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An air mass is a large volume of air defined by mostly uniform qualities in both temperature and moisture content. Air masses are categorized by their temperatures and the areas over which they acquire their properties and play a major role in weather characteristics. The four main types of air masses are continental tropical, continental polar, maritime tropical, and maritime polar. An air front can also be categorized by its stability in relation to the air underneath it. Warmer air masses tend to move towards cooler areas and vice versa; when two masses meet, an air front is formed.
A continental air mass forms over land, and is generally dry due to limited amounts of moisture as opposed to a maritime air mass, which forms over large bodies of water. A tropical air mass contains warm air, whereas a polar air mass contains cold air. The stability of an air mass is also taken into consideration; masses with cooler air underneath tend to be more stable than masses with warmer air below them, as the mass tends to drop in altitude given these conditions. Meteorologists further categorize the air masses according to the combination of moisture and stability conditions. Superior air masses are both dry and stable, whereas monsoon masses are moist and unstable.
Air masses move according to their temperature and density. Warm air moves towards the poles, while cooler air moves towards the equator. In addition, dense air masses tend to be heavier and move downward. This movement is responsible for different weather conditions, dictating the wind conditions, temperature and humidity experienced on the surface.
The movement also allows for the formation of air fronts, points at which masses of differing qualities share a boundary. The dynamic motion caused by the difference in density, moisture, and temperature in these areas is the cause of weather phenomena such as typhoons and cyclones. Although wind shifts are a constant feature of air fronts, precipitation only occurs when there is sufficient moisture at the junction. The air masses will eventually move far enough apart from each other to eliminate the front and its resulting weather conditions.
Several factors can affect the formation of air fronts. Volcanic eruptions, for example, can rapidly heat the air in the region, increasing the temperature of the air mass above it. The melting of the polar ice caps can likewise cause a significant increase in the moisture of the air mass overhead. At the same time, the weather conditions created by the changing air fronts and masses can affect the development of the masses that come afterwards.