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  1. 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.
  2. Dear Friends, October has fallen prey, yet again, to that persistent pattern, with low-pressure stubbornly dominating west and north-west of Scotland whilst high-pressure dominates over central and Eastern Europe and over the Azores: This regime' is occasionally interrupted by the wet, windy blast of some ex-hurricane that skirts the western coasts and brings horribly warm sticky air- totally inappropriate for October - to northern Britain as much as anything else! But it is the relentless prevalence of the High-Pressure over Europe and Low-Pressure South Of Iceland, and the persistent muggy, yucky and damp South or South-westerly Winds this Stubbornly Persistent Weather-Pattern brings that is the greatest cause of concern. These winds bring warm humid air up from the Azores, plenty of dankness and the wind and cloud-cover at night, not to mention the warmth of the airmasses, prevents anything like frost at night. The reasons for this pattern at this time of year are fairly obvious: The North Atlantic is just past it's warmest in October (so strong high-pressure seldom persists over it), whilst seasonal net radiative cooling over Central Europe encourages high-pressure to form there instead. The other main influence on the weather-patterns in mid and higher latitudes in October is the sharp seasonal cooling of the High-Arctic which quickly becomes very cold, though the Arctic cold is seldom extensive enough to help precipitate cold Arctic outbreaks these autumns it is sufficient to cause a big increase in the atmospheric temperature and pressure gradients between the Arctic and still-warm mid-latitude oceans so as to encourage some deep depressions to form. With high-pressure over Europe and the three-wave pattern of the upper Circumpolar Vortex these depressions have a tendency to linger south of Iceland whilst high-pressure dominates Europe. Another elemental reason why we have south-westerlies for much of the year, with the possible exception of late-spring is due to the fact that the Earth rotates. Easterly trade winds in low latitudes constantly impart westerly Atmospheric Angular Momentum to the atmosphere as a result of the winds blowing in a direction opposite to that in which the Earth rotates. This Westerly Atmospheric Angular Momentum does not dissappear as this would transgress the fundamental physical Law of Conservation of Angular Momentum, which stipulates that the total Angular Momentum of a rotating body must remain constant unless acted on by an outside force: Tidal influences from the Moon and Sun are so small as to have no meaningful impact on Westerly Atmospheric Angular Momentum that could really affect our weather. What goes up must, therefore, eventually come down and under current climatic conditions the excess Westerly Atmospheric Angular Momentum in stiff west and south-west winds that dominate in higher latitudes. Problem is, we are getting more of the south-westerly variety rather than north-west or even westerly winds that would be just as (if not more) effective at removing excess Westerly Atmospheric Angular Momentum from the global atmospheric circulation. For us who live in Scotland and the North of England, in particular the causes of the recent autumn weather-patterns are maybe a little academic, but the persistent mildness, damp air and the proliferation of bugs brought by the muggy October weather that results from persistent south-westerlies is starting to grate! It is utterly shocking that we should have to wait until November for the first air-frost! Even at my home in the North Pennines, at over 400 metres above sea-level, these are the extreme minima that I have recorded in October since 2010: 2010:-2C, 2011: 0C, 2012: -2C, 2013: 3C (!!), 2014: 0C, 2015: 0C, 2016: 0C, and the weather -pattern remains most unconducive to frost to the end of October 2017 (extreme low so far just 2C).
  3. Just a thought: IF ONLY the Earth rotated on its axis at half its current speed (i.e there were just 183 48-hour days each year) Britain would have much more extreme weather. For starters the long, long days of Summer would get hot given clear skies but clear nights (with a 16-hour period without the Sun) would get colder. The slow rotation of the Earth would have other implications in that the NE and SE Trade winds in low latitudes would be less affected by the Coriolis Effect and would come in more directly from north and south respectively; the easterly component of Trade Winds would be swamped by the stronger heating/cooling of land-masses on a diurnal basis crating stronger sea-breezes with westerly (as well as easterly) components. Hence the need for Westerlies in higher-latitudes to counterbalance tropical and polar easterlies (to satisfy Conservation of Angular Momentum laws) would be much-reduced and so blocking-highs with cold north or north-east winds would be much more frequent in the winter-half-year in the latitudes covered by Britain: Depressions would often be confined to the North Atlantic and they may still be intense but small in area of extent; it would more often be the case that blocking highs in the Arctic would entrench cold over Europe so that moisture would only come up against very cold air leading to snow over Britain, France and Norway. This set-up would lead to frequent frigid NE winds, clear skies and extremely low-temperatures occurring during the 32-hour long winter nights, occasional snowfalls would lead to a covering of snow reflecting away the weak sunshine during 16-hour winter days so they would still be very cold. The Arctic would be a bit less cold than nowadays in winter owing to the more-persistent loss of cold airmasses (compared to nowadays) but clear skies with 48-hour darkness each day would counterract this to some extent by strengthening radiative heat loss. For Britain, persistent NE winds from the Russian Arctic would still lead to the North Sea freezing over and clear still nights would lead to widespread minima below -40C (-50C in parts of Scotland). Both day and night would be below -20C across the entire country in December and January. Spring would warm slowly with prevailiing icy NE winds maintaining the reflective snow-cover and the frozen North Sea would only slowly thaw and warm up: Typical April conditions would be long nights falling to -25C in Manchester and -30C in the upper Dee Valley in Aberdeenshire with longer sunny days but with icy NE winds reaching 5C in Manchester and -5C in the upper Dee Valley (Aberdeenshire), so the snow would thaw very slowly from the lowlands (freezing at night largely undoing this). The subtropical high would form further north in summer because a strong subtropical jet would then struggle to form south of 50N; this would guarantee more fine and sunny weather in summer with the very long sunny days getting really hot but the still long hours of night getting cold and causing ground-frost in southern Britain. Hot days above 30C would be widespread but close to the North Sea (which would still be cold after being frozen hard in winter) sea breezes would ensure maxima above 15C were a rarity in places like Bridlington and Berwick on Tweed. Minima below 0C would be common in the Peak District, the Yorkshire Dales and in much of Scotland and permafrost would be extensive across the North Pennines and much of inland Scotland. Rain would be unknown in the summer months and savage-drought would be the normal state of affairs over Britain. The change of the radiation balance to negative in September would be associated with less wind and rain but more high-pressure from Arctic Scandinavia: Hard night frosts would return with avengeance with more than 24 hours uninterrupted strong radiation cooling on clear nights with icy NE airstreams; temperatures below -10C would occur at times almost anywhere. Long sunny days of almost 24 hours would be 15 to 20C with light NE winds across the English Midlands, much colder in Scotland and NE England though. October with weaker sun and even longer nights would bring days (as well as nights) below freezing point though occasional depressions pushing into the west would yield occasional snowfalls as milder Atlantic air pushes against cold air over Britain [the North Atlantic would not be so mild; cold airmasses from the Arctic would cross it more frequently whilst warm air from south of 45N will rarely reach it because the subtropical high-pressure belt would be forming much further north- as such the North Atlantic Drift would no longer bring warm surface waters from the Gulf of Mexico]. So October would get very cold quickly with snow-cover established over the whole of Britain (not a lot though because cyclonic activity would be infrequent); nights below -30C would occur on the stillest very-long nights following more direct Arctic blasts in Scotland and parts of northern England by the end (the 15th) of October. And so another long brutal winter would set in across Britain; by November the North Sea would have frozen over entirely........ Now, if only we could find a way of halving the Earth's rate of rotation!!!
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