What must occur for air masses to form fronts?

a. they must increase in strength

b. they must collide with each other

c. they must move from west to east

d,. they must create air-pressure differences

npi

Explanation:

They must collide with each other to form fronts of air masses.

Explanation:

The ''air mass'' is the volume of air fixed by the temperature and the volume of water vapor. The ''front'' is the division between two air masses of discrete density and it is the source of meteorological origin of outside the region. As long as the warm air cools down and the water vapor liquefy, the air can become cloudy with falling rain or snow.  

So they must collide with each other while the air masses are directed at the fronts and the dividing line between the fronts can be disfigured sometimes. This formation can be caused by the surface features such as mountains or strong winds of current flow.

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