Rainfall

• Precipitation levels in subtropical deserts average less than 10 inches per year. With temperatures capable of reaching over 120 degrees Fahrenheit, even this small amount of rainfall rarely remains on the desert's surface, evaporating into the air. As such, humidity in subtropical deserts can be as much as 20 percent.
  
  

Atmospheric Circulation

• The lack of rainfall in subtropical deserts is due to the way tropical weather systems operate. Warm, moist air at the equator rises and is blown over the tropics. Here it falls as rain. As the now-dry air moves on, over the subtropical areas, it sinks. Not only does this dry air constitute the climatic environment for the subtropical deserts, it also encourages, as it gets closer to the ground, evaporation of any water on the desert's surface and inhibits condensation, compounding the dryness.
  
  

Solar Radiation

• While the global atmospheric circulation of air is the primary cause of the lack of rainfall over subtropical deserts, the issue is compounded by the relative lack of clouds that results from moisture-deficient air. This means that radiation from the sun is unimpeded from source to ground surface, baking the ground and increasing water evaporation.
  
  

Topography

• The lack of rainfall can also be affected by the environment proximate to the desert. For example, subtropical deserts that abut the coast are deprived of moisture by coastal wind patterns preventing clouds moving over the land. Furthermore, mountains that border subtropical deserts force any clouds upwards where they deposit their rain. Thus, mountains serve as a barrier and influence the amount of rainfall on the desert.
  
  

Adaptations

• Plants and animals in subtropical deserts have had to adapt to the lack of regular, available moisture. Most plants, such as cacti, have developed leaves that retain what little rainwater does reach them and long roots that reach down into substratum water sources. Animals are almost always active at night when it is cooler and moisture in the air is more likely to condense on plant and rock surfaces and become available to drink.