BrickFielder

One thing you can be sure of is that the models will not have the details nailed done until very close to the event. The best chance of snow is for north east Scotland where showers coming from Scandinavia are likely to bring a little snow. For other areas then we have to look to Wednesday and Thursday. Looking a little deeper into the possibilities then we can have a good guess about snow and at the least look at the relevant criteria so that closer to the time we know what to look out for. Wednesday sees an upper level lobe of vorticity (300hPa) cross the southern half of the UK with a small low pressure system forecast to cross just to the south of the UK. Matching 850hPa temperatures and precipitation then things are not looking hopeful with temperatures at this level above -5C where precipitation is falling. Surface temperatures look a little high for snow as well. I don't think this feature looks like giving much in the way of snow. Thursday during the early hours I think is a different prospect. Again you would have to say that the 850hPa temperatures below -5C and precipitation do not align very well and hence the reason that people are talking about a leading edge of snow. The difference here though is the surface temperatures with quite a hard frost ahead of the front taking temperatures down. Soil temperatures do not look to rise much above freezing for the midlands northwards. The zeroDegreeIsotherm remains quite low for more northern areas. My opinion at the moment is that because this is happening overnight then the prospect of snow for the midlands northwards may be a little underestimated by the models.I doubt whether the snow will last long and should start to melt during the morning. Because details are likely to change with more accurate perameters being available to the models as we get closer then my opinion will change.

Stratospheric modelling at the moment will be difficult for the models with the stratospheric vortex (This is different to the low level polar vortex) being displaced. See on this chart how there is warm air over north east Canada. Then on the next chart the warm air has largely gone. Certainly warm air at this level cools but these charts almost suggest the vortex behaves as the polar vortex with air flows from one area to another. I don't really like this concept and tend to think of the stratospheric vortex as a column of cold air. It can bend and stretch or split in two , or develop tendrills ,but is not quite like a flow of air at a level in the troposphere. Accepting that the model is accurate means the tropospheric polar vortex developing over the north east Canada and US will be short lived and focus of the polar vortex will be solely over china and east asia. At lower levels models are not really suggesting a strong high over Siberia , which I would be expecting under the circumstances. Ensembles show that a fair amount of uncertainty comes into the models at around the 6th of February. This I interpret as down to how the jet stream will react to the cold air displaced south over the US. Here cold air will meet warm and we should expect the jet stream to speed up. Will it come blasting across the Atlantic giving us an almost Bartlett high situation with a north south divide in weather or will the jet tend to wrap around the low towards Canada creating a mid Atlantic ridge. GFS ensemble panels show some of the uncertainty about how the jet will behave. This tends to be an area where GFS can be a little weak and I would tend to go along with ECMWF view. At the moment this looks like being a fairly short cold spell and might even miss the UK to the east, but with my unease about the stratospheric modelling proving correct things may change. Another important factor with respect to the jetstream will be how much low pressure systems undercut the ridge to the west of the US. This seems likely to me given the strength of the Pacific jet and the polar vortex out towards china. My feeling is that this will put energy into the southern arm of the jet across the Atlantic allowing the northern arm to drift more northwards. Not much point looking beyond 5 days on the forecasts at the moment given the uncertainty , but there are some chances of a return to cold weather.

Dewpoints can be a good indicator of humidity, so that when people say the dewpoint is too low they really mean the air is too dry for snow. Humidity is usually a reflection of the difference between the actual temperature and the dewpoint temperature. In real terms if the dew point is low then we need a very low surface temperature as well to get snow. Because snow comes from clouds and clouds keep temperatures up and we are surrounded by sea which also tends to keep temperatures up ,very low surface temperatures and snow are unusual in the UK. Low dewpoints become a good approximation for it is too dry to snow.

