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Dear fellow Weather Observers In middle latidude islands and some continental areas in various parts of the World hundreds of miles of warm seas with average temperatures well above 0C in the coldest months separate the region from any source regions from which extremely cold airmasses (with a mean temperature well below 0C at sea-level) may originate. Such areas of the World include the UK, but also Ireland, various Mediterranean islands, southern Japan, Bermuda, New Zealand and Tasmania. Also included are southern Argentina/ Chile as well as southern Australia and South Africa because the continents to which they are attached are only large in latitudes too low to support serious continental cooling and so Antarctica is the only real source region for frigid air- meaning advection across the Southern Ocean is required for this air to reach these continental localtions- so for the purposes of the dynamics of very cold airstreams the southern tips of the Southern Hemisphere continents are included. Quite apart from the modifying influence of much warmer sea-surfaces in warming very cold airmasses passing over them: An airmass of 3 km depth in the lower atmosphere is warmed by roughly 10C in 24 hours if the temperature differential between it and the ocean surface is 20C to begin with because a temperatre differential of 15C leads to a 300 Watts per square metre net heating of the air by the warmer ocean (and the warming will be more than that to begin with), thouh in winter this is offset (to a rather smaller extent) by net radiative heating of the lower atmosphere. In addition, there will be a significant amount of heat (latent heat of condensation) as the water vapour pressure from the warm sea surface greatly exceeds that from the cold atmosphere above- with the result that moisture enters the atmosphere and vigorous convection soon results in cumulonimbus and showers of rain or snow. Thus, you end up with a situation whereby an airmass that starts off at -10C and spends 24 hours over a sea-surface of +10C is likely to be about 2C in the lowest layers after 24 hours. This is why a frigid Siberian airmass, crossing Scandinavia at a temperature of -20C in the lowest layers in winter, is hardly ever likely to be below -5C at sea-level by the time it reaches the coast of eastern Scotland and North East England after its passage across the northern North Sea, the surface of which in January is typically about 8C. But this obvious warming is not the only way warm, ice-free high-latitude seas protect middle-latitude regions from the fiercest onslaughts of Arctic, Siberian or Antarcic cold: It is very clear that the vapour-pressure of warm sea-surfaces compared to a very cold airmass over-running it pumps large amounts of moisture into the air. A strong dry and icy wind at -10C over a sea-surface of 10C can extract over 1cm of water-equivalent, which is more than sufficient to result in deep cloud-cover in an airmass at (say) -5C in the lowest layers and -30C at 3,000 metres (the air would warm more near the surface though less quickly aloft and that would encourage strong convection currents in the still-frigid air). Much of the moisture would freeze out in cloudy convection resulting in sharp snow-showers, which are a feature of very cold airsreams over warm seas, but not all of it. With the moisture chiefly freezing out at higher levels the higher-level air (at 2 to 3 km) warms quite rapidly so after 24 hours this reduces the atmospheric temperature gradient to the extent that convection then proceeds at a lower rate. However, cloud-cover at 2-3 km would act as a stronger surface from which radiative heat-loss maintains the middle-level air at a low-enough temperature to ensure convection continues. Continued below

Hello, thought I'd introduce a subject close to my heart because where the prevailing wind comes from in Britain (and in what seasons) has a pretty massive impact on our prevailing weather and climate. Like a number of us on this Forum I prefer severe cold and snow in winter (preferably something below -10C), cool fresh springs with snow lingering on the northern hills and a few frosty nights, warm dry summers with enough rain to ensure things grow (not too hot and humid) and crisp frosty mornings and bright days in the autumn. Unfortunately, the prevailing winds in northern Britain - fresh south-westerlies from autumn through spring and westerlies in high summer tend to preclude such sharp contrasts between the seasons. The reason why we have persistent airstreams off the North Atlantic with depressions passing to the north-west to bring wet windy weather for much of the year (particularly the case for those of us, like myself, living in northern England or Scotland) is because of the mid-latitude Westerlies- so called (they tend to angle in from the south-west much more than north-west in Britain). This is part of a major system of the global winds called the Ferrel Cell (or Ferrel Westerlies, and yes sometimes these winds are feral as well!) because the wind-circulation was founded by William Ferrel in the mid nineteenth century. These Westerlies blow extensively over middle to high latitude regions for most of the year and they are part of the vast global circulation that distributes heat from low latitudes (where more heat is absorbed from the Sun than radiated to space over the course of a year) to higher latitudes where there is a net radiative heat loss during the course of a year. Hence arguably, weakening this Ferrel Cell and the surface Westerlies that are an integral part of it would reduce this heat transport so that high latitudes would cool (particularly in the winter half-year). Leaving aside the technological feasibility of achieving such a feat, would there be other outcomes for the prevailing weather, particularly for the UK if the Ferrel Cell could be weakened or destroyed. The strong jet-stream that crosses the North Atlantic and which assists the development of depressions that drive the Westerlies would, by