iapennell

@ArHu3 If you can make an possibility of anything remotely like a Sudden Stratospheric Warming over the Arctic from that chart you are an absolute genius! That looks to me like a very strong polewards temperature gradient between the Arctic and mid-latitude stratospheres, a temperature of close to -90C at 10 mb over the central Arctic would be colder than normal even for late December and the stratosphere temperatures (at 10 mb) of -40C in the latitudes of Spain and California are, if anything, a little warmer than usual. This temperature pattern in the Stratosphere would support very strong circumpolar Westerlies at this height on the fringes of the Arctic- as indeed is confirmed by maps of the wind-profile on this very thread; this tends to lock-in the very cold Arctic stratosphere with the strong circumpolar Westerlies inhibiting the penetration of warmer air from lower latitudes. That is the last thing you need if you want SSW's to occur: It is my contention we are unlikely to see anything like that for a month or so (and there is a real likelihood now that they won't happen at all this winter). Another wet and milder than usual winter overall is on the cards!

Have to say its pretty naff having to report on boring weather- I have had almost three weeks without an air frost (and that is at over 400 metres above sea level in the North Pennines, four miles from the little market town of Alston)! At such times, we could do reports on our Utopian Weather for each day, which I am sure would make much more interesting reading (and cheer us up in time for Christmas). So here's mine: Woke up at 8 am to a glorious pink dawn with the sparkling deep powdery snow covering the land to a depth of one metre. The frost was thick on the inside of the bedroom window. I looked at the weather readings transmitted from my AWS on the wall, the bitter north-east wind that had brought the heavy powdery snow yesterday has died away overnight and clearly the skies had cleared overnight as the temperature was -25C and the barometer had risen to 1040 mb- this is looking to be a truly fantastic Christmas! I wiped away some of the frost from the window and a glistening silvery pink ice-fog was drifting over the snow-bound meadows. Filled with excitement I put my fur coat on (the one that I bought when I went on holiday to Alaska in November 2008), get some gloves and thick fur boots on, and grabbing my camera headed outside. The frigid air bit into my lungs and thick clouds of steam emanated from me as I breathed out- the scene was fantastic. The deep snow was sparkly and pink under a deep indigo sky in which a crescent waning moon shone low in the south. The trees were filigrees of pink, glowing in the fiery red light of the dawn from low in the south-east (above which Venus, the morning star, still shone brightly). The thick ice-fog filled the valley, a glittery silver-pink miasma against the morning light, which merged with the pink glistening snow. My nose went numb with the cold and I realised there was a little icicle forming at the end of it so I decided to head back home and indoors. Outside the snow-shrouded wood I saw a little fox sitting down in the snow, he leant back and let out a wail in hunger. I felt a little sorry for the fox and hope he finds something to eat soon, it can't be easy catching rabbits when they are all burrowed deep in the frozen earth! The Sun came up, a pink glowing ogan on the south-east horizon, the pink snow glittered along with the ice-fog stalking the meadows and the air was full of sparkly ice-crystals that looked like a billion minute diamonds. I took some pictures, though the camera was a little stiff with cold despite my keeping it my coat pocket. I got home at 9 am and the thermometer registered -26C- it felt so great to be alive this morning! Then my mother told me that large amounts of hot water were needed for the chickens as all outdoor water supplies had frozen up and Malcolm (who has his sheep on our land which he rents off my parents) needs assistance in locating some of those buried beneath drifts after yesterday's heavy snows. Sure this cold brings some hard work, but the sparking snow and ice is magical.

There is a high moving in according to longer-term outlooks, it looks set to move over the South before turning east into the continent and pulling up mild air from the Med. Yikes!