The Polar Stratospheric Vortex has become displaced from the pole and now centers over Scandinavia and Russia. I think this will tend to lift up the tropopause and the differences in temeperature between 500hpa and 100hpa be high enough for low pressure to form. Around the edges of the vortex high pressure is likely to form in contrast. Forecast would tend to suggest the high pressure to our west will tend to move eastwards as the stratospheric polar vortex moves towards Siberia. However things become a little tricky after that with either the still quite strong vortex migrating back to the pole (return to wet mild atlantic conditions) or a split of the vortex and early demise for it (a more blocked pattern).

Yes this is essentially correct. One mistake people often make is confusing the tropospheric polar vortex with the stratospheric polar night vortex although to a large extent the polar night vortex dictates how any polar vortex may behave. The polar vortex is usually associated with cold air at the surface which is dense and sinks. This creates a pressure pattern of high pressure at the surface but low pressure from mid levels upwards. Looking at 850hPa (low level) you should see cold temperatures and high pressure where as at 500hpa (mid level) you should expect lower heights. Above in the stratosphere is the polar night vortex which if strong allows the tropospheric polar vortex to form but if weak lets the polar vortex break down occasionally. Stratospheric Polar Night Vortex forecasts

Current weather conditions with temperatures topping out at about freezing would suggest you should not have any problems. Check out the webcam to see what it is like. Web Cam

I always get a bit worried when I see comments that CO2 warming is proven. That the temperatures have risen over the last few decades is proven but the exact mechanisms of how are a little more difficult to pin point. A CO2 molecule and a H20 molecule have absorption and radiative spectrums which are very close and in affect act very similarly in terms of greenhouse affect. CO2 concentrations in the atmosphere range between 300 to 600ppm where as H20 concentrations range from 10000 to 16000 ppm. Sunspot activity can change H20 concentrations by as much as 1000ppm. You might wonder with those numbers why any increase CO2 is not dwarfed by changes in water vapour, but the answer is not quite so simple. Typically climate scientists refer back to work done by Ramanathan and Coakley in model simulations during the 1970's which in recent times look to be somewhat questionable. The basic premise that water vapour forms cloud which reflects light is correct and this compensates for the greenhouse affect of water vapour. Well thats not exactly true either because it depends on the depth of cloud and height of the cloud and the amounts of cloud. To some extent these are governed by ionisation patterns which can be linked back to sunspot activity. Typically during high sunspot activity there is more high level cloud which has a net greenhouse affect rather than light reflective affect. Where it is claimed that CO2 contributes 35 % and water vapour 65 % to the greenhouse affect these values are likely to fluctuate depending on the sun cycle such that during active sun periods such as now CO2 contributes 40% but at quieter times this may reduce down to 20%. I have not disproved that CO2 is causing global warming but just wanted to show that the amount may be questionable and even latitude in some of the basic ideas can lead to dramatic differences in conclusions when dealing with such a complex system as the earths climate. Solar Changes affects on Climate (Hadley Center)

Abrupt changes have occurred in weather. Most noticeable was the regime change in the Pacific around 1976 to 1977. Before which we had equal numbers of El Nino and La Nina episodes and since mainly El Nino. The 1970s North Pacific climate regime shift is marked by a notable transition from the persistent warming (cooling) condition over the central (eastern) North Pacific since the late 1960s toward the opposite condition around mid-1970s orchestrating with a deepening of the Aleutian Low and strengthening of the midlatitude westerlies (Mantua et al.1997; Zhang et al. 1997). This abrupt change shows up in changes to ENSO, the PDO index and AO index. More specifically the AO index has become more positive since 1976 with the result that the UK is now warmer and conversely much of Alaska has been cooling since 1976. This change is the most notable one but there have been others although the statistical evidence is less compelling. 1989 and 1998 seem to also have seen slight regime shifts and maybe the fact that there were El Ninos in 1976-1977 1997-1998 is significant. Similarly the NAO index shows tendencies to shift rather abruptly although not necessarily at the same point as Pacific regime shifts. Current thinking is that the regime shifts in both the Atlantic and Pacific basins are attributed to the delayed adjustment of the subtropical ocean circulation to the persistent wind stress anomalies prior to the climate regime shift. In other words long periods of anomalous high or low pressure in the northern subtropical basins of the Atlantic and Pacific can set off regime changes (lurches). Which is a bit worrying because we have had a long period of low pressure around Iceland this autumn. This does not mean that all of the warming in the UK is attributable to this or that global warming is not taking place. Changes in our weather can have complex and multiple origins and these regime changes may well be linked to global warming through increased anomalous conditions in the northern oceans.

