The surface of the earth is covered with air masses – layers or sections of the atmosphere that have similar temperature and/or humidity. Air masses can be classified by source, which in turn determines their characteristics. The primary types of air masses are maritime (ocean) polar or mP, continental polar (cP), maritime tropical (mT), continental tropical (cT), arctic (A) and highland (H).
Fronts occur when different air masses meet, particularly at the edges of polar air masses. An approaching front is typically heralded by a change in the wind direction and speed and falling barometric pressure. This is due to the low pressure area that develops at the front. An area of low pressure is generally associated with both a cold front and a warm front. Occluded or stationary fronts may also develop.
The type of front is determined by which air mass is dominant; that is, by the one that is pushing against the other one. For example, when cold, dry (cP) air pushes against warm, moist air (mT) a cold front results. The intersection of the two is the frontal boundary, and if conditions are right, clouds and precipitation -- perhaps even severe thunderstorms – will develop. The most common cloud type during the passage of a cold front is the cumulonimbus, a tall cloud which typically contains lightning, rain, and possibly hail or tornadoes.
A warm front is formed when a warm, tropical air mass (mT or cT) pushes against a polar air mass. In the United States mT air from the Gulf of Mexico may move north and collide with polar air moving south. Warm fronts may have moderate to heavy precipitation, but severe weather is less likely than with cold fronts. Nimbostratus, a thick dark cloud which hangs low in the sky, is most common during the passage of a warm front.
Cold fronts often move faster than warm fronts, and when the two are linked by low pressure, the cold front may catch up to the warm front. The warm air is lifted by the colder air on either side, and this merger is called an occluded front. These fronts are associated with continuous precipitation (light, moderate, or heavy) and any or all of the cloud types found with cold and warm fronts.
Finally, a stationary front is just what it sounds like – a front that is not moving. This occurs when the air masses on either side of the front have equal force so one cannot push the other. Often there is a high pressure cap just beyond the front which is difficult to dislodge. Stationary fronts can remain in the same area for days, with continuous precipitation that may result in serious flooding. Sometimes they will reverse themselves, turning the former cold front into a warm front or vice versa.