So we have two fundamental controls on our weather at this time of year, firstly increased CO2 levels have led to an increase in the outgoing radiative heat being retained by the Earth (i.e. absorbed in the atmosphere and re-radiated back down towards the surface); hence for quite a few decades now the picture has been an excess of incoming radiation from the Sun over outgoing radiative heat loss to space as averaged over all latitudes during the course of a year. This extra warmth is stored in the oceans and transported by the global winds to high latitudes in winter- leading as it does to anomalously high temperatures, particularly in regions where the sea nearby fails to freeze over. Consequently, airstreams reaching Britain during the winter months have been warmer than the corresponding airstreams in the past. The Arctic, for example, has lost some of its potency as a source region of extremely frigid air-masses (in early winter especially). The fact that the Earth rotates and the need for the Earth-Atmosphere system to conserve Angular Momentum results in surface Westerlies in the winter months that need to counterbalance easterlies elsewhere in the Northern Hemisphere: Physical forces from outside the Earth are insignificant in these recent times of global warming and with warm higher-latitude oceans (with their strong baroclinic zones on their northern borders) enticing excess Westerly angular momentum in the atmosphere towards the surface (rather than pushing it up into the mesosphere) in the winter months it means north-west Europe will inevitably be in the firing line for strong mild west or south-west winds coming off the North Atlantic at this time of year. And with the effects of global warming meaning the North Atlantic is overall warmer than usual for December this means particularly mild (and wet) weather reigns supreme: Temperatures above 10C have occurred far more frequently over lowland Britain in late December than temperatures below 0C over the last 30 years! So, is there anything that can be done to stop and hopefully reverse this trend? The obvious solution is for the entire Global Community to work towards the decarbonisation of their economies and the reduction of carbon dioxide levels in the atmosphere in an attempt to stop global warming getting out of hand but (understandably) most countries are reluctant to impose measures that would stifle their economies and threaten the stability of their power-supplies. Planting trees on a very large scale (preferably ones that have light-coloured leaves) will go some way towards mopping up excessive amounts of CO2 in the atmosphere but is not going to reverse the trend towards higher CO2 levels alone. I have discussed geoengineering on other threads on this Site in the past and aside from planting light-coloured trees on a grand scale the only other cost-effective solution that has a real chance of success (and which I have gleaned from extensive discussion on this Forum, though Environmentalists will vehemently object!) is the regulated injection of sulphur dioxide into the Stratosphere where it can reduce the amount of insolation reaching the surface of the Earth. None of these measures are a permanent fix however and in the longer term using technology to switch to a zero-carbon economy (without damaging the economy) and helping other countries do the same is the best chance we have of safeguarding our futures and enabling our grandchildren (and their children) to experience the joy of snow at Christmas. Wind, rain and mild weather at Christmas have become more frequent in recent years (overall); as the climate has warmed and the edges of Arctic ice at this time of year receded polewards the Westerlies have pushed northwards and have become stronger: The more northward-displaced Westerlies have to blow stronger and harder to counterbalance easterlies elsewhere (in order that there is a balance with Westerly angular momentum being removed from the global atmospheric circulation at the same rate it is added) because the Westerlies blow even closer to the axis of the Earth's rotation- the effect is analogous to that of a see-saw- a light person far from the centre can hold up a heavy person the other side of (but close to) the centre of the see-saw. Helping reduce CO2 levels will encourage the polar ice to creep a bit further south once more which would push the Westerlies into somewhat lower mid-latitudes in winter- these Westerlies, then blowing further from the axis of the Earth's rotation will not then need to blow as hard to counterbalance easterlies elsewhere across the Northern Hemisphere. Britain then might get more respite from Christmas wet and wind, along with more chance of that white stuff that makes Christmas much more joyful! There is a serious side to tackling global warming because not only is persistent wind and rain at this time of year a nuisance for many, but greater storm frequency means more flooding, more coastal erosion, more sinkholes opening up under unsuspecting communities (these seem to be happening more frequently in recent years) and this is a major threat to coastal communities even without the consideration of what happens when the Greenland Icecap goes into terminal decline because of modest global warming. Another solution to mild, wet and stormy winters is using the best technology available to tackle the Westerlies head on by addressing an underlying cause; the need for the atmosphere to shed excess Westerly momentum accumulated elsewhere. The most practical solution is the planting of trees in locations prone to frequent Westerly gales in winter (the trees need to be quite sturdy)- obvious locations include western Scotland, western Ireland and coastal areas of British Columbia (including Vancouver) in Canada. The mountain chain that separates Spain and Portugal also gets strong westerly winds at times in the winter, particularly towards the northern end of these mountains so the planting of trees here (ones able to cope with hot dry summers) would also help to relieve the atmosphere of excess Westerly momentum. These trees would serve a dual purpose- that of removing carbon dioxide from the air (ideally heir leaves should be light in colour so they don't absorb too much heat from the Sun and so add to the global warming problem rather than being a solution to it), whilst acting as a major sink to accumulated Westerly Momentum in the atmosphere by impeding the force of strong Westerly winds. This in turn would lessen the "need" for strong/persistent westerlies elsewhere in higher latitudes at this time of year. If all this could be done in the next 15 to 20 years then future generations might be spared a long run of dreary mild and wet winters and (crucially) spared the likes of what happened in December last year when much of northern Britain was under water at times!

Late December not stormy? I live in the North Pennines and can recall west or south-westerly gales in late-December in 1983 (very bad ones), 1986, 1991, 1997, 1998 (severe gales), 2011 and (particularly) 2013. Late December last year was very wet as well as stormy.