First sign that the stratospheric cooling may be coming to an end. Indications are that any warming will take at least a month and a half to reach the bottom of the stratosphere. This means any large scale blocking or high pressure pattern is unlikely until towards the end of January. Signs of a change coming up.Strong Atlantic jet seems set to weaken for a period while the Siberian high seems set to move back west. The jet is also set to put on a burst as it exits china. Leading to an amplified pattern over the United States and sending the jet north in our location. I think it is a pattern which will reset back to Atlantic weather for us as canadian cold builds high pressure and straightens the jet out by mixing cold and warm air of the east coast of the United States. The jet burst off the Chinese coast is likely to repeat so our weather is likely to flip flop between Atlantic wet and windy weather and murky foggy high pressure with brief northerlys. There is an interesting little low pressure forecast to develop over the Azores which by its nature of having some tropical properties may prove unpredictable and a bit of a spoiler or unknown quantity. It might take of though the Med or the continent rather than fading as currently forecast (Not likely to head our way but worth watching). I expect it to throw the forecasts off a little ,so expect forecast changes over the weekend. Some chance of snow where we have had very little so far but I doubt that we will have a white christmas.

Excellent analysis by GP but I have a feeling that large scales changes are more likely to occur in January and into February and I will explain why later, although this does not exclude short cold spells happening before then. I think looking at correlations between winter and various indices is a slight mistake in that you probably should have treated early and late winter separately. My reasoning is linked to how stratospheric warming events work. Let us have a quick look at the state of play currently in the stratosphere. No warming looks to be happening at the moment although the stratosphere typically cools at this time of year it is particularly cold now. This equates to a strong stratospheric vortex and very limited high pressure high latitude blocking with little chance of easterly winds bringing cold air from the continent our way. Put simply this is not a good sign for snow lovers and looks set to continue for the time being. I do expect this to change but not till later on. My understanding of how stratospheric warming events work starts with the conditions of high latitude blocking. High latitude blocking can lead to cold for the UK when we get high pressure to our north ,over Scandinavia or mid Atlantic linking to a Greenland high. This high pressure blocking is significantly enhanced by a weak stratospheric vortex and warmer than normal air just above the tropopause (low stratosphere). We know that these warming start off in the top level of the stratosphere and move downwards over a period of just less than a month. Looking at the current state with little warming in the upper stratosphere there is not much hope of a warming reaching lower levels in the next few weeks. Put simply a stratospheric warming moves down from the upper stratosphere to turn the AO (Artic Oscillation Index) or NAM (northern annular mode) negative and stratospheric cooling truns them positive. Many readers will perhaps be familiar with all that I have mentioned so far, hopefully this bit will be a little new as I try to explain some thoughts about how the warming events start. The source of stratospheric warming events seems likely to be planetary waves which start in the upper troposphere. Here is what a planetary wave looks like in the upper stratosphere identified concentrations of trace elements. Not only do these waves have a latitude and longitude element but as they break they transfer heat upwards. I think we are porbably around day 30 on the downward transfer of a cold event (another 30+? days to go) From this we can see that these waves start very low down in the atmosphere and travel all the way up and back down again. These waves do not always behave like this and will quite often be reflected back down from the tropopause . Current explanations for whether the waves are reflected or not center of the strength and direction of prevailing stratospheric winds. Statistics show that during strong westerly QBO (Quasi Biennal Oscillation) stratospheric winds in early winter all planetary waves are reflected back from the tropopause and no stratospheric warming events occur. This changes later in the winter with statistics showing the QBO has no affect in late winter. This is why I think the polar vortex is unlikely to weaken much during December. Comparing planetary wave ones from this year and last we can see that there is a distinct lack of wave activity this year( Note Wave 2 also looks meagre this year). Here is a quick quote from some reference material on the subject. One thing you should pick out from that quote is the warming by decelerating the mean flow and it is by looking at the heat flux at the lower stratosphere that we get a real clue as to the source of planetary waves. We can link the heat flux directly to the NAM and here are a few charts and slides from Polvani and Waugh 2004 which help to explain the link. To sum up we should be looking at the current AO for clues to the AO in about sixty days except when there is a strong westerly QBO in early winter as now. A positive AO now and strong vortex in theory might equate to the opposite in February. Put simply conditions look favourable for a cold February.Interestingly this would seem to show that we should have two month phases of weather and that seasonal weather should repeat its self every year until we have a strong westerly QBO. Please note that this post is for peoples enjoyment and is exploring avenues of understanding that I am not an expert on, so I am more than likely a little wrong in my conclusions.

