The daily rotation of the Earth is something very much taken for granted. It gives us day and night very obviously. However it also does much more. It is one of the main ‘drivers’ of our weather systems.
The Sun heats up the Earth’s atmosphere, an envelope of gases 500 miles deep. These gases are densest at ground level and in fact 90% of the atmosphere, by weight, is in the lowest 9 miles above the surface. As the Earth is a rough sphere, the atmosphere at the equator receives the Sun’s rays at a more direct angle that it does at the poles. The heating at the equator is therefore much greater than at the poles. Hot air rises from the equator and flows towards the cold poles whilst cold, polar, air flows south to replace it. If the Earth did not rotate, this would take the form of a fairly straight south to north and north to south circulation, interrupted only by mountain ranges; but this is not the case.
Air does not flow directly south to north or vice versa because of the Earth’s rotation. This is much faster at the equator than at the poles. In the northern hemisphere, air moving north from the equator is moving faster eastwards than the underlying surface of the planet and is pushed to the right as it proceeds. Polar air coming in the opposite direction is moving more slowly than the underlying surface and is also therefore pushed to the right. (In the southern hemisphere the air flow is curved to the left). This is known as the ‘Coreolis Effect’ (named after French physicist Gustav de Coriolis 1792-1843, who discovered it in 1835) and becomes more significant the further north or south the wind is from the equator, as the following equation reveals: ( the value of the sine of latitude at the equator will be zero.)
Coreolis Effect…2xOmega V sine Phi….(Where Omega is rotation of Earth, V is wind speed
Phi is angle of latitude.)
The Coriolis effect also causes air to rotate clockwise around depressions (lows) and
anticlockwise around anticyclones (highs).
The differential heating of the atmosphere and the rotation of the Earth produce the Trade Winds north and south of the equator. First described in 1753 by the English scientist George Hadley (1686-1768) Trade Winds are caused as hot humid tropical air rises, causing low pressure and flows towards the Poles, sinking down at about 30 degrees latitude to create stable areas of high pressure (anticyclones). Some of this air moves back towards the equator’s low pressure systems, blowing NE to SW in the northern hemisphere and SE to NW in the southern hemisphere.
Hadley had described the operation of one of the three major circulations of air in each hemisphere, the Hadley Cell. One of the two others is the Ferrel Cell, between about 30 degrees and 60 degrees latitude, first explained in 1856 by the American William Ferrel (1817-1891). The other is the Polar Cell above 60 degrees. On the boundaries between these three, hemispheric air circulations, Jet Streams occur. There are two Jet Streams in each hemisphere, super-fast streams of air running westwards at the boundary points between Hadley and Ferrel Cells and Ferrel and Polar Cells, where temperature differentials are high.
The sub-tropical jet is located about 7 miles up in the atmosphere and reaches 300 miles an hour (500kph). The polar jet is about 5 miles up. The direction of their winds is caused by the Earth’s rotation. It is now recognized that these Jet Streams mark fronts or boundaries where weather systems evolve, especially the depressions or low pressure systems which are such a feature at mid latitude. Although it was once thought that weather systems grew from the surface of the planet and rose into the atmosphere, it is becoming clearer now that the reverse is in fact often the case, with eddies and disturbances in the Jet Streams playing a major role.
The Earth’s spin has a major impact on the planet’s weather. Much more of the detail of this has been discovered in the last 50 years as Humankind has been able to apply increasingly
sophisticated technology to the study of meteorology and earth sciences but it seems almost certain that there is still a great deal yet to be found out.