@Ed Stone PS. The name of the book I was referring to is called "Countdown: How Nigh Is The End" written by the famous Patrick Moore (who used to present "The Sky at Night" on BBC2, I recall; sadly he died a year or two back). He makes reference to the Moon slowing down the Earth's rotation ever so slightly but that in 50 billion years our days would be 46 days' long and that this would be the time it takes for the Moon to orbit the Earth by then- the Earth loses angular momentum through interaction with the Moon but that this is transferred to the Moon whose orbital angular momentum about the Earth increases by the same amount. By AD 50,000,000,000 the Moon's orbit will have receded to about 330,000 from the Earth; Patrick Moore in his chapter on The Menacing Moon then explains that the Moon will not be slowing the earth's rotation down by this point because the duration of the Moon's orbit and the length of day would be the same length, but Patrick Moore then explains the Moon is "not the only factor slowing down the Earth's rotation" and that tidal interaction with the Sun would continue to do this, leading to the Earth taking longer to rotate on it's axis than the Moon takes to orbit the Earth. Once that happens, the Moon will try to speed the Earth up and in the process loses absolute orbital momentum and this means the Moon draws in again towards the Earth. As the Moon draws in it orbits faster and tries to drag the slowly-rotating Earth around (with little effect) and eventually after billions more years the Moon would get to within the Roche limit-leading to huge forces between Earth and Moon with the result being that the Moon ends up breaking into little fragments rather than colliding directly with the Earth (which would destroy both the Earth and the Moon). However, Patrick Moore then points out the one obvious flaw in this theory that there will not be an AD 50 billion because the Earth will be destroyed by the Sun turning into a Red Giant Star long before then!! The central Law of Conservation of Angular Momentum states that the total angular momentum of any rotating body remains constant without any outside force acting upon it. This means the Earth and atmosphere have to maintain the same total Angular Momentum in the absence of outside influences. However there are outside forces that act chiefly to slow the Earth's rotation down, the tidal effects of the Sun and Moon being two of them. Other forces operate such as the diffusion at the top of the atmosphere to space (mostly of helium and hydrogen), if the diffusing gases at the top of the atmosphere have stronger westerly momentum then that constitutes a loss of total Angular Momentum of the Earth and its atmosphere (though if one also factors in the gases that have escaped to space the total Angular Momentum of the Earth, atmosphere and the diffused gas- with reference to the axis of Earth's rotation will not have changed- the Law of Conservation of Angular Momentum holds). Other forces acting from outside the Earth-Atmosphere system include the impact of Solar Storms which- through the interaction with Earth's Magnetic Field- tend to have a small effect in adding to the total Angular Momentum of the Earth Atmosphere system; meteorites large and small slam into the upper atmosphere at great speed and these will either impart to (or subtract from) the total Angular Momentum of the Earth Atmosphere system. Neither is deep space totally free of trace gases and dust particles and these will have a small effect on our rotating Earth in slowing the Earth's rotation down (and also reducing by a small amount the orbital Angular Momentum of our planet as well, though this is likely to be more than countered by the fact the Sun rotates on its axis every 28 days whilst the Earth takes a year to orbit the Sun- the Sun must be imparting a very small amount of this momentum to Earth's orbit pushing our orbit outwards). It is believed by some astronomers that our Solar System- in its 330 million-year orbit around the centre of our galaxy travels through dust-lanes in the spiral arms of The Milky Way. These astronomers believe this is a possible cause of Ice Ages because the dust falls on the Sun, causing it to run riot whilst Earth is shielded from it and then go "off the boil" as it were (with subdued convection); this could cause the Sun's total radiation to drop 2 to 3%- leading to an Ice Age (this, by the way, was a theory around in the 1970's and how much verified this is by more recent findings it would be interesting to discover!). However, if the Earth did encounter excessive quantities of dust in its orbit coming from outside our Solar System the impact on the total Angular Momentum of the Earth Atmosphere system would be substantial and there would be a considerable force slowing the Earth down. Even if it lasted just a year, the impact on our weather-patterns would be very dramatic indeed.

