Fronts are boundaries between air masses of different densities, moisture, or temperatures. There are four main types of fronts: Cold fronts occur when cold air cuts under warm air, forming cumulonimbus clouds that can cause thunderstorms or tornadoes. Warm fronts happen as warmer air advances over colder air. Occluded fronts include three air masses where colder air forces warm air upward. Stationary fronts remain stationary, causing light winds and precipitation.