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Air Masses and Weather Fronts

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  • 0:04 Air Masses and Weather Fronts
  • 1:47 Weather Patterns
  • 3:40 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what an air mass is and how the movement of weather fronts leads to weather patterns you see every day.

Air Masses and Weather Fronts

The properties and movements of air masses are responsible for all the weather patterns we experience in daily life. An air mass is a large body of air, with a similar temperature and moisture content throughout. Air masses can be small or they can be gigantic, covering hundreds of thousands of miles. Air masses over water are called maritime air masses and contain a lot of evaporated moisture. Air masses over land are called continental air masses and are much drier.

A weather front is a boundary between air masses with different properties. There are four main types of weather fronts: cold fronts, warm fronts, occluded fronts and stationary fronts. The diagram below provides the symbols that are used to represent the different types of fronts that we will examine.


Symbols Used to Represent Fronts: 1= warm, 2=cold, 3=occluded, 4=stationary
Symbols Used to Represent Fronts: 1= warm, 2=cold, 3=occluded, 4=stationary


A warm front is an area of warm air displacing cold air.

A cold front is an area of cold air displacing warm air.

An occluded front is where a cold front catches up with a warm front. This usually happens when two fronts are heading northwards at angles. To help imagine this, think of it as a sandwich of air temperatures. The warm front is warm air displacing cooler air. Then a cold front catches up from behind, displacing the warm air. So when an occluded front is about to form you have cold air behind, warm air in the middle, and cooler air ahead. The air ahead may be colder than the air at the back, or may be warmer. But it's always a cold-warm-cold sandwich.

A stationary front is air of different temperatures that are alongside each other and not moving relative to each other.


The Formation of an Occluded Front
The Formation of an Occluded Front


But what does all this have to do with the weather? Exactly what weather occurs when these fronts are around? Let's go through some of the possible scenarios.

Weather Patterns

The movements and collisions of fronts are the main cause of weather patterns, including rain and snow. When a cold front or cold occlusion goes under a warm, moist air mass, the warm air rises and rain clouds or even thunderstorms result. If the warm air is dry, the air will still rise but no clouds will form.

Cold fronts move faster than warm fronts and tend to cause the largest storms. This happens because warm air is suddenly forced upwards by the cold air, forming giant clouds. Behind the front, there are usually clear, sunny skies.


Formation of Rain Where a Cold Front Advances
Formation of Rain Where a Cold Front Advances


When warm fronts are approaching, fog is commonly seen and rainfall gets more severe as the front approaches. But it's nowhere near as dramatic, because warm fronts move slowly. It's more like a consistent rain and overcast skies. If the warm front is unstable, it may lead to thunderstorms, but relatively mild ones. Behind the front, you find scattered clouds.

Not only do these fronts bring precipitation, the air also heats and cools the region it passes through. This can lead to more evaporation of surface water, which can feed or diminish the fronts.

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