Fronts form when air masses collide because air
masses do not mix unless they are similar in temperature and moisture
content. When warm air displaces cold air at the ground, the front is
referred to as a warm front. When cold air replaces warm air at the
ground the front is referred to as a cold front.
If the boundary between warm and cold air does not
move, it is called a stationary front. When a cold front meets a warm
front, and goes under it, the fronts are occluding.
Weather map symbols for fronts