Fronts and Mid‐Latitude Cyclogenesis

When two different air masses meet, a front is formed. The boundary between these masses is called a stationary front. The air between the two masses never really combines. The stationary front may only last for a few days. One of the air masses, usually the colder and drier mass, pushes to the south, while a warmer and moister air mass moves up from the south. Because of the Coriolis Force, the masses start to move to the right of the direction that they were originally moving. A counterclockwise rotation is started. A low‐pressure center forms along the stationary front. As the system spins and moves to the east (in the United States), a cold front and a warm front are created. The cold front is the leading edge of the cold air and is fast moving. The cold air acts as bulldozer, pushing the warmer air. The air in the warmer sector rises and forms clouds.

Thunderstorms and other violent storms are characteristic along cold fronts. They are usually quick‐moving storms. When the cold front passes, the winds shift from a southerly direction to the northwest. The air also becomes drier. As the warm air moves along, the front edge of it tries to move the colder, denser air ahead of it. It cannot and rides up the “bubble” of air. As the warm air rises, it cools and forms clouds. Because the warm front is slow moving, it can cause several days of steady rain. The air at the surface is cool, making for dreary days. As the warm front passes, the air temperature rises and the winds come from a southerly direction. Eventually the cold front catches up to the warm front, squeezing the air of the warm sector upward. This forms an occluded front, and usually precipitation falls on the Earth below. This is near the end of the life cycle of the low‐pressure center. These are called mid‐latitude lows and the process is called cyclogenesis. The side view of fronts, labels for fronts, and steps of cyclogenesis can be seen in Figure .

Figure 1 
Side view of fronts.

The winds around a low‐pressure center move inward and counterclockwise. This is the opposite for a high‐pressure system, where the winds move outward from the center and clockwise. The winds are generally lighter in highs and the skies are generally clear.