Looking at satellite images is a great start for every forecast, but more information is always helpful. Let's review some symbols commonly seen on weather maps, starting with fronts. Fronts are boundaries between two different air masses.

A blue line with triangles on one side is a cold front. When you see a cold front on a weather map, it means a colder air mass is trying to replace warmer air. The front marks the leading edge of the cold air. The blue triangles always point in the direction that the front (and the cold air) is going.

A red line with half-circles signifies a warm front. A warm front shows the leading edge of warmer air trying to replace a colder air mass. The half circles always point in the direction that the front (and the warmer air) is moving.

It is common to see a cold front or a warm front on a weather map. You might also see symbols for stationary fronts or occluded fronts (see examples on the right) which indicate that the weather is moving slowly in that area.

Fronts don't just exist at the surface of the earth, they have a vertical structure extending up into the atmosphere as well. When air masses behind fronts collide, warmer air is always forced up and over the colder air. When the warm, moist air rises, it cools, and water condenses out into clouds and precipitation. By showing where the clouds are, satellite images outline the approximate location of fronts.

Why do air masses move? The fact that the Earth is nearly round causes the sun to warm areas near the equator more than areas near the poles. Differences in heating are also due to the orientation of the Earth in its orbit around the sun (it's tilted) and the irregular shape of the orbit. All the while the Earth is rotating on it's axis every day. This constant motion combined with differences in heating causes air to move along the surface of the Earth (advection) and rise in the atmosphere (convection).

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