Search the Community

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