@Ed Stone First of all, all the best for Christmas for you and your family. You certainly have a point about the tidal impacts of the Sun and Moon but this is very slight. I recently read a book written by the late Patrick Moore on the subject of different ways in which our planet could meet its end, one of these being the likelihood of the Moon colliding with the Earth. In the chapter on The Menacing Moon he discussed that lunar and solar tidal influences would take so long to slow the Earth's rotation that it would take 50 billion years for the length of day to increase to a length of 47 of our current days. This means a rate of increase in day- length of just a few millionths of a second annually. In other words that is neither here nor there. On an annual basis seasonal changes in global winds, cooling at high latitudes leading to strong Westerlies aloft after increases in easterlies over low latitude continents, leads to increases in the length of day by 1 to 2 milliseconds but this reverses in the spring as the low latitude easterlies weaken and excess Westerly Momentum is returned to the surface in higher latitudes. Significant changes in the Length of Day occur in association with periods of sharp global cooling when tropical easterlies strengthen but the colder oceans of higher latitudes discourage intense cyclogenisis over them. In such situations the westerlies strengthen aloft in lower latitudes leading to mountain torque removing much excess Westerly Atmospheric Momentum in the winter months. However, in this situation with more extensive cold -air highs near the surface at high latitudes there is convergence aloft and the dynamic creation of lower pressure and depressed thickness heights right up into the mesosphere over mid-latitudes which means excess westerly atmospheric momentum is transferred upwards to the edge of space, whereby some could be lost to atmospheric diffusion to space. However, most Westerly Atmospheric Momentum shunted up into the mesosphere in winter at times of global cooling is returned to the surface (lower mid-latitude mountains) in spring. Unfortunately this is not the situation with the global climate at present, unfortunately being for those wanting snow and crackly hard frosts at Christmas. There is far too much higher latitude ocean warmth that actively invites excess Westerly Atmospheric Momentum to make surfacefall upwind of Britain!!

(CONTINUED Now the higher-latitude south-westerlies impart westerly atmospheric momentum to the underlying surface through friction whilst at the same time the north-easterlies in the tropics and at very high latitudes result in the atmosphere overall gaining Westerly momentum- again through friction with the underlying surface: If there are more (or stronger) easterlies over the Northern Hemisphere there is a net increase in westerly momentum for the atmosphere as a whole, this process cannot go on for long because the resultant very strong westerlies aloft at higher latitudes results in deeper depressions forming with the result being this excess and accumulating westerly atmospheric momentum is transferred back to the surface. This, in essence, explains why we always have this problem with strong Westerlies coming off the North Atlantic at this time of year! Of course, this is not the whole story. The strength of the NE Trade Winds is greater in winter, the Polar Easterlies are stronger and there are easterlies over Russia and Mongolia at this time of year due the Siberian High. Thus a lot of Westerly atmospheric momentum accumulates into the atmosphere and so this demands for a zone of stronger Westerlies in the fewer locations not affected by easterlies. With favourable baroclinic temperature gradients between the Arctic and far North Atlantic this country is thus right in the firing-line for strong Westerlies off the North Atlantic in December!

This is an appropriate point in the year to have this topic when the strengthening Circumpolar Vortex and the consequent increase in vigour of mild south-west winds wreck the potential for another cold snowy Christmas/Winter (Yet Again)!! All that hope for increased snow-cover over Eurasia to bring about a hard winter- dashed once again. Storm Barbara is about to descend upon the North of Britain to give a reminder of last December's catastrophic flooding over the Festive Period. And this keeps happening every December! A little look at some basic meteorology and some physics is in order to get to a correct prognosis: Once you have a prognosis you can then think about whether we have the technology to stop this happening in future so that future generations can (more than Bing Crosby sang in his song) not only dream of a White Christmas (like the ones we used to know) but to make them a possibility for our grandchildren and their children! Two major (and fundamental) controls on the global weather systems have conspired against White Christmases in recent times: Firstly rising CO2 levels have led to a warming of the planet as more heat is absorbed globally over the years than has been lost to space; this is a consequence of CO2 being a greenhouse gas that re-absorbs long-wave heat radiation emitted by the Earth's surface and re-radiating much of this heat downwards leading to a small warming effect of the surface and lower atmosphere. As the Earth gets warmer the extra heat finds its way into the oceans and is transported to high latitudes leading to milder winters. At the fringes of the Arctic (and Antarctic) the reduction in sea-ice takes place earlier in the summer due to this warming and the darker ocean and land surfaces absorb the Sun's energy rather than reflecting it back into space- making oceans and lands at higher latitudes warmer still. The extra warmth arising from more solar energy absorbed by ice-free sub-arctic seas in late spring/summer means these waters stay warmer and fail to freeze until well into December, the open water warming rather than cooling any very cold air-masses passing over them. This all means that even the Arctic loses some of it's potential to deliver very cold conditions further south even if the pressure-patterns were to be favourable for Arctic winds to blow southwards: And the indications are that recent winters (2009/10 and 2010/11 notwithstanding) have been even less favourable for the right weather-patterns to deliver cold and snow than even their earlier unfavourability for cold in Britain; this naturally brings us onto the second major control on the weather in early winter over Britain. The fact that planet Earth rotates once every 23 hours and 56 minutes (our days are only 24 hours' long by virtue of the fact that the Earth moves in its orbit round the Sun and so it takes 24 hours for the Earth to catch-up its position relative to the Sun, as it were) has major implications for our weather: If the Earth did not rotate (or only rotated very slowly with a day lasting a month, for example) cold air over the Poles would flow directly towards the Equator and return to high latitudes aloft; there would be no depressions in middle or high-latitudes- only a broad area of high-pressure which would be highest at the surface in the Arctic and Antarctic. However, since the Earth does rotate at the rate it does the air moving north and south away from the Intertropical Convergence Zone (the zone of hot moist rising air near the Equator) retains the west to east movement it had when first rising in the Intertropical Convergence Zone (ITCZ) as it moves to higher latitudes where the Earth rotates more slowly. The result is a band of very strong westerly winds at high altitudes over about 30N (and 30S), which is the subtropical jet-stream and air coming from the Equator to the south of it aloft (in the Northern Hemisphere) cannot penetrate this zone of very strong westerly winds and is forced to descend. This results in the subtropical high-pressure belts near the surface centred on roughly 30 to 35N (in the Northern Hemisphere). Further north towards the poles any further northwards movement of air over areas where the Earth's rotation speed is much lower results in very strong Westerlies, so strong in fact that descent of this air would carry the westerly momentum towards the surface and frictional drag on the air below would result in lower-level air moving towards the subtropics (this is similar to the mechanism by which strong Westerlies blowing over the ocean leads to ocean currents moving from west to east at the surface but more north-west to south-east some ten or more metres below the surface of the ocean. The progressive change in ocean-currents at depth -and of wind-direction descending through the troposphere- is called the Ekman Spiral and it is a feature of fluid dynamics on any large rotating body). This initial process results in surface air-pressure falling north of subtropical high-pressure belts (in the Northern Hemisphere) and with lower pressure to the north surface winds blow northwards from the subtropical-high to become increasingly deflected by the Earth's rotation as the air moves over land and seas that rotate closer to the axis of the Earth's rotation and so they become surface south-westerlies. It is these south-westerlies that bring mild moist air to Britain at this time of year and wreck so many of our Christmasses!! However, at high-latitudes surface pressure is higher over very cold and frozen lands and seas and air flows equator-wards from these and, deflected by the Earth's rotation, become the Polar Easterlies and the is a return flow of air aloft from sub-polar regions towards the central Arctic; these Polar Easterlies meet the surface south-westerlies further south along what is called the Arctic Front but there are other constraints (again to do with the Earth's rotation) that greatly limit how far south these Polar Easterlies can get although other factors such as sea-surface temperatures and Arctic pack-ice extent also play a big role too! (CONTINUED BELOW

