Flexi Says: When cold air masses move south from the poles, they run into warm air masses moving north from the tropics. The boundary between two air masses is called a front. Air masses usually don’t mix at a front. The differences in temperature and pressure cause clouds and precipitation. Types of fronts include cold, warm, occluded, and stationary fronts. At the cold front, clouds form, and precipitation falls. Cold fronts often bring sudden changes in the weather. There may be a thin line of storms right at the front that moves as it moves. In the spring and summer, the storms may be thunderstorms and tornadoes. In the late fall and winter, the storms may bring snow. After a cold front passes, the cold air mass behind it brings cooler temperatures. The air is likely to be less humid as well. The warm air mass then flows up over the cold air mass. As the warm air rises, it cools. This brings about clouds and sometimes light precipitation. Warm fronts move slowly and cover a wide area. After a warm front passes, the warm air mass behind it brings warmer temperatures. The warm air is also likely to be more humid. Cloudy weather and precipitation along the occluded front are typical. Sometimes two air masses stop moving when they meet. These stalled air masses create a stationary front. Such a front may bring clouds and precipitation to the same area for many days.
