These data are plotted on maps of the United States. On the map, each NWS Office appears as a small circle; weather observations are systematically arranged around the circle in what is known as the station model format. Meteorologists carefully analyze the weather map to determine the location of large-scale weather systems such as Highs, Lows, and fronts. Simplified versions of these maps appear in newspapers and on television.
Lows: Low pressure centers, called "Lows," form along the boundary between different air masses. Lows appear on the weather map as large "L's." Lows are usually associated with cloudiness and precipitation. Winds blow counterclockwise around Lows.
Highs and Lows influence the weather across broad geographic regions.
Fronts: Warm and cold air masses do not readily mix; the boundary between different air masses is called a front.
When a warm air mass displaces a cold air mass, the boundary is called a warm front. The advancing warm air is less dense than the retreating cold air, so the warm ai r aloft overruns the cold air near the earth's surface, resulting in a widespread layer of clouds ahead of the weather map warm front. Steady precipitation usually falls before a warm front arrives. After the warm front passes, the air temperature rises.
When a cold air mass displaces a warm air mass, the boundary is called a cold front. The advancing cold air is more dense than the retreating warm air, so the cold air forces its way under the warm air, resulting in brief, showery precipitation. After the cold front passes, the air temperature falls.
Sometimes the boundary between different air masses isn't moving; this is called a stationary front.
