Air can be cold or warm, wet or dry. When different kinds of air masses meet a front forms. When a warm front approaches , air starts to rise and forms clouds . Light rain usually comes with a warm front.
About a day later a cold front follows. Cold fronts move faster than warm fronts and often catch up with them. The cold air moves under the warm front and pushes the warm air up. Clouds and rainfall are the result. Sometimes such a front can even cause short showers and thunderstorms.
A warm front causes air to rise