Here's why:
* Fronts are boundaries between air masses with different temperatures and densities.
* Warm fronts occur when a warm air mass moves into a region occupied by a cold air mass.
* Cold fronts occur when a cold air mass moves into a region occupied by a warm air mass.
* Stationary fronts form when the boundary between the two air masses is relatively still, with neither air mass having enough force to push the other aside. This can happen when the winds on either side of the front are weak or blowing parallel to the front.
Stationary fronts can bring prolonged periods of rain or snow, as the warm, moist air rises along the front, cools, and condenses. They can also lead to periods of fog and low-level clouds.
