As air masses move across the land and the oceans, they bump into each other, but the air masses do not easily mix. Why is that? Think about oil-and-vinegar salad dressing. The less dense oil floats on top of the more dense vinegar.

Something similar happens when two air masses with different densities and temperatures meet. The area where the air masses meet is called a front. Because cold air is denser, it sinks beneath warm air. The warm air is forced upward and winds develop. Fronts usually bring a change in temperature, and they always bring a change in wind direction. There are four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts. Fronts are named for the air that is behind the front.

Watch this video for an introduction to the types of fronts.

Cold Front

• A cold weather front is a boundary between a cold air mass and a warmer air mass.
• Cold weather fronts usually move from northwest to southeast.
• Cold, dense air pushes warm air out of the way.
• Cold fronts move very quickly and bring short periods of heavy rain, thunderstorms, or snow.
• Lower temperatures are behind the front.
• Blue arrows pointing in the direction the front is moving are used as a symbol for a cold front.

Warm Front

• A warm weather front is a region where a warm air mass is replacing a cold air mass.
• Warm fronts usually move from southwest to northeast.
• Warm air moves up the cold front as it slowly displaces the cold air.
• Warm fronts move slowly and bring many days of steady precipitation. After the front passes, weather conditions are clear and warm.
• Higher temperatures are behind the front.
• Red half-moons in the direction the front is moving are used as a symbol for a warm front.

Stationary Front

• A stationary front is where a warm air mass and a cold air mass meet, but neither has enough force to move the other.
• Stationary fronts can remain in the same location for several days.
• The air from the warm front and cold front meet, but do not move.
• Warm and cold fronts are moving in opposite directions, thus making a stationary condition.
• These fronts have the same weather as warm fronts.
• Alternating red half-moons and blue arrows show the directions the fronts are moving.

Occluded Front

• Occluded fronts happen when two cold air masses trap a warm air mass and force it upward.
• The temperature on the ground becomes cooler.
• The warm air mass is cut off, or occluded, from the ground.
• Weather is similar to a cold front.
• Purple alternating half-moons and triangles on the same side of the line are used as a symbol for an occluded front.

Name that Front! Study the following diagrams and identify what type of front is being displayed.