When cold air meets warm air, frontal wedging occurs. In frontal wedging, cold air forces warm air upward; the warm air then forms clouds, resulting in rain.
When cold and warm air collide, one of four types of frontal wedging occurs: warm, cold, stationary or occluded. A warm front occurs when warm air pushes away cold air. These usually come in from the southwest. When cold air pushes out warm air from the northwest, this is called a cold front. When a stationary front occurs, warm and cold fronts move in and push against each other, but neither manages to push the other one out, resulting in clouds and storms.
An occluded front occurs when both warm and cold air are moving in the same direction and one overtakes the other. If the cold air overtakes the warm air, then it is considered a cold occlusion. If the opposite happens, it is called a warm occlusion. Experts believe that an occluded front is a sign that a storm is almost blown out and weakening.
Not all air collisions occur between warm and cold air. In some cases, two air masses with the same temperature collide in a process called convergence. When this happens, both air masses rise and form clouds.