For a front to occur, two different masses of air need to collide with each other.
Explanation:
The fronts formed by colliding air masses are literally the zones of transition where two air masses flowing towards each other meet and stop each other from gushing further horizontally.
These colliding air masses are generally warm and cold in nature.
The cold air masses usually flow from temperate zone towards the tropical zone, whereas, warm air masses flow from the tropical zone towards the temperate zone.