This is a little clearer showing the altitude-latitude wind-profile expected for the Northern Hemisphere only (and predicted for nine days' time). The deep reds indicate locations of strong westerly winds and these are beginning to extend to the mid-troposphere in higher latitudes. If we want a really cold winter you want an extensive depth of easterlies at high latitudes and for the upper Westerlies to be further south (and stronger at the 300 mb level and about 30N- so that these strong Westerly winds hit the Himalayas and excess Westerly AAM is removed from the atmospheric circulation). It also helps if you have strong easterlies high up over the Equator rather than Westerlies in the preceding months. The current set up is not good for really cold weather to occur.

This map shows the current situation with very strong Westerlies in high northern latitudes and at high elevation indicating strong Circumpolar Vortex. Notice too, the sizeable blue blob in the centre of this altitude-latitude wind-profile which shows the Westerlies of the QBO high above the Equator. This tells you much about what we can expect in Britain in the next couple of months!

The other parallel with Winter 1988/89 was that the QBO was westerly through the autumn of 1988 at the 50 mb level over the Equator and it was westerly at 30mb over the Equator until July 1988 becoming only weakly Easterly for the remainder of that year (not good news if you want a cold winter in western Europe). The Arctic sea-ice extent was greater for comparative seasons than nowadays (but still slightly below the long-term normal extent) but the North Atlantic a little warmer than usual through autumn 1988- cue strong baroclinicity. Sunspot activity was increasing after the Solar Minimum of the mid-1980s- this tends to increase the intensity of the Circumpolar Vortex in winter in the Northern Hemisphere. There are certainly parallels for this winter though the QBO has been strongly Westerly at both the 30 mb and 50 mb levels through October and November and we are heading towards Sunspot Minima (though it's not due yet), on balance these would tip the odds in favour of mild wet weather. The strength of the Westerlies at very high altitudes and high latitudes going into winter are not encouraging for cold-weather lovers either. The only crumbs of comfort is the early build-up of snowcover over Eurasia and the fact that depression tracks are well north of their long-term normal latitudes (the centre of storm Barbara is to the north of Iceland), and this does increase the scope for colder high-pressure air to build in from eastern Europe at some point during the 2016/17 winter. I recall that in Winter 1988/89 there was often extensive high-pressure over Europe and it became so dry in places that there were forest fires in parts of the Alps, put that high-pressure a few hundred miles further north then there is a chance of dry frosty weather over England and (as I point out in my winter forecast) that is an outcome that will happen on two or three occasions over the next couple of months Of course, high-pressure, does not bring snow and I am fairly certain that little (if any) snow will fall at low levels in the South or Midlands this coming winter.

