What Causes Air Masses?

Features

• Air masses are vast volumes of air that feature almost uniform levels of temperature and moisture throughout an individual mass. An air mass covers a large area, up to thousands of square miles. The exact characteristics of any one specific air mass are determined by the surface conditions of the area of Earth that it formed above.

Formation

• Air masses are formed when a body of air remains static in any one location for a period of between three days and one week, particularly if that location on the Earth's surface has fairly consistent, relatively flat geographical features. A still, stable atmosphere also contributes to the formation of air masses. The regions above which an air mass is formed are called source regions. Ideal source regions include deserts and oceans. Features such as mountain ranges tend to break up air mass formation, preventing the bodies of air from becoming as large as they may otherwise become.

Different Types of Air Masses

• The classification of a particular type of air mass is based upon the temperature and moisture conditions of the source region at the time of formation. An air mass is classified as continental if it was formed over land or maritime if the air mass was formed above sea and this part of the classification also infers the level of moisture within the body of air. Secondly, air masses are classed as tropical, equatorial, arctic or polar, depending on the temperature and geographical location, particularly the latitude of the source region. The classification of an air mass is written with a lower case letter to describe the moisture level and an upper case letter to describe the temperature level. For example, cT, or continental tropical, air mass contains warm air with very little moisture.

Fronts

• Once air masses have been formed, they may change location as a result of being carried by wind. When this happens, they may meet another air mass. When two air masses with different temperature and moisture conditions meet, the differences in their conditions appears to be amplified and a front, or frontal zone is created. The term front is derived from the term battlefront, in which two opposing sides meet each other in battle during a war. Different types of fronts can create a range of weather conditions, depending on the types of air masses that meet each other, and can warn of certain weather conditions that are approaching.