When considering this you need to think about what warm and cold core mean. Firstly lets look at high pressure systems and the difference between a cold and warm core high. A warm core high is deep and involves subsidence from the top of the troposphere and these tend to evolve nearer the tropics. A cold core high builds from the surface and although moderately deep involve surface subsidence.So a cold core high tends to be shallow and involve artic air. Looking at low pressure systems then cold core lows tend to develop at mid latitudes are deep and tend to have a cold pool of air aloft.These have rising air at all levels in the atmosphere. Warm core lows tend to be shallower and involve lifting only to a limited level (typically mid levels). Your statements then are largely correct except when you start looking at the details of warm core tropical cyclones where you may get subsidence at the center.We won't even mention the differences between barotropic and baroclinic. Below is a link which I hope explains it a little better than I can. Habyhints high and low pressure types

There is also some unusual cold building up on the border of eastern Alaska and western Canada. -23C at Watson Lake -18C at inuvik The normals this time of year are 13 degrees above zero by day and 4 degrees below at night. Similarly cold is building in Siberia -24C at Tiksi and Pevek. Artic Temperatures

Gust fronts don't really extend in front of a Thunderstorm they mark the boundary of the downdraft and updraft regions and can be a few minutes ahead of the rain.First you get an updraft area of rising warm air ,then you get the gust front and roll cloud and finally the rain.Where a thunderstorm has become outflow dominant (no updraft) then I guess a gust front could extend a couple of miles out from the thunderstorm.

SST's in the pacific look like playing a large part in the weather. Typical El Nino conditions where the jet sinks south over the US and low pressure is located in the Gulf of Alaska. The passage of storms has created a further band of warm water further north which is creating high pressure towards the aleutians. Closer to home and the jet is likely to want to climb north along the eastern seaboard of the US which will create periods where we have a mid atlantic blocking pattern. It looks like being too far west though for the associated trough over western europe to be positioned so that we are in the cold air. Ireland looks in a better position but this pattern will probably retrogress west as el nino matures. At the same time or in combat with the above scenario warm seas south of greenland are creating high pressure ovr greenland. Short term we should expect mild wet weather from low pressure systems parked to our west with short northerly spells as atlantic blocks break. Interspersed with this should be atlantic low pressure systems blasting across to the north of the UK. Longer term the mild wet weather will sweep warm air to our north warming the seas there. This increases the likihood of high pressure developing to our north bring cold continental wind from the east. Retrogressing pattern due to maturing el nino may cause ridging towards the UK. Of course it never works out as simply as this and these are just indicators.

