Lesson Plan - Get It!
Shh. Act natural. You're surrounded. It's all around you - to your left, to your right, above you...even inside of you. What is this mysterious substance?
Just like a fish swimming in water, we are humans "swimming" in air.
There's a gigantic bubble of air around you right now. You can't see it, but you can feel it. What's the air in your bubble like today? Is it hot or cold, wet or dry? Don't get too used to it, though. A different air bubble might move into your area tomorrow and with it would come different weather conditions. Maybe today's air bubble is warm and humid, but the one that moves in tomorrow could be cool and dry.
A single one of these air bubbles is called an air mass. An air mass is a large area of air that is all very similar. For example, if it's a wet, hot air mass, then all of the air in it would be similarly wet and hot.
- How are air masses created?
Air is constantly moving because the sun doesn't heat the Earth evenly. This uneven heating causes temperature and density differences resulting in convectional movement.
To learn more about convection currents and heat transfer, check out the Elephango lesson under Additional Resources in the right-hand sidebar.
Sometimes, though, air moves so slowly that an air mass spends enough time in the same place to take on the characteristics of that place. It's kind of like when your friend visits France and comes back with a French accent! The place where the air mass forms is called the source region.
- What do you think an air mass formed in the Sahara desert would be like?
- What about one formed in the northern Pacific Ocean, near Alaska?
When these air masses eventually move to a different location, they keep the same weather conditions as the source region. Once the air mass is moving, it does not take on the conditions of its new locations because it's not staying there long enough to change.
- Where do you think the source region of the air mass you're currently in is located?
Classifying Air Masses
Air masses are classified by two characteristics: their relative temperature and moisture level.
First, let's discuss moisture level. There are only two options here: wet or dry. Wet (humid) air masses were likely formed over the ocean and are, therefore, classified as Maritime meaning "related to the sea."
Dry air masses are formed over land, so we refer to them as Continental, meaning formed over a continent.
The temperature of air masses isn't quite as straightforward. We do not name air masses after their actual temperature but rather after the type of climate where they formed. There are three categories that are most commonly used: arctic, polar, and tropical.
Arctic air masses are extremely cold air masses formed around the North Pole. Similarly cold air masses are also formed near the South Pole. These are called Antarctic air masses.
Polar air masses are frigid air masses and are formed in cold areas far from the equator.
Tropical air masses are formed, unsurprisingly, in or near the tropics. The tropics are defined as the area between the Tropic of Cancer (23.5º N) and the Tropic of Capricorn (23.5º S). Tropical air masses are warm but not necessarily wet. Remember, tropical refers only to the temperature, NOT the moisture level.
Image by KVDP, via Wikimedia Commons, is licensed under the CC BY-SA 3.0 license.
Still have questions? Watch What are air masses? from Met Office - Weather to review what we've learned.
Now that you know all about air masses, learn how to name specific air masses based on their properties in the Got It? section.