What is a rain shadow?

How it works

Wind blowing in from the sea carries lots of invisible water vapour. When mountains get in the way, the air is forced up the slope. Higher up, the air pressure is lower, so the air spreads out and cools. Cool air can't hold as much water, so the vapour turns into droplets, builds clouds and falls as rain on the windward side, the side facing the wind.

By the time the air crosses the top, most of its water has been wrung out, like squeezing a sponge. As it sinks down the other side, it is squashed by the air around it and warms up. This warm, dry wind brings little rain and even dries out the ground, so plants struggle to grow.

The name paints a picture: the mountains act like an umbrella, and the land behind them sits in the rain's 'shadow'. Many of the world's deserts and dry grasslands lie behind tall mountain ranges, cut off from the moisture that comes from the sea.

Where to see it in New Zealand

New Zealand is a textbook example. Westerly winds from the Tasman Sea hit the Southern Alps / Kā Tiritiri o te Moana, so the West Coast is very wet and covered in rainforest. East of the mountains, the Canterbury Plains and Central Otago are much drier. Canterbury's hot, dry nor'wester is air that has come down from the Alps.

Try it together

On a cool day, breathe onto a cold window or the side of a glass of iced water and watch tiny droplets appear. Cooling air turns water vapour into water, just like on a mountain's windward side. You could also compare the weather forecast for Hokitika and Christchurch on the same day.