In meteorology, a cold front is an area where cold air is pushing into warm air and displacing it. Because warm air is less dense than cold air, the cold air generally presses under it in a wedge, forcing it upward rather severely. When the air is pushed upwards, it cools and expands, and if there is enough moisture in the air it creates clouds, and often rainfall (or snow). Cold fronts are common in mid-lattitude areas where cold air from the north may be pushed south by the jet stream.

Cold Fronts are usually associated with precipitation which is more severe, but shorter in duration, than Warm Fronts. Often they are associated with thunderstorms, especially in the summer. They are followed by cold air which is often dry, but sometimes contains showers and unstable air, especially in coastal areas. Cold fronts are usually depicted as a blue line with triangles pointing its direction of motion. They are preceeded by high clouds such as cirrus, quickly lowering to clouds like cumulonimbus or nimbostratus.

The wind picked up the other night.
I thought for a moment it was a train,
because the ground shook slightly and
there was the low familiar rumble I know with my eyes closed.

The wind came in slow waves and was a sign the weather has changed.
Winds no longer mean thunderstorms and breezes after hot afternoons.
They mean cold fronts and wind chills.

The wind picked up the other night.
I stood in a hallway in the dark and waited for awhile for it to ease.
A half moon shone through a western window
with no clouds to obscure its blue light.

Only the tree limbs, swaying in the breeze:
Weary dancers, out too late.

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