7.4.5: Tropical Desert Climate

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Figure \(\PageIndex{1}\): Grand Erg Desert.

Geographical Distribution

Deserts, mostly classified as BWh climates, are typically found in the continental interiors of the subtropics and on the lee side of mountains in the midlatitudes. Cool coastal deserts are found where cold water upwells along a coast, stabilizing the air and preventing moisture formation, as seen in the north of Chile. Vast deserts cover much of north Africa (The Sahara), Saudi Arabia to Iran, Pakistan and western India. Tropical deserts are found in Baja California and interior Mexico in North America.

Distinguishing Characteristics

Temperature

The tropical desert has the highest mean annual temperature of any climate on Earth. The high temperatures are a result of the high sun angles throughout the year and having the highest percentage of sunshine of any climate. No month has an average temperature below 18^oC (64.4^oF) and many places have consecutive average monthly temperatures in the mid 30^os Celsius (90^oF). Daytime temperatures can reach 50^oC (120^oF) in low elevation inland deserts.

The sky in the tropical desert remains cloud-free due to the subsiding air of dominant high pressure resulting in large amounts of insolation. The cloudless skies during the day let insolation in, but also let much heat out at night. Without the absorptive blanket of clouds, longwave radiation emitted from the Earth readily escapes to space, chilling the nighttime desert air. The high energy input during the day and large loss at night results in an extremely large daily temperature range.

Precipitation

Precipitation in the tropical desert is very irregular and unreliable. Low latitude deserts average less than 25 cm (10 in) in a year. An entire year's worth of rain may fall in one downpour. The continental location of many tropical deserts places them far from a source of moisture, the ocean. Combine continentality with the strong subsidence of the subtropical high and you have one of the driest places on earth. Air subsiding from the subtropical high is adiabatically warmed, reducing the relative humidity of the air. Relative humidity can drop to 10% or less. The extremely low relative humidity causes evaporation of what little surface water there is. The subsiding air also promotes atmospheric stability, further inhibiting precipitation.

Figure \(\PageIndex{2}\): Oasis in Mauritania. (Image courtesy FAO)

Climatologists describe the desert as an "arid" climate. An arid climate, as defined on the basis of the soil moisture balance, is one in which the annual precipitation is less than half of the annual potential evapotranspiration. In the tropical desert, the only substantial source of surface water other than exotic streams is an oasis, where the groundwater table is near the surface.