A system for describing climates, devised in 1969 by A. N. Strahler, in which world climates are related to the main air masses that produce them, as: (a) equatorial/tropical air masses, producing low-latitude climates; (b) tropical and polar air masses, producing mid-latitude climates; and (c) polar and arctic air masses, producing high-latitude climates. Subsets of these are based on variations in temperature and precipitation to give 14 regional types, plus upland (highland) climates which are regarded as a separate category. See also köppen climate classification; thornthwaite climate classification.