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Wind

Air and fluids · level 1, lesson 2 of 6

A question to start

You cannot see the wind. So how can you tell when it is windy outside?

The idea

Wind is moving air. When air stays still, there is no wind. When a lot of air moves along, it pushes on everything in its way, and that is wind.

You cannot see the wind itself, but you can see what it does:

  • leaves and grass sway;
  • flags and washing flap;
  • clouds drift across the sky;
  • a windsock at an airport fills up and points the way the wind is going.

The Sun warms some places more than others, and the air moves around because of it. That is where most wind comes from.

no windwindywind
Wind Mars Earth
A usual day 2 to 7 metres each second, measured by the Viking landers in summer from still air to a strong breeze
The strongest 17 to 30 metres each second, in a dust storm up to 100 metres each second

A child running fast covers about 5 metres each second. So a usual Mars wind moves about as fast as you can run.

Worked examples

1. Which way? A windsock points away from where the wind comes from. If it points to the trees, the wind is coming from the other side.

2. How strong? Leaves wobble a little in a light wind. Branches sway in a strong wind. In a storm, big trees bend.

3. No wind. On a still day, smoke from a candle goes straight up and a flag hangs down.

Try it

  1. What is wind?
  2. Name two things you can see moving in the wind.
  3. A flag hangs straight down. Is it windy?
  4. On a usual Mars day, is the wind faster or slower than you can run?
Answers
  1. Moving air.
  2. Any two: leaves, grass, flags, washing, clouds, a windsock.
  3. No, the air is still.
  4. About the same: 2 to 7 metres each second.

Common mistake

"Wind is made by trees waving." It is the other way round. Moving air pushes the trees, and they wave.

Where it leads