On a still day, clouds move slowly across the sky. How much of this is due to Earth鈥檚 rotation? Do clouds seem to move faster at the equator?
Peter Bursztyn, Barrie, Ontario, Canada
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Clouds move in response to the local winds. Although the air immediately around you may be still, the winds are far stronger thousands of metres higher up. That is why clouds are usually in motion, even on apparently windless days. But part of a cloud鈥檚 motion is indeed governed by Earth鈥檚 rotation.
Strong solar heating at the equator causes the air above it to expand, forming a zone of low pressure, causing air to move in from the north and south. These 鈥渢rade winds鈥� are then deflected due to Earth鈥檚 rotation, in a process known as the Coriolis effect that explains how objects move on the surface of a rotating body.
The equatorial surface moves faster east than the wind 鈥� the velocity of the land at the equator is an astonishing 1670 kilometres per hour 鈥� so what began as a north-south wind appears to be blowing from the north-east. The same effect occurs in the southern hemisphere, so the southern trade winds blow from the south-east.
Anthony Woodward, Portland, Oregon, US
Earth鈥檚 rotation has some influence on the direction in which the clouds move, but isn鈥檛 the cause of their movement. Fortunately for us, the main bulk of the atmosphere moves in sync with Earth itself. Otherwise, we would be swept off our feet when trying to stand up.
I live in Portland, Oregon, which is close to 45掳 latitude, so I am moving at about 1180 kilometres per hour around Earth鈥檚 axis. I don鈥檛 notice this because the atmosphere is moving at the same speed as I am. Yet portions of the atmosphere can move in relation to the surface of Earth, giving us winds. The major cause of these winds is the differential heating of Earth鈥檚 surface.
Mike Follows, Sutton Coldfield, West Midlands, UK
People used to believe that Earth was the centre of the universe, and reasoned that a wind would blow in the opposite direction to that of a moving Earth. Some who think we live on a flat Earth may share this belief. The clouds, however, allow us to see global air circulations, particularly if we have time-lapse satellite images. They show us that air moving across Earth鈥檚 surface is subject to the Coriolis effect, one of the most persuasive pieces of evidence that we live on a spinning globe and not a flat Earth.
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