Hi Douglas, I realise your question about the wind was a "for discussion" type question, but it seems to imply a fallacy. The atmosphere and the earth are part of the same system and rotate together, so if everything was static there would be no friction between the atmosphere and the surface of the Earth.
However the atmosphere is a fluid and there is unequal heating between the equator and the poles. The unequal heating causes air density and pressure differences, and the atmosphere develops a circulation. Because the Earth is a rotating sphere the motion of these circulations are deflected to preserve angular momentum. The resulting pattern of atmospheric circulation becomes quite complicated. In the simple case of a non rotating Earth air would rise at the equator and descend at the poles, and at the surface of the Earth there would be a pole to equator ward return flow, that is a northerly wind in the northern hemisphere and a southerly in the southern. This would make a single "cell" of circulation equator to pole in both hemispheres. However as the Earth is rotating, rather than one circulation cell, we have three. The full hemisphere cell mentioned above reaches only to about 30° latitude and is called the Hadley cell. The rotation of the Earth affects the cell by deflecting the surface return flow to the west in both hemispheres. These are the NE trades of the northern hemisphere and the SE trades of the southern hemisphere. The other two circulation cells are the Ferrel and the Polar cell. The Ferrel Cell runs in the opposite sense to the direct thermally driven Hadley cell, so the surface winds run the opposite way as well, and are deflected in the opposite way. So in the northern hemisphere in the mid-latitudes the average/predominant wind would be a SW wind. However the Ferrel circulation is much more complicated than this simple picture suggests, and there are flows both north and south, though both are generally from the west. Cheers Hank ________________________________ From: sundial [mailto:sundial-boun...@uni-koeln.de] On Behalf Of Douglas Vogt Sent: Thursday, 11 April 2013 10:57 To: sundial@uni-koeln.de Subject: Re: Is East/West always at exact 'right-angles', to North/South? Reminds me of someone a year or so ago who developed a method of determining the east of Mecca for prayer purposes regardless of how far north or south the location of the person is. In this case, east varies quite a bit. Another slightly off topic question your students may want to consider: if the earth rotates west to east, one would think there is some friction between the earth and the air causing the air to turn at a slower rate. This suggests winds should be out of the east. However, prevailing winds are often out of the west. Does this make sense? ________________________________ From: Beverly Stimpson <beverly.stimp...@gmail.com> To: sundial@uni-koeln.de Sent: Wednesday, April 10, 2013 5:41 PM Subject: Is East/West always at exact 'right-angles', to North/South? Dear List Members (and experts!), I am a new member to this Mailing List - a school-teacher who can see the several curriculum benefits, of using sundials in (mainly Primary) schools. One of my pupils recently asked me a question, which seemed simple - but after thinking about it, now I am not so sure it has an obvious answer! Basically, is geographic East/West always at exact 'right-angles' to the direction of North/South - or is this affected by Latitude of location? On magnetic compasses, plus most maps (depending on the projection used), East/West is certainly at 90 degrees to North/South - but at (say) the North and South Poles there is no East or West direction at all, or are those locations just 'special cases' (being the exception to the rule)? That is what made me wonder, if East/West might depend on the Latitude. To keep things simple, I am only thinking about horizontal surfaces. I shall look forward to your replies, explaining whether East/West is always at 'right-angles' to North/South (or not, as the case may be). Thanks, Bev Stimpson. -- --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
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