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9.21: Air Masses

Difficulty Level: At Grade Created by: CK-12
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Why do these air balloons rise?

Warm air rises and cool air sinks. In a hot air balloon, a heater heats the air inside the balloon. When the weight of the warm air plus the balloon is less than the weight of the cooler air outside the balloon, the balloon will rise. Air masses work on the same principles, rising and falling when they confront an obstacle, such as another air mass.

What is an Air Mass?

An air mass is a batch of air that has nearly the same temperature and humidity (Figure below). An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days or longer, it picks up the distinct temperature and humidity characteristics of that region.

Air Mass Formation

Air masses form over a large area; they can be 1,600 km (1,000 miles) across and several kilometers thick. Air masses form primarily in high pressure zones, most commonly in polar and tropical regions. Temperate zones are ordinarily too unstable for air masses to form. Instead, air masses move across temperate zones, so the middle latitudes are prone to having interesting weather.

The source regions of air masses found around the world. Symbols: (1) origin over a continent (c) or an ocean (m, for maritime); (2) arctic (A), polar (P,) tropical (T), and equatorial (E); (3) properties relative to the ground it moves over: k, for colder, w for warmer.

What does an air mass with the symbol cPk mean? The symbol cPk is an air mass with a continental polar source region that is colder than the region it is now moving over.

Air Mass Movement

Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it.


Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms. How would a moving air mass form an inversion? When a warmer air mass travels over colder ground, the bottom layer of air cools and, because of its high density, is trapped near the ground.

Moderate Temperature

In general, cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles. This brings heat to cold areas and cools down areas that are warm. It is one of the many processes that act to balance out the planet’s temperatures.

Figures and animations explain weather basics at this USA Today site: http://www.usatoday.com/weather/wstorm0.htm.

An online guide from the University of Illinois about air masses and fronts is found here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/af/home.rxml.


  • An air mass has roughly the same temperature and humidity.
  • Air masses form over regions where the air is stable for a long enough time that the air can take on the characteristics of the region.
  • Air masses move when they are pushed by high level winds.


Use this resource to answer the questions that follow.


1. What is air mass?

2. What determines the types of air masses?

3. What is continental air?

4. What is maritine air?

5. What is tropical air?

6. What is polar air?

7. List the four types of air masses and give the abbreviation for each.

8. What characterizes arctic air?

9. What characterizes equatorial air?


1. How do the movements of air masses moderate temperature?

2. Why do air masses form mostly in high pressure areas?

3. What is the relationship between air masses and storms?

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air mass A large mass of air with the same temperature and humidity characteristics.

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Difficulty Level:
At Grade

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Date Created:
Feb 24, 2012
Last Modified:
Aug 06, 2016
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