Since 2002, NASA's Aqua Satellite has been orbiting Earth, taking thousands of high-resolution photos of our planet in order to help scientists better understand our water cycle and weather.
Recently, NASA compiled all this data to produce the gorgeous map above — a map of the average cloud cover everywhere on Earth for the last 13 years.
Dark blue areas see very little cloud cover. White areas are cloudy almost all the time.
When you look closely, you can see all sorts of interesting patterns going on. For instance, there are a striking number of cloudless deserts all over the planet at roughly 30° north and south:
The Sahara, Arabian Desert, and Kalahari are all at roughly 30°N or S. (NASA Earth Observatory)
These are the result of Hadley cells: winds patterns that blow dry, high-altitude air masses from the equator to the tropics, where they descend, sucking up moisture and preventing cloud formation.
On the flip side, the same patterns form a distinct band of cloudiness at the equator. Here, the same cells blow low-altitude, moist air. When it rises upward, it causes water vapor to condense into clouds.
On a more local scale, you can see cloud patterns that closely match mountain formations.
In the US, for instance, clouds regularly form along the ridges of the Rocky Mountains (and to a lesser extent the Appalachians) because of the air masses that get blown eastward and pushed up the mountains. When that happens, they cool, and can't hold as much moisture — so their water vapor condenses to form clouds.
Hat tip to Slate's Phil Plait for spotting this fascinating map.