December is rarely very cold in Britain because the Circumpolar Vortex tends to be at its strongest in late December: The Arctic (particularly NE Canada and Greenland) is very cold by this time whilst the North Atlantic still has considerable warmth and this creates the conditions for persistent deep cyclogenesis in the far North Atlantic with deep depressions racing east/NE in the vicinity of Iceland. In recent years the greater warmth of the northern North Atlantic has provided even more moisture in the air (to provide even more latent heat fuel as the air rises and condenses its moisture) to power greater depressions; the more northerly path of depressions following the retreated edge of Arctic ice demands of even stronger higher-latitude Westerlies to occur in to counteract the input of Westerly AAM elsewhere (due to the interaction of tropical and subtropical easterlies with the surface in low latitudes) to satisfy Conservation of Angular Momentum laws. That is because the Westerlies coming across the North Atlantic blow closer to the axis of the Earth's rotation. It is this strength of deep depressions-and attendant Westerlies- that has contributed to the devastating floods and storms seen in northern Britain in December 2015 and throughout the winter months (and nationwide) throughout Winter 2013/14 December is very unlikely to be very cold for the next few years and for the above reasons but there is an increased chance of something like December 1981 or December 2010 as the Grand Solar Minima gets underway over the next 15 to 20 years and more so should some of those Decembers follow years during which major volcanic eruptions occur with excessive amounts of dust and SO2 in the Stratosphere helping to cool the lower atmosphere earlier in the season in higher latitudes whilst stratospheric dust leads to absorption and warming of weak sunshine at elevation. Such a development during the coming Grand Solar Minimum may overwhelm the effects of increased CO2 - induced warming leading to a slight global cooling. More important such a development- warming of the Stratosphere in early winter at higher latitudes, and cooling below is much more likely to encourage high-pressure than low-pressure at high latitudes going into winter and set the scene for Sudden Stratospheric Warming type events. In this situation with stronger seasonal cooling setting in sooner along the margins of the Arctic- encouraged by more frequent north-easterly winds- the storm tracks would move into lower latitudes and the Westerlies would weaken (blowing as they would be, further from the axis of Earth's rotation, they would not need to blow strongly to counteract the frictional effects of tropical and polar easterlies so as to satisfy Conservation of Angular Momentum Laws). This would set the scene for much colder northerly and easterly airstreams to reach the UK in early winter. Furthermore, if we are to get really cold weather in December, it is helpful if the QBO is in a strongly "easterly" phase over preceding months and the entire pattern of winds aloft should have strong Westerlies, if there are to be any at all, at the altitude and latitude of the Himalayas so that the strong mountain torque removes excess Westerly AAM before it reaches higher latitudes. None of this was the case going into winter in 2016/17. Even during the coming Grand Solar Minimum we are likely to have to wait until after Christmas for serious cold to set in; the strong baroclinic gradient between the frigid NE Canada/Greenland and the relatively warm North Atlantic in December will continue- for the main- to ensure very severe wintry spells of any duration during December will remain rare in lowland Britain (i.e. a one-in-20 year occurrence). On that note, just to wish you all a happy and peaceful Christmas (even without the snow).

I am plumping for a CET mean of 4.5C for January 2017, if I may. That is in accordance with what I have for my seasonal prediction for this January: Mainly mild and wet with west/south-west winds but with a couple of cold dry spells associated with high-pressure over Europe bringing hard frosts to England and Wales- at least to locations away from the coasts. Snow on northern hills at times. I am also going for rainfall totals averaged over England and Wales during January 2017 being 90 mm.

Dry clear and still with high-pressure extending across Britain from northern Europe would be my favourite type of weather if snow was not a possibility in winter: such a weather-pattern would bring strong net radiative cooling day after day and with no wind to interrupt the cooling air near the ground it would get progressively colder. Temperatures below -10C would occur widely, freezing fog would occur locally and a thick rime would cover the landscape since even daytime temperatures would remain below freezing point after several days of such settled weather. A frozen, rime-covered landscape under a deep blue sky has to be the next best thing. However, as with snowfalls across Britain, this is a weather type that is unlikely to feature much this winter. There will be a couple of dry frosty spells in January/February but -10C and a thickly rime-covered landscape, probably not this year. Sadly!

