What's a Weather Front?
An air mass is a large body of air that has similar moisture (density) and temperature characteristics. A front is a transition zone between two air masses.
A cold front is a warm-cold air boundary with the colder air replacing the warmer. As a cold front moves into an area, the heavier cool air pushes under the lighter warm air that it is replacing. The warm air becomes cooler as it rises. If the rising air is humid enough, the water vapor it contains will condense into clouds and precipitation may fall.
As the cold front moves, warm, moist unstable air is usually replaced by cold, dry stable air.
A warm front is the boundary between warm and cool (or cold) air when the warm air is replacing the cold air. Warm air at the surface pushes above the cool air mass, making clouds and storms. Warm fronts often bring stormy weather. Warm fronts often form on the east side of low-pressure systems where warmer air from the south is pushed north.
A warm front typically replaces cool dry air with warm moist air.