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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 "Bwk". (Tropical and Subtropical Desert Climate).
The average temperature for the year in San Juan is 64.2°F (17.9°C). The warmest month, on average, is January with an average temperature of 80.6°F (27°C). The coolest month on average is June, with an average temperature of 46.0°F (7.8°C).
The highest recorded temperature in San Juan is 104.0°F (40°C), which was recorded in November. The lowest recorded temperature in San Juan is 20.0°F (-6.7°C), which was recorded in July.
The average amount of precipitation for the year in San Juan is 3.5" (88.9 mm). The month with the most precipitation on average is January with 0.7" (17.8 mm) of precipitation. The month with the least precipitation on average is April with an average of 0.1" (2.5 mm). There are an average of 25.6 days of precipitation, with the most precipitation occurring in January with 4.0 days and the least precipitation occurring in June with 0.6 days.