Earth
An air mass is a large volume of air defined by temperature and water vapor content.. An air mass can cover many hundreds or thousands of square km and will take on the characteristics of the surface below them. They are classified according to latitude and continental or maritime source. Cold air masses are said to be polar or arctic and warmer masses are referred to as tropical.
Continental and superior air masses are dry. Maritime and monsoon air masses are moist. Weather fronts are able to separate air masses with different density characteristics. These density characteristics are due to temperature and water t. Once an air mass moves away from its source, underlying vegetation and water bodies can quickly modify its character.
The Bergeron classification is the most widely accepted way to classify an air mass. Air mass classification involves three letters. The first letter describes its moisture properties. C is used for dry continental and M for moist maritime air masses. The second letter shows temperature characteristic of its source region: T for tropical, P for polar, A for arctic or antarctic, M for monsoon, E for equatorial, and S for superior(dry air formed by significant downward motion in the atmosphere). The third letter is used to designate the stability of the atmosphere. If the air mass is colder than the ground below it, it is labeled k. If the air mass is warmer than the ground below it, it is labeled w.
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of weather occurrences. The air masses separated by a front usually have different temperatures and humidity. Cold fronts may have narrow bands of thunderstorms and severe weather and can be preceded by squall lines. Warm fronts, on the other hand, are frequently preceded by precipitation form stratus clouds and fog. Some fronts do not produce precipitation and there is very little cloudiness, but there is always a shifting in the winds. After a front passes, the weather will quickly clears.
Air masses can be modified in a variety of ways. Surface flux from underlying vegetation can act to moisten the overlying air mass. Heat from warmer underlying water can quickly modify an air mass over short distances. Temperature difference causes warmth and moisture to be transported upward, condensing into vertically oriented clouds which produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes. Who thought a mass of air could change so much over an area of our planet.
We have written many articles about the air for Universe Today. Here’s an article about the composition of air, and here’s an article about air pressure.
If you’d like more info on the air, check out the National Oceanic & Atmospheric Administration (NOAA) Homepage. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about the Atmosphere. Listen here, Episode 151: Atmospheres.
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