One of the most important things about climate change is that climate scientist must have credibility. When the press over emphasize a certain area of climate investigation or quote out of context or just plain misunderstand the research we get into the dangerous position where the general population start to loose faith in what the climate scientists are actually saying. The problem is that as climate science progresses then certain aspects become more or less important and that new research tends to contradict the findings of previous research. The general population then sees the previous claims as wildly outrageous and loose faith in any new predictions. Lets just take a look at the case of CFC's and HCFC's which are being phased out and banned because of the serious impact on climate.Some research now raises doubt about what sort of impact with suggestions that the net effect of CFC's may be to reduce surface temperatures. Next week new research may suggest it is the most important component of global warming.Whether or not it contributes to global warming I consider that it was right to ban CFC's because of a multitude of other smaller effects like increasing UV rays. For those who are interested then you may want to investigate the greenhouse affects of HFC's the replacement to CFC's (stronger affect) or how CO2 could be used instead of HFC's. The affects of CFC's Next we need to understand a little about climate modelling and about how it has changed over the last few years. One of the most important changes to the models is that they are starting to model the behaviour in not only the troposphere but the stratosphere and mesosphere with some significant changes in results( ionosphere to be modelled as well soon). The next important development is the inclusion of a chemical coupled model which looks at the chemical interactions and their affect on climate. These advances have lead to increased confidence in the model outputs and the tentative claims of climate scientists that global warming is definately taking place. The trouble is that there is very little information from these new models and people tend to quote details from the older inaccurate models. Two particular models seem to be leading the pack , firstly the NASA GISS model and the european MA-ECHAM models.State of the art modelling only a few years ago was still able to throw up some surprising results. Take the following reports which show CO2 causes a warming in the stratosphere (which means the surface cools). MA-echam4 stratospheric warming This is an important point because many climate models suggest that most of the warming due to CO2 would be at the troposphere lower stratosphere boundary which does not necessarily imply surface warming (actually it suggests cooler surface temperatures). Stratospheric Cooling For those of you reading this and perhaps thinking I am going against the conventional thinking then you would be wrong. The latest model result from MA-ECHAM5 and the HAMMONIA system does suggest that CO2 will cause a surface warming. HAMMONIA The problem is that these are models and there is research out there which suggests the models may need changing and that the effects of one forcing may not be as strong as another. Aerosol Forcing New evidence comes to light like the fact that the mixing of increased CO2 is limited to the troposphere and lower levels of the stratosphere (might current models give the same result as the MA-ECHAM4 under this scenario). CO2 Mixing The most important point I think is too look at this realistically and say a combination of our actions is causing climate change. We should be looking at all forcings and start thinking about all emissions.Finally climate scientists recently came to the opinion that AGW (Anthropological Global Warming) is probably taking place.Is that probably 51% certainty, I think not, probably closer to 70-80% although if you asked them a different more specific question like 'Do CFC's definately cause climate warming' then you may get a different result. There is plenty to worry about and a lot of things need changing from emissions to land usage, but I worry when people become fixated about a single aspect of climate change that they may become disallusioned at some further date.Lets have the facts without the sensationalism and be open to new research and ideas.

Interesting report in the times today about the sources of Carbon Dioxide emissions. By far the largest emissions are by Industry, transportation as a whole was calculated at producing 14% of carbon dioxide of which a lower percent is produced by personnel travel. I can not really argue with arguments that we will all have to pay towards combating climate change although I would still argue we should be tackling all forms of emission not just carbon dioxide. I would argue that taxing personal motoring may have very little affect and that the politicians will continue to shy away from the difficult policies which will achieve results despite this report.

500hPa is mid level in the troposphere at about 18000 feet and the temperatures at this level give a fair view of the synoptic pattern. You should be able to get a rought idea of where the jet stream ought to be by looking at the steepest temperature gradients. You should be able to get a feel for how air is moving and from that the general synoptic pattern. Heres a quick first look at a 500hPa chart. Not too far out when looking at the more detailed charts at that level. Note that where the 500hPa winds are strongest is where rain is likely to occur most. 850hPa is a lower level in the troposphere at about 4500 feet. This gives a reasonable indication of low level temperatures with -8C at this level equating to about 0C at ground level. It also is a good level to get of feel of where the frontal systems are. Warm air and strong temperature boundarys being likely places for fronts. Here is another quick interpretation of a chart. A little out but not too far off the detailed chart. As easy quick glance charts these charts give a lot of information and really cannot be beaten ,however delving into some of the other charts will tell you a lot more if you are so inclined.

