The word monsoon may conjure in your mind torrential rains. But monsoon actually comes from the Arabic mawsim—meaning not rain but season.

In the tropics, there are only two seasons, known as dry and wet monsoons. They’re brought about by equatorial winds that blow in each spring and fall—predictably enough that sailors have depended on them for centuries.

During wet monsoons, the winds bring rain in tropical places around the world, like India and Southeast Asia. But also in the subtropics—like the American Southwest, Central America, northern South America, and West Africa.

When humid monsoon winds meet highlands, like in the Tibetan plateau or the Himalayan foothills, the moist equatorial air rises and more water vapor condenses out to produce those famous torrential downpours.

The largest wet monsoon rains ever recorded happened in India in the summer of 1860, with over one thousandinches of rain.

Wet monsoons recharge many of the great rivers of the world, providing water for millions of people and their agriculture.

But they can also produce devastating floods and cropland erosion and bring water- and insect-borne diseases.

Dry monsoons provide a welcome relief from the rains but sometimes cause drought.

These hardships aside, the wet–dry monsoon cycle is essential to life in the tropics.