When a thunderstorm forms over the desert, it may not produce rain—but could form a deadly dust blizzard called a haboob.

When rain droplets condense within a thunderhead, they cool the air, which rushes toward Earth with the rain.

If this happens over a desert, the rain will often evaporate before reaching Earth. The evaporative cooling further chills the air, which falls faster.

Eventually the current of air strikes the ground, at high speed, and expands outward.

The blast often carries dust and sand into the air, to form a huge dust storm that advances in front of the thunderhead.

These were first studied in the Sahara, hence were given an Arabic name, haboob. But they form across arid regions of the U.S. as well.

Sand, dust and soil carried by a haboob can sandblast vehicles and buildings. Worse, haboobs often carry bacteria and viruses from the desert floor, causing respiratory problems and skin irritation for those exposed to them.

Even more dangerous, haboobs can suddenly drop visibility to zero. Highway pileups are common in these storms, and the dust can blind pilots and interfere with airplane engines.

If areas like the American West become drier, we could see more frequent haboobs.

If you’re caught in one, first cover your nose and mouth. If driving, honk your horn and pull far off the road. Then turn off all your lights so other cars don’t steer toward and hit you.