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Dear Readers, I would like to explain how a fundamental feature of the Global Circulation, that of the tendency of the atmosphere to rotate with the solid Earth around it's axis of rotation accommodates changing seasons functions. I also discuss how it accommodates some of the extreme conditions deduced by paleo-climatologists to have occurred across a large area of planet Earth during the most severe phase of the last Ice Age. Drought and severe cold seem to have featured heavily in both lower and higher latitudes during the most severe phase of the last Ice Age whilst drought was a feature feature of climate in the tropics and sub-tropics. This can only be explained in terms of persistent weather-patterns that have been observed to bring such conditions during the period of modern civilisation, that of strong high-pressure over higher latitudes keeping rain (or snow) bearing weather systems away, with the high-pressure delivering cold dry north-easterly winds across large areas of planet Earth. In the tropics too, drought is today associated with a strong sub-tropical high-pressure belt that brings dry, quite strong north-east winds (or south-east winds in the Southern Hemisphere) on the equatorward side. This is associated with the zone of hot, steamy rising air associated with Intertropical Convergence Zone (ITCZ) being restricted to being very close to the Equator with monsoons failing to penetrate tropical continents to bring seasonal rains. This is also a feature of Ice Ages where West Africa and southern Asia fail to get monsoon rains because they stay cooler (or at least fail to heat up more) than the hot equatorial seas to the south. The modern understanding of how the atmosphere gains and loses Westerly Atmospheric Angular Momentum (AAM) by frictional interaction with the underlying surface is not consistent with a global climate whereby cool dry north-easterly winds can prevail over more than about 65% of the Earth's surface because of the Law of Conservation of Angular Momentum. It is then argued that because the atmosphere continues rotating with the Earth and because outside forces (from lunar tides, meteorites, etc.) are miniscule compared to the mass of the Earth's atmosphere and the momentum exchanges with the underlying surface, that what Westerly AAM is gained by the atmosphere in the tropics, subtropics and polar regions (due to the Polar Easterlies) has to be returned to the surface in middle latitudes. The atmosphere in low and very high latitudes does gain Westerly AAM through the frictional impact of surface easterlies on the seas and lands over which they travel and, indeed, Westerlies in higher latitudes, again through their frictional interaction with the underlying surface over which they blow, make those middle latitude locations the sink of Westerly AAM. However, strong Westerlies in higher latitudes in autumn and winter are consistent with deep depressions which draw not only on sharp upper atmospheric temperature and pressure gradients aloft (i.e. through a strong Circumpolar Vortex) but also a source of warmth and latent heat to fuel these depressions effectively. And paleo-climatologists have deduced that winters at the height of the last Ice Age were dry and bitterly cold in the North whilst there were also strong dry North East Trade Winds bringing drought across Africa, southern Asia and central America with greater volumes of dust transported across the Atlantic from the Sahara to Brazil (Nigel Calder's 1974 publication on "The Weather Machine and the Threat of Ice" covers all these details). And the Law of Conservation of Angular Momentum does not preclude the possibility that much stronger Westerlies could dominate in the Stratosphere and upper atmosphere whilst Easterlies predominate in the lower atmosphere and the Earth's rotation slows down enough to add a few milliseconds to the Length of Day: What matters is that the entire Earth Atmosphere system conserves axial Angular Momentum in the absence of outside forces. Under such conditions, with very strong Westerly winds aloft high mountain areas alone (like the Himalayas) would become the sink for Westerly AAM as very strong Westerly winds blasted them at times. In the absence of mountains, Westerly AAM would be brought down to the surface in areas of strong atmospheric subsidence when and where day-time solar heating of the surface and lowest layers of the atmosphere is sufficent to bring about stronger surface boundary layer convection- i.e. subtropical land areas in the Spring and Summer (beneath the descending air of a strong subtropical high-pressure belt). Such westerly (or more likely with the air moving equatorward with higher surface pressure to the north, north-westerly) winds would be very strong and return to the Earth Westerly AAM imputed to the atmosphere by north-easterlies over the rest of the Northern Hemisphere over the preceding year. Such a situation would be more likely to develop in a severe Ice Age winter when extremely cold, dense air covers all middle and high latitude areas, the troposphere would correspondingly become a bit thinner north of the sub-tropics then the upper-air thermal and pressure- gradients become further strengthened around 25 to 30N: That would further increase the speed of the subtropical jet-stream and the westerly speed of the subsiding air beneath it. Either way, in a severe Ice age Winter we have a situation whereby Westerly AAM is transferred from the surface into the atmospheric circulation (due to extensive Easterlies) , then is transferred upwards rather than polewards. That is not inconsistent with the Law of Conservation of Angular Momentum, but the sinks for Westerly AAM will also (likely) be in the tropics or sub-tropics (due to very strong Westerly jet-streams blasting subtropical mountains or (with daytime heating below) brought down to the surface in violent dusty squalls beneath the strongly- descending subtropical high-pressure belt (at about 25N). The Ferrel Cell in middle- latitudes, with surface south-westerlies and winds returning from higher latitudes would almost certainly cease to exist in a severe Ice Age winter (with all seas and lands frozen north of 30N)- to be replaced by a single Direct Cell extending from the Polar Regions to south of the Equator. Today, it is the returning flow of air from high-latitudes aloft- becoming relatively less "Westerly" as it moves into lower latitudes that today weakens the Westerlies moving north aloft from the Hadley Cell (by colliding into them), so that the air descending in subtropical highs does not have a strong Westerly component by the time it reaches the surface. If the mid-latitude and high-latitude oceans and lands are all frozen there is no Ferrel Cell, the air descending from beneath the subtropical highs moves strongly from the West as it descends- and it becomes subtropical Mountain Ranges like the Himalayas, Karakoram mountains and the High Atlas that the Westerlies hit (and are slowed down rapidly by). Unless the upper- air is much colder still (so as to encourage convection and cyclogenesis), frozen lands and seas with all the low- atmosphere being well below freezing-point do not lead to depression formation (or, with it, strong Westerlies on their southern flanks). continued below.