Air masses carry large bodies of air with similar temperatures across regions; cold air masses bring lower temperatures while warm air masses raise temperatures, and depressions form as warm and cold air masses interact, with cold fronts pushing out warm air and warm fronts lifting warm air over colder air.

1. Air masses and depressions

2. Image source: http://www.bbc.co.uk/news/magazine-24305230

3. Air masses
Cold air
Warm air

4. Cold front
Cold air

5. Warm front
Warm air

6. The depression
Cold air
Cold front
Warm front
Warm air