Deserts can take many forms — including sweeping sand dunes, rocky canyons, sagebrush steppes and polar ice fields. But they're united by one thing: a lack of rainfall. Generally speaking, anywhere that gets less than 10 inches (25 centimeters) of rain a year counts as a desert, said Lynn Fenstermaker, an ecologist at the Desert Research Institute in Reno, Nevada.
Of course, that lack of rain means deserts are, famously, dry. But why do some places on Earth get much less rain than others? In other words, why are deserts dry?
