Fronts are boundaries between two air masses that collide but do not mix. There are four main types of fronts: cold fronts move quickly and cause thunderstorms and heavy rain or snow as cold air pushes under warmer air. Warm fronts move slowly and result in drizzle, rain, and fog as warm air moves over cold air. Stationary fronts hardly move and bring many cloudy, wet days as warm and cold air remain separated. Occluded fronts can produce clouds, rain, or snow when warm air gets trapped between two cold air masses and is pushed up.