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Global winds

Air and fluids · level 8, lesson 5 of 6

A question to start

Earth's trade winds blow steadily towards the equator, year after year. Mars has a wind Earth does not: every year, a large part of its air flows towards one pole and freezes there. What drives the winds of a whole planet?

The idea

The Hadley circulation. Sunlight heats the planet most where the Sun is highest. There the air warms and rises. High up, it spreads towards the poles, cools and sinks again further out. Near the ground, it flows back towards the warm belt. The Coriolis effect turns this returning flow sideways: on Earth, that makes the trade winds. Mars has the same kind of circulation, driven by its own sunlight and seasons.

Mars's carbon dioxide flow. In each polar winter on Mars, carbon dioxide freezes out of the air onto the cap: 3 to 4 trillion tonnes, 12 to 16% of all the air. Air from the rest of the planet flows towards the pole to replace it. In spring the ice turns back to gas, and the flow reverses. Earth has nothing like it: its main gases never freeze.

warm belthigh up: towards the poleMars: air to the capwinter pole

Worked examples

1. How much air flows to the cap? Suppose 3.5 trillion tonnes, \(3.5\times10^{15}\) kg, freeze out over about half a Mars year: \(\tfrac12 \times 687 \times 86{,}400 = 2.97\times10^7\) s. Average flow: \(3.5\times10^{15}/2.97\times10^7 = 1.2\times10^8\) kg/s.

2. Across the 60° circle. Its length is \(2\pi R \cos 60° = 2\pi \times 3.39\times10^6 \times 0.5 = 1.07\times10^7\) m. So \(1.2\times10^8/1.07\times10^7 = 11\) kg of air crosses each metre of it each second.

3. As a wind. Each square metre of Mars carries 171 kg of air above it. Moving 11 kg/s across each metre needs the whole column to drift at only \(11/171 = 0.065\) m/s on average. Small, but it is planet-wide and lasts for months.

Try it

  1. Where does air rise in a Hadley circulation, and why?
  2. What turns the returning flow into the trade winds?
  3. Why does Mars have a seasonal flow of air towards a pole, but Earth does not?
  4. If 4 trillion tonnes froze out over the same time, what would the average flow in kg/s be?
Answers
  1. Where the Sun heats the ground most; the warm air is less dense and rises.
  2. The Coriolis effect.
  3. Mars's main gas, carbon dioxide, freezes at its winter poles; Earth's nitrogen and oxygen never do.
  4. \(4\times10^{15}/2.97\times10^7 = 1.3\times10^8\) kg/s.

Common mistake

"Wind blows straight from the equator to the poles." Rising, sinking and the Coriolis effect break the flow into circulations with winds that blow mostly east or west.

Where it leads