What is a rain shadow?

A rain shadow desert is the result of the rain shadow effect. This occurs when a mountain range lies between an area that has very wet air like a coast line and an area of very dry air like a desert.
When the warm wet air moves from the ocean towards over the mountains it cools and the water in the air (water vapor) is squeezed out as rain or snow. When the air finally reaches the inland rain shadow desert it has no moisture left and therefor no rain in it and so the area is considered a rain shadow.
If anyone is interested there are some physics and chemistry conceptual links. The physics of why warm air holds more moisture than cool air has to do with kinetic theory.
Remember that air is a bunch of molecules spread out so far that they are a gas. Air is a substance that like any other substance responds to heat energy by moving faster and bouncing off in all directions. When the air is warm there is more space for water to hang out between all the other molecules but when the air gets cooled at high altitude the molecules slow down and get closer so the water sticks together (polarity of water) and becomes liquid (rain) or even snow if it’s cold enough. When the air warms again over the rain shadow it’s still dry since the water was left behind.
The chemistry involved has to do with the polarity of water and how polar covalent bonds form.