature low-pressure systems, and the meeting point is called the triple point. A stationary front is one that has stalled. If the temperature contrast fades, for instance after crossing a uniformly warm ocean, a front can dwindle into a shear line that only marks a change in wind speed. Meteorologists also distinguish unstable anafronts, with vigorous rising air and heavy rain, from feebler katafronts.
Forecasters describe the air masses themselves with the Bergeron system, a three-letter code. The first letter marks moisture, c for dry continental air or m for moist maritime air; the second marks the source region's temperature; and the third says whether the mass is colder, k, or warmer, w, than the ground beneath it. Mountains and large lakes or seas can alter a front's path and character along the way.
Source: Weather front