By the IPCC own admission there are a number of areas where climate modelling is not to the standard that scientist would be happy with. Modelling of ENSO events is poor. All models underestimate the increase in vegetation feedbacks. Models underestimate natural climate variability. Modelling of snow, soil moisture energy fluxes and forcing is poor. Modelling of ocean ventialltion rates is poorly parameterized. Sea ice distributions and modelling is poor. Affects of Hurricanes and cylones is not included in climate models. Solar variability and cosmic radiation changes are poorly modelled. Radiative forcing is poorly modelled. IPCC 2001 Their conclusion was What stands out to me is the poor modelling of vegetation feedback (Take up of CO2 by plants) ,which I read as the modelling of CO2 affects is poor. The other point is the definate affects of volcanic activity showing that aerosols really do have a big impact on climate and the affects of solar variability. A discussion on climate model performances There have also been some publicised occasions where climate models have been shown to match earth history climate only to find that the data of earth histry climate was wrong in some respect by finding a new source of information. This does not mean we should ignore the results of climate models or not take the risks of emission seriously, just that we don't have the whole picture yet and should be tackling all potential causes of global warming.

Detailed SST's Partial SST archive CDC SST's IRI SST Forecasts The Gulf Stream Artic Sea Ice coverage There are also some good charts in the data center (see at the top of the page). Unseasonal warmth in China is showing up as lack of ice to the north of china which tends to confirm your idea that the cold is building on our side of the artic.

It is not really fair to dismiss all his thinking based on some incorrect assumptions about the THC. Even here it is not so clear cut about what is or is not happening with bryden et al reporting some remarkable changes to the THC even if the gulf stream is very stable and the total flow remains the same. A number of question marks hang over the bryden data in terms of whether a long enough monitoring period was used and whether a seasonal difference has been picked up. Byrden et al discussed at the Realclimate Looking at what Gray says specifically The above are valid facts, whether you interpret them as Gray does is questionable. His comments about the performance of GCM's have some basis in fact although I am not sure that you could argue that the GCM's may have equally underestimated warming. I don't think I can really agree with him because he has made too many assumptions like assuming resolving cumulus scale convection will make an appreciable difference to the modelling results. I do think he has a valid point in that we do not have a handle on how much AGW is occuring. In the end his conclusions are not that AGW is not happening just that the possible affects are being exagerated. Lets remember he has been a expert on cumulus scale convection so at least some of what he says may have some bearing even if there are holes in some of his thinking.

There is no specific answer. Upper levels are usually up around the jetstream at above 350hPa. 500hPa is often considered a mid level. 850hPa is getting toward low level. 700hPa may be mid or low level depending on your definition.

I am not sure that is what GP meant. I think he might have been aluding to the North Atlantic SST Tripole where in recent years we have had a cool warm cool tripole as opposed to a warm cool warm tripole (as now). In which case we have returned to the SST patterns of the 1940's - 1960's. SST'S (Pacific and Atlantic) are really only half the story for the NAO with the other half being sea ice and snow cover, which is why a number of people are looking toward the early build up of cold in certain areas of the artic. Clarification from GP required I think.

The diagram is taken from 'ATMOSPHERIC GCM RESPONSE TO EXTRATROPICAL SST ANOMALIES: SYNTHESIS AND EVALUATION' by Yochanan Kushnir at Lamont Doherty Earth Observatory I would agree that Pacific conditions can largely override Atlantic ones.

