The Rain Shadow Effect

Tomas Pueyo at Uncharted Territories: I’ve discussed this in the past, and you might have learned about this in your geography classes, but I wanted to dedicate a short, clear, beautiful article to it. I wrote it first as a Twitter version, and made it even better for the article.

This gif of the Rock of Gibraltar gives an intuition for why many of the world’s deserts are next to rainforests:

What’s happening here?
How can you use that to predict where there will be deserts or rainforests?

Look at the map below: In some places, deserts and lush forests are side by side. Why?

The mountain chains between them:

The effect is called the Rain Shadow:

  • Wet air comes from the sea
  • It hits mountains and goes up
  • Higher altitudes are cooler, so the air cools
  • That condenses water (like the droplets on your Coke glass)
  • Rain falls
  • The air that gets past the mountains is dry

The left part of this diagram is what’s happening in Gibraltar: You can see the water condense as the air lifts up. But the Rock of Gibraltar is not tall enough for that to become rain, so the cloud keeps going.

The dry air past the mountains is called Foehn Wind, and you can get an intuition for it here:

These mountains are tall enough to create condensation, but not tall enough to catch all their moisture, so some of it remains in the wind that falls down. As it falls, water evaporates again. The wind after the mountain is drier than before the mountain.

You can see this in this video from South Africa, where the taller mountains produce a drier wind afterwards (leeward).

More here.