implication, have to be weaker and interrupted by long periods with high-pressure over and to the north of Britain which would result in north-easterly winds: This would mean less rainfall and sunny warm (but not hot) days in summer would be frequent. Clear skies combined with frequent incursions of air from high latitudes would result in frequent spells with very severe frost any time from October through the winter until April. The weakening of the North Atlantic Drift that a wholesale weakening of the Westerlies across the North Atlantic would entail combined with more frequent frigid airstreams from Russia over the Norwegian Sea would encourage pack-ice to extend south to the north coasts of Britain by February and this would persist until April. This extra ice-cover combined with clear skies would result in a strong surface net radiative heat loss that would make the region much colder than at present. There would be warming effects at the highest latitudes caused by weakening or destroying the Ferrel Cell: The middle atmosphere over the Central Arctic would be 10 to 15C warmer in winter because of the frequent outflow of very cold air-masses from the surface and its replacement with warmer air from lower latitudes. This relatively warmer air would come via high elevations and subside through the atmosphere courtesy of persistent large surface anticyclones but the clear skies and dry atmosphere caused by such subsidence would permit intense surface radiation cooling over the Arctic pack-ice so winter surface temperatures would be little warmer there. In summer this is another matter because such displacement of cold surface air over the Arctic due to anticyclonic subsidence would bring unbroken sunshine from clear skies which, combined with relatively warm-air advection from above would raise average temperatures. Weak/absent westerlies in higher mid-latitudes implies few snow-bearing depressions for the Greenland ice-cap and for Canada and Russia. Little or no snowfall means that sunny summer conditions over the Greenland ice-cap could hasten its net melting although an absence of warm south-westerly winds over it might also lessen the rate of demise of the Greenland ice-cap and (in any case) clear skies at night help whatever thaws on the surface of the ice freezes at night. Over Russia and Canada little snow-cover means that when the Sun gets strong in spring the bare land is soon exposed absorbing much more heat from the Sun, however persistent north and easterly winds would bring cold air from the Arctic to delay thawing. The clear skies, light winds and persistence of dry frigid air-masses from high latitudes would still bring about extremely severe winters across Russia and Canada and without significant snow-cover the frost would penetrate deep helping to preserve the permafrost. In order to consider how the Ferrel Cell might weaken we need to understand what causes it in the first place. As to how the Ferrel Cell and mid-latitude Westerlies are maintained we need to realise that it is a by-product of the fact that our planet rotates once a day on its axis as well as being caused (indirectly) by the imbalance of Solar heating between high and low latitudes and the (consequent) need for air to move when it acquires different densities. In the tropics and sub-tropics winds converge towards the zone of maximum heating and rising air from the north-east (a consequence of air moving south over land that rotates progressively faster the further south it gets- south of the Equator the corresponding winds are south-easterly). Thus in the tropics and subtropics the atmosphere is gaining westerly angular momentum as surface easterlies are slowed by friction with the underlying surface and this excess is transferred polewards, significantly in the vicinity of the sub-tropical jet-stream, at 12 km elevation at around 30N (and 30S). There has to be a sink for this accumulated atmospheric westerly momentum so as to satisfy Conservation of Angular Momentum laws (pp 140-142, "Atmosphere, Weather & Climate" (8th Edition), Barry & Chorley (2003)). The sink for the excess westerly atmospheric angular momentum is all those strong westerly winds that blow across the North Atlantic and North Pacific and (all too often) afflict Britain and much of NW Europe. The surface waters of the North Atlantic (and North Pacific) do not have a very high coefficient of friction (unless of course whipped into a stormy chaotic mass of waves by high winds!) and thus the Westerlies often have to pass over extensive land areas with higher frictional coefficients before the net westerly AAM accumulated in lower (and high) latitudes is satiated. This is particularly true from autumn through spring when subtropical and polar easterlies are stronger and there is thus a corresponding greater "need" for Westerlies in higher mid-latitudes. All of this means that the key to getting Britain proper seasons involves finding the global circulation another (extra) sink for net westerly atmospheric angular momentum accumulated by virtue of the NE Trade Winds and Polar Easterlies. Thousands of stiff west-facing large steel "brushes" placed between 45 and 65N in the North Atlantic might do the trick as might building a 12 km high wall (with artificial trees pointing west to absorb the force of the wind) north to south over the southern USA and Mexico to intercept and slow the subtropical jet-stream. Another idea might be pumping trillions of tons of cold deep water to the surface over the subtropical North Atlantic in order to weaken the baroclinic gradient that drives the Westerlies but this would also strengthen the NE Trade Winds- the Westerlies needed to counter the NE Trade Winds might then be deflected to the Med to ruin many folks' holidays! I am not seriously suggesting that we should try these suggestions (some folk might consider it insane!) but it's a fact that if we are ever to weaken the Westerlies (so we can have "decent" seasons in Britain and prevent the Arctic ice melting!) you have to tackle the root causes of their existence.