To promote some further discussion I thought i would bring up the subject of the other major greenhouse affect. Here I am talking about water vapour which probably accounts for 60-95% of all the greenhouse affect. A number of people have argued that water vapour rather than CO2 is the prime forcing for climate changes. Water Vapour not CO2 Climatic effects of Water Vapour Of course there are a number of holes in the argument and the realclimate discusses a number of these, although it should be noted in the comments below the real climate report that even they have made a number of mistakes. Real Climate Calculating the greenhouse effect Before we look at the rights and wrongs of this lets just look at some of the science behind it and more specifically the radiation budget. The radiation budget A large amount of uncertainty surrounds whether clouds act as a greenhouse gas warming the climate or as a reflective blanket preventing sunlight reaching the surface and cooling the climate.Current thinking is that high thin clouds produce a net heating affect while low thick cloud produce a cooling effect. Clouds and Climate There is strong evidence that inceased water vapour at higher levels has produced a net warming over Europe and this is why places like the UK have seen much more of an increase in temperatures than other parts of the northern hemisphere. Water Vapour heats Europe This may be attributable to emissions of methane although there are arguments that volcanic eruptions may play a part. Methane is the prime forcing Water Vapour and Volcanoes I am not sure about these arguments but I do go along with the idea that we are producing more aerosols and particulates. These are the small nucleii around which water condenses out to form cloud. Particulates and Clouds Some of the latest thinking on water vapour suggests that this is particularly sensitive to air quality changes over asia and particularly southern china. Recent Thinking In all this I have not mentioned HFC's the replacement for the dreaded CFC's which may ironically be causing more warming than CFC's ever could. The point of the dicussion is not to belittle the affects of CO2 but to open up new avenues of discussion showing that we need to control all emissions rather than just CO2 and perhaps argue that European emission regulations may have a number of holes in them. Bill Gray's stance on AGW is fairly well known and a number of people have pointed out errors in his thinking. I should also point out that his detractors have also made a number of errors. It is interesting that he has picked up increased northern blocking observed recently as a sign of a period of cooling as this has been much talked about. Bill Gray Talk The real Climate on Gray

Where air of one temperature meets air of another temperature then you will tend to get wind moving along the boundary. These winds are part of the explanation behind vorticity. Lets take a step back though and think about something a little simpler like a merry go round in a park. If you run past the merry go round and give it a quick shove with your hand then it will rotate. If we then start to think about lots of people running past on one side and giving the merry go round a shove and others walking past on the other side and giving a lighter shove then the merry go round will rotate according to the shoves of the faster moving people. It is the same with a parcel of air with wind moving faster on one side of the parcel giving the air parcel a spin in a particular direction. Winds can differ at both different levels and in a horizontal plane and the amount of relative spin (Compared to earth spin) determines whether air is likely to rise or sink at a synoptic level. Generally low pressure systems have areas of higher relative vorticity and high pressure has lower relative vorticity which equates rising air and cloud, to higher relative vorticity. Vorticity charts are looked at in a number of ways. Firstly in conjunction with jetstream charts where areas of divergence (jetstreak left exit and right entrance) are likely to increase vorticity. Here we are looking for a general lifting of the atmosphere due to approaching higher vorticity (PVA) or the oposite sinking air (NVA). Thunderstorms and clouds are less likely to form in areas of NVA or receeding vorticity.Put simply approaching vorticity increases the risk of cloud and showers. Secondly we are looking at vorticity to give an indication of the boundaries between one air mass and another. Quite often cooler air coming in aloft will be preceeded by increased vorticity due to the increased wind at the air boundary. Here we are looking at vorticity to mark a trough or possibly changes in the stability of the atmosphere due to temperature differences between different levels in the atmosphere. Put simply vorticity can mark cooler air and increased instability(depending on height - upper cool air/vorticity) with again a greater risk of cloud and showers. Low level vorticity (850hPa ,700hPa) also marks the boundary between different air masses and these charts are used more to detect frontal systems. Put simply increased relative vorticity can show the location of cold fronts.