air mass

air mass

air mass, large body of air within the earth's atmosphere in which temperature and humidity, although varying at different heights, remain similar throughout the body at any one height. Air masses form over parts of the earth's surface called source regions, which are large bodies of water or landmasses with relatively uniform topography, often ranging hundreds of thousands of square miles in area. When a body of air remains over a source region for days or weeks, it reaches an equilibrium with the surface. Radiation and convection exchanges between the surface and the air determines the air mass characteristics. Air masses formed over oceans generally contain more moisture than continental ones; air masses formed in polar latitudes are colder than those from the tropics. As an air mass moves away from its source region, it brings its particular weather conditions to areas over which it travels. At the same time, its characteristic properties are slowly modified by exposure to new environments. The boundaries between air masses, called fronts, are, typically, zones of rapid transition from cold to warm or from dry to moist air. Turbulence at the boundary often breeds low-pressure storms.
An Air-mass thunderstorm, also called a garden variety thunderstorm, is a thunderstorm that is generally weak and usually not severe. These storms form in environments with low Convective Available Potential Energy (CAPE) values, low wind shear, and low helicity values. The lifting source, which is a crucial factor in thunderstorm development, is usually the result of uneven heating of the surface when it comes to air-mass thunderstorms. This is opposed to a lifting source being a cold front, a strong upper level jet streak, or a strong low level convergence boundary which is usually the case with other types of stronger storms. The energy needed for these storms to form comes in the form of insolation, or solar radiation. Air-mass thunderstorms do not move quickly, last no longer than an hour, and have the threats of lightning, moderate light or heavy rainfall. On unusual occasions there could a weak downburst, and small hail (There could be 3/4 inch diameter (severe) hail in very rare cases). They are common in a maritime tropical air mass, around the gulf coast states, during the summer, and in the afternoon.

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