Wiktionary
n. (context meteorology English) A particular volume of air, especially one having a roughly uniform temperature, pressure and water vapour content. (from 19th c.)
WordNet
n. a large body of air with uniform characteristics horizontally
Wikipedia
In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature and/or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass' characteristics, and well as modification.
In astronomy, air mass (or airmass) is the optical path length through Earth’s atmosphere for light from a celestial source. As it passes through the atmosphere, light is attenuated by scattering and absorption; the more atmosphere through which it passes, the greater the attenuation. Consequently, celestial bodies at the horizon appear less bright than when at the zenith. The attenuation, known as atmospheric extinction, is described quantitatively by the Beer–Lambert–Bouguer law.
“Air mass” normally indicates relative air mass, the path length relative to that at the zenith at sea level so, by definition, the sea-level air mass at the zenith is 1. Air mass increases as the angle between the source and the zenith increases, reaching a value of approximately 38 at the horizon. Air mass can be less than one at an elevation greater than sea level; however, most closed-form expressions for air mass do not include the effects of elevation, so adjustment must usually be accomplished by other means.
In some fields, such as solar energy and photovoltaics, air mass is indicated by the acronym AM; additionally, the value of the air mass is often given by appending its value to AM, so that AM1 indicates an air mass of 1, AM2 indicates an air mass of 2, and so on. The region above Earth’s atmosphere, where there is no atmospheric attenuation of solar radiation, is considered to have “ air mass zero” (AM0).
Tables of air mass have been published by numerous authors, including Bemporad (1904), Allen (1976), and Kasten and Young (1989).
The air mass coefficient defines the direct optical path length through the Earth's atmosphere, expressed as a ratio relative to the path length vertically upwards, i.e. at the zenith. The air mass coefficient can be used to help characterize the solar spectrum after solar radiation has traveled through the atmosphere. The air mass coefficient is commonly used to characterize the performance of solar cells under standardized conditions, and is often referred to using the syntax "AM" followed by a number. "AM1.5" is almost universal when characterizing terrestrial power-generating panels.
Usage examples of "air mass".
During this time, the enemy continued his steady methodical advance well behind the front of the cold air mass.
And in Ratharkin itself, the soi-disant Queen Caitrin of Meara met with the remnants of her family and their key leaders for one final conference before riding out to the open-air Mass that would send Meara's hopes on their way.
There were white cirrus streaks clawing up the azure sky over Mount Remillard, and I hoped that a cold, clear air mass was moving in on us.
He improvised an altar in the square and on Sunday he went through the town with a small bell, as in the days of insomnia, calling people to an open-air mass.
Over the Atlantic a low-pressure air mass was advancing in an easterly direction toward Russia.
All that remained of the Gulf storm was an enormous energy differential between the hundred-degree air mass over the desert and the sixty-five-degree air mass of the front.