SEATTLE - A front is a boundary between warm and cold air. With a cold front, you have an area of cold air moving into an area of warm air. You can pick them out on weather maps as a blue line with blue triangles. The triangles point in the direction the front is moving.
Since cold air is heavier than warm air, the cold air pushes underneath the warm air, causing the warm air to rise. The warm air then condenses as it reaches cooler altitudes, creating clouds and, eventually, precipitation.
If the difference in temperatures is very large, you can get a strong front that can create thunderstorms, or even tornadoes. Luckily around here, the Pacific Ocean keeps the temperatures differences to a minimum.
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