When air masses meet, they create weather fronts. The air masses typically have different temperatures and densities so their impact on weather will depend on the type of front created.

## What is a Front?

Sam is sitting on his front porch, watching his dog play. Suddenly, he gets shivers from the cold air. Before he knows it, the weather has changed and his dad is calling him inside before it rains. ‘What happened?’ asked Sam. Sam’s dad responds that he just experienced a front.

A front is when two air masses meet and create some type of weather or precipitation (water we get from the atmosphere). The density (or how much mass and volume something takes up) of an air mass are different so it causes a change in the air. When the air becomes warmer, the molecules spread out and cause the air molecules to go up. When air becomes colder, the molecules come closer to together and cause the air molecules to go down. When air masses meet, we get a few different types of fronts.

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## What is a Cold Front?

A cold front happens when a colder air mass meets and pushes a warm air mass up. When the two meet, the density of the cold air mass pushes the less dense warm air mass up. Imagine flipping a glass plate and a paper plate upside down and ramming them into each other, the paper plate will slide over top of the glass plate because it weighs less. This is like a cold front.

The temperature of the air becomes colder because the warm air is pushed up higher into the atmosphere and replaced with the colder air. A cold front also causes thunderstorms and heavy precipitation.

## What is a Warm Front?

A warm front happens when a warm air mass takes over a colder air mass. The warm air mass replaces the cold air molecules in the cold air mass. The temperature of the air becomes warmer as the cold air is pushed back. The amount of water in the air (or humidity) will usually increase causing precipitation. The precipitation is usually lighter than in a cold front.

## What is a Stationary Front?

A stationary front is when a warm air mass and a cold air mass meet. The warm air and cold air sit against each other and do not move. The cold air is not able to make the warm air mass move up so the air stays in the same spot. If the air masses start to move, the front will change to either a cold front or warm front depending on the movement of the cold air.

## What is an Occluded Front?

An occluded front happens when a cold front over takes a warm front in a low pressure area, which is an area that has spread out air molecules. The movement of the warm air up into the atmosphere helps build energy so the low pressure system can build. When a low pressure system builds up enough, it can become a tropical storm and possibly a hurricane.

## Lesson Summary

Fronts are created when two air masses meet. When a front is created, weather such as storms and precipitation happen. A cold front is when the cold air mass takes over the warm air mass causing it go up into the atmosphere. A warm front is when a warm air mass takes over a cold air mass leaving warm air. A stationary front is when neither the warm air mass or cold air mass moves. And, an occluded front is when a cold front is created in a low pressure area.