What Are the Lines That Are Drawn on Weather Maps to Separate Different Air Masses?
- A cold front is the boundary of a cold air mass that is displacing a warmer air mass. In North America, a cold front usually moves from northwest to southeast, and can cause temperatures to drop 15 degrees or more as it passes. The air behind a cold front usually is drier than the air in front of it, which can create rain or storms along the cold front. On a weather map, a cold front is represented with a blue line marked with blue triangles pointing in the direction the front is advancing.
- A warm front occurs when a warm air mass moves in to replace cold air. Warm fronts usually move from southwest to northeast. Warm air masses tend to be more humid than the cold air masses they replace, and they typically move more slowly than cold fronts, leading to gentle rain or drizzle instead of heavy precipitation. A warm front is indicated by a red line, with red half-circles pointing in the direction of travel.
- A front is stationary if it is moving at less than five knots (about five miles per hour). While the air masses do not replace one another, they may still move parallel to the front line. Severe weather, often flooding, can occur along a stationary front. On a weather map, a stationary front is represented with a red and blue line, with red semi-circles pointing away from the warm air, and blue triangles pointing away from the cold air.
- As a storm system develops, fronts begin to rotate around a low-pressure area. When a cold front rotates around to catch up to a warm front and the cold air mass ahead of it, it creates an occluded front, a boundary between two cold air masses. Temperature and humidity will be different across the front, and moderate precipitation and funnel clouds are possible along the occluded front. The occluded front is represented with a purple line of alternating semi-circles and triangles.