Köppen climate classification
Group B: Desert and semi-arid climates

Desert and semi-arid climates are defined by low precipitation in a region that does not fit the polar (EF or ET) criteria of no month with an average temperature greater than 10 °C (50 °F).

The precipitation threshold in millimeters is determined by multiplying the average annual temperature in Celsius by 20, then adding:

If the annual precipitation is less than 50% of this threshold, the classification is BW (arid: desert climate); if it is in the range of 50%–100% of the threshold, the classification is BS (semi-arid: steppe climate).

A third letter can be included to indicate temperature. Here, h signifies low-latitude climates (average annual temperature above 18 °C (64.4 °F)) while k signifies middle-latitude climates (average annual temperature less than 18 °C). In addition, n is used to denote a climate characterized by frequent fog and H for high altitudes.
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