When warm air replaces a cold air mass, a warm front forms. In general, this weather pattern moves from southwest to northeast, and air behind the front is warmer and more moist than the air ahead of it. The most significant signs of a warm front are higher temperature and humidity. On weather maps, a warm front is symbolized by a solid red line.
A cold front occurs when colder air replaces a warm air mass. Cold fronts generally move from northwest to southeast across the United States, and air behind the front is colder and drier than air ahead of it. Temperatures drop significantly when a cold front passes, often by more than 15 degrees. On a weather map, a cold front is symbolized by a solid blue line with triangles pointing toward the warmer air.
When either a warm or cold front stops moving over a region, a stationary front occurs. This weather pattern may bring days of cloudy, wet weather and, in the case of a cold front, snow. If low pressure areas form during a stationary front, new storms are possible, which may bring heavy precipitation and winds. A stationary front is symbolized by both cold and warm front lines with alternating triangles pointing toward the front's eventual path.
When warm and cold air masses meet, an occluded front forms. During an occluded front there are changes in temperature and wind speed. When a depression (low-pressure system) forms, a warm front moves faster than a cold front. As the system intensifies, the cold front catches up to the warm front and creates the occluded front. An occluded front is symbolized by a purple line.
