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Table 1 Air mass types in the SSC

From: Heat/mortality sensitivities in Los Angeles during winter: a unique phenomenon in the United States

Air Mass Definition
Generally Non-Oppressive Air Masses
 Dry Polar (DP) Arrives from polar regions and is usually associated with the lowest temperatures observed in a region for a particular time of year as well as clear, dry conditions.
 Dry Moderate (DM) Consists of mild and dry air. Often occurs when air warms as it descends mountain ranges.
 Moist Polar (MP) Typically cloudy, humid, and cool. MP air appears when air over the adjacent cool ocean is brought inland, frequently during stormy conditions.
 Moist Moderate (MM) Considerably warmer and more humid than MP. The MM air mass typically appears in a zone south of MP air, near an adjacent stationary front (an area where warm air moves over a cooler air mass).
 Moist Tropical (MT) Warm and very humid. It is typically found in warm sectors of mid-latitude cyclones or in a return flow on the western side of a high-pressure area.
 Transition (TR) Defined as days in which one weather type yields to another, based on large shifts in pressure, dew point, and wind over the course of the day.
Oppressive Hot Air Masses
 Dry Tropical (DT) Represents the hottest and driest conditions found at any location. There are two primary sources of DT: either it is transported from the desert regions, such as the Sonoran Desert, or it is produced by rapidly descending air.
 Moist Tropical Plus (MT+) Hotter and more humid subset of MT. It is defined as an MT day where both morning and afternoon temperatures are above the MT averages, and thus captures the most “oppressive” subset of MT days.