A side view of a warm front (A) and how it is represented on a weather map (B)
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Windows to the Universe image by Lisa Gardiner
A warm front is where a warm air mass is pushing into a colder air mass. Warm fronts move more slowly than cold fronts because it is more difficult for the warm air to move against the cold, dense air.
You will often see high clouds like cirrus, cirrostratus, and middle clouds like altostratus ahead of a warm front. These clouds form in the warm air that is high above the cool air. As the front passes over an area, the clouds become lower and rain is likely. There can be thunderstorms around the warm front if the air is unstable.
On weather maps, the surface location of a warm front is represented by a solid red line with red, filled-in semicircles along it, like in the map on the left (B). The semicircles indicate the direction that the front is moving. They are on the side of the line where the front is moving. Notice on the map that temperatures at ground level are cooler in front of the front than behind it.
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