What happens when two air masses collide?

• When two different air masses collide, they don’t mix. They push against each other along a line called air front. 
• When a warm air mass meets a cold air mass, the warm air rises as it is lighter and the water vapour in it condenses at higher altitudes. This type of front is called a warm front. It generates nimbostratus clouds, which can result in moderate rain. 
• On the other hand, when a cold air mass catches up with a warm air mass, the cold air slides under the warm air and pushes it upward forming a cold front. As the warm air rises, it cools down rapidly giving rise to cumulonimbus clouds that are often associated with heavy precipitation and storms.

Further Reading:

• General Circulation Of The Atmosphere
• Heating And Cooling Of The Atmosphere