The Quasi Biennial Oscillation became even more "Westerly" in November: At 30 mb over the Equator the average wind-speed is over 14 metres/second (near 32 mph) from the west and at 50 mb the November average was Westerly 7 m/s (16 mph). This is liable to feed into the general atmospheric circulation over the next few months, and this means mild conditions with strong west or south-west winds will gain the upper hand towards the end of the winter. The westerlies in the Equatorial stratosphere have been truly exceptional and they extend through a considerable depth of the high atmosphere there; such a large quantity of Westerly AAM accumulated high over the Equator does not, at any level, look good at all for prospects of extensive blocking with easterlies in higher latitudes come February-March 2017.

http://www.wetterzentrale.de/pics/Rtavn3361.gif There are hints in some of the longer range forecasts of high-pressure with cold and frosty weather (see above link to Wetterzentral). But indeed, as I predicted from the outset December is not going to average out colder than usual and wet, windy (and mild) conditions will seldom be far from the north-west of Britain. After suggesting cold dry weather would gain the upper hand the long-range forecast from the BBC is now suggesting that mild and wet conditions will win out in the New Year. They say a week is a long time in politics, the same is true of the medium-term weather prospects in this country!

THIS is an illustration of expected wind-speeds thoughout the depth of the atmosphere from Equator to North Pole: All that red is Westerly winds. Notice the red in the Stratosphere over the Equator and tropical latitudes (70 mb to 15 mb), which ties in with the QBO being strongly Westerly at 30 mb through both September and October and Westerly at 50 mb in both those months. There are also westerly winds at 200 to 300 mb in all tropical latitudes which is unusual. There are also very strong Westerlies developing at stratospheric/mesospheric levels on the fridges of the Arctic and over higher mid-latitudes. Sooner or later all this Westerly AAM will reach the surface. NOT good news for cold-weather lovers!

Interesting to note the Stratospheric(70 mb to 15 mb)) Westerlies high over the Equator and also between 150 and 300 mb in all tropical latitudes as well as the very strong Westerlies above 15 mb in the Arctic and sub-arctic Stratosphere. None of this portends anything like Sudden Stratospheric Warmings over the Arctic and the Westerlies high over the Equator show very clear the strong Westerly phase of the QBO, another indicator that this winter is unlikely to be a cold one in Britain. The only straw at which cold-weather lovers could clutch is the strength of the Westerlies at the latitude and height of the northern Himalayas/Pamirs, which suggests that strong mountain torques could remove Westerly AAM from the Northern Hemisphere Circulation later this winter to permit greater scope for easterlies reaching the UK later. However, the subtropical jet-stream is not, if this illustration is anything to go by, either stronger than usual nor in lower latitudes/elevations than normal for the time of year- and it is that which would be required for big mountain torques removing Westerly AAM. It looks like the Westerly AAM in the Northern hemisphere atmosphere is building up strongly and in the absence of a strong sink for this over mountains (or elsewhere) in lower latitudes it looks like higher latitudes are due for a resumption of the weather we would more normally associate with our winters in Britain,- i.e relatively mild and (at times) stormy.

Refer to the prediction I made for the winter back in October: https://www.netweather.tv/forum/topic/86410-winter-20162017-preliminary-prediction/ The warmth of the NE Atlantic, the QBO being Westerly (quite strongly at 30 mb but also westerly at 50 mb through both September and October), and to add to that the likely strong baroclinic temperature/pressure gradient between the warmer than usual northern North Atlantic and the (become) very cold NE Canada and Greenland- not to mention, of course, the fact that strengthening NE Trade Winds in the tropics and subtropics going into winter (because of the cooling subtropical continents and oceans) results in an increase in the Westerly AAM of the Northern Hemisphere atmospheric circulation: That means there is a corresponding need for stronger Westerlies in higher latitudes to counter-balance the tropical and subtropical easterlies by removing this accumulated Westerly AAM (again through friction with the underlying surface). Again, these westerlies are going to align with the depressions forming along these major baroclinic zones and with warmth over the northern North Atlantic these depressions will be fuelled by extra moisture and latent heat of condensation so the Westerlies will be quite strong. All that can be certain is that these depressions will travel a bit further north than usual allowing some cold dry weather (with night frosts) over Britain (much as has been seen already), but it would be naíve to think cold dry and frosty weather (with high-pressure in charge) would persist throughout the winter and that deep depressions would not bring spells of rain, strong west or SW winds and mild temperatures for a good deal of time. A few did get carried away by blocking highs and snow-cover over Eurasia and are now facing a reality check; higher-latitude Westerlies almost always assert themselves strongly in December over and west of Britain (Decembers 2008-2010 were very unusual) and warmer than usual waters north of the UK make it more (rather than less) likely that Westerlies will predominate. Despite sea-ice being seasonally well below the normal, it's current extent is still close to the long-term normal for the second week of November- and in most Novembers' past that was enough ice with the become very cold Greenland/NE Canada (with sea-surface temperatures just above average in the northern North Atlantic) to create the necessary baroclinicity to generate deep depressions moving north-eastwards in the vicinity of Iceland. To paraphrase the above, it is the tendency towards strong baroclinicity between Greenland/the Arctic and the North Atlantic combined with the constraints of the Law of Conservation of Angular Momentum that results in a Corollary that the increased strength and extent of tropical, subtropical and Polar Easterlies at this time of year must be counterbalanced by stronger Westerlies elsewhere that (taken together) must not be underestimated. The North Atlantic, being a large body of relatively warm water extending to high latitudes and juxtaposed with very cold frozen Arctic wastes just a bit further to the north of it, is simply placed in the right position to assist the development of frequent and strong cyclogenesis. I am not predicting a colder than normal December this winter and though I did provide an update knocking about 1.5C off my November prediction for England and Wales and 1C off December I see no reason to amend it further: I predicted that we would get a good deal of mild and wet weather in December in interspersed with some dry frosty weather and the most recent outlooks (notwithstanding that November has actually been a degree colder than usual) seem to vindicate these predictions.

