ANSWER:
For a front to be formed, the air masses must "collide with each other".
Step-by-step explanation:
- In general, when a mass of warm air collides with a mass of cold air, a front is formed. Whenever a front passes, weather changes take place in the specific area. Fronts formed are of different types.
- Cold front for instance is formed when "cold air mass" pushes a warm air mass and brings about cool air and also cold weather.