What must occur for air masses to form fronts? They must increase in strength. They must collide with each other. They must move from west to east. They must create air-pressure differences.
For a front to be formed, the air masses must "collide with each other".
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.
