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This climate is dominated in all months by the subtropical anticyclon, or subtropical high, with its descending air, elevated inversions, and clear skies. Such an atmospheric environment inhibits precipitation. Most of Earth's tropical, true desert climates occur between 15° and 30° latitude. The most extreme arid areas also are far removed from sources of moisture-bearing winds in the interiors of continents and are best developed on the western sides of continents, where the subtropical anticyclone shows its most intense development. An exception to the general tendency for aridity to be associated with subsidence is in the Horn of Africa region, where the dryness of Somalia is caused more by the orientation of the landmass in relation to the atmospheric circulation. Both the high- and low-sun monsoonal winds blow parallel to the coast, so that moisture-laden maritime air can penetrate over land only infrequently.
The Köppen Climate Classification subtype for this climate is "Bwh". (Tropical and Subtropical Desert Climate).
The average temperature for the year in St.-Louis is 79.0°F (26.1°C). The warmest month, on average, is September with an average temperature of 84.0°F (28.9°C). The coolest month on average is January, with an average temperature of 73.0°F (22.8°C).
The highest recorded temperature in St.-Louis is 113.0°F (45°C), which was recorded in April. The lowest recorded temperature in St.-Louis is 45.0°F (7.2°C), which was recorded in January.
The average amount of precipitation for the year in St.-Louis is 14.9" (378.5 mm). The month with the most precipitation on average is August with 6.1" (154.9 mm) of precipitation. The month with the least precipitation on average is March with an average of 0.0" (0 mm).