Air Masses and Fronts

One possible situation is that in which a mass of cold air moving across the Earth's surface comes into contact with a warm air mass. When that happens, the cold air mass may force its way under the warm air mass like a snow shovel wedging its way under a pile of snow. The cold air moves under the warm air because the former is more dense. The boundary formed between these two air masses is a cold front.

Cold fronts are usually accompanied by a falling barometer and the development of large cumulonimbus clouds that bring rain showers and thunderstorms. During the warmer seasons, the clouds form as moisture-rich air inside the warm air mass, which is cooled and water condenses out as precipitation.

Cold fronts are represented on weather maps by means of solid lines that contain solid triangles at regular distances along them. The direction in which the triangles point shows the direction in which the cold front is moving.