At just over 400 metres above sea-level in the North Pennines, I record more air-frosts than the surrounding lowlands (typically 70 to 80) and in 2010 logged 119 air-frosts. However, extreme minima are not so spectacular as my weather-station is on a hillside (where the very cold dense air drains off on clear nights), I logged nothing below -11C in December 2010 but at the hotel where I work in the valley below (just one mile away) it dropped to -15C and Haydon Bridge (in the South Tyne Valley less than 20 miles away but at lower elevation) -19C was recorded that month and water running down the weir (under the bridge that gives Haydon Bridge its name) froze solid!

It is Newport (Shropshire) that also holds the all-time minimum air temperature for England, -26.1C early on 10th January 1982, almost one month after the -25.2C recorded at nearby Shawbury. Minima below -20C occurred widely across England during these extremely cold clear and calm periods during December 1981/January 1982- locations as far south as Grendon Underwood (Bucks) reporting such minima. Given the right conditions, a very cold dry airmass, deep powdery snowcover clear skies and an absence of wind very low temperatures can occur anywhere away from the Coast and with the proviso the locations concerned are not on steep hillsides above about 50 metres- from where rapidly-cooling air drains away. I expect we will see a few winter spells like this once more in the coming Grand Solar Minimum when the QBO is strongly easterly over preceding months.

What are you expecting- high pressure extending from the Canary Islands to Denmark funnelling warm air from the Azores throughout the month? That could produce a mean temperature like that in December but I cannot imagine such a set-up persisting like that- not even in my worst nightmares!!

I will plump for a CET mean of 5.1C if I may- I am NOT expecting a very cold month. December will be replete with some cold frosty spells and lengthy mild wet and windy conditions from the south-west. Indications are afoot of Westerly AAM being transferred to higher latitudes and a strengthening of the Polar Vortex. Those anomalous strong Westerlies high-up over the Equator (as noted in September and October) have to come down and spread their influence northwards sooner or later!

It is also likely that the definition of the seasons that we have, what month falls into which season, is a product of our temperate rainy climate: The warmth of the North Atlantic, it's high specific heat capacity and the prevailing south-westerly winds (particularly autumn and winter) means that the transition seasons of Spring and Autumn are lengthened and all seasons are of the same length. In other locations at the latitude of northern England such as the interior of Manitoba (Canada) or Siberia east of the Urals winters start earlier, are much colder and last later but they have hot summers. I don't know what their seasons would be but they might have something like: December-January (Mid-Winter), February-March (late Winter), April-May (Spring), June-July (Summer), August-September (Autumn) and October-November (Early-Winter); or perhaps the same definitions for Spring, Summer, Autumn but Winter being a six-month season of October through March because their typical weather is with very hard frosts and light snowfalls (with persistent snow-cover) throughout that period of the year. Such a definition would also reflect that there is little lag between insolation levels and temperatures at the surface- temperatures are clearly dropping by mid-August and severe cold sets in by mid-October in most years (such are the features of northern continental climates). It is certainly the case that the period of cold with bare trees, no flowers, frequent gales and rain (but also the potential for extreme cold given the right conditions) commences in November in Britain- but especially in the North. However, in these globally-warmed times there have been times with markedly above-normal temperatures and warmth in early November (last year being the classic example when 22C was recorded in Wales) and the severe cold-snaps of Novembers' past have become rarer. On that basis it is probably churlish to argue with the official designation of months into seasons as they are, which as mentioned above, is convenient for calculating averages, etc. The seasons as we have them defined divide neatly into four a year, are of the same length and it makes calculating average temperatures, rainfall, wind-speeds for months, seasons and years EASY!