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Polar Vortex

Azores Hi

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  • Location: Ashford, Kent
  • Weather Preferences: Anything
  • Location: Ashford, Kent

    Hi folks,

    I've been trying to follow the charts for a while now and I'm starting to get to grips with it. One thing which has been puzzling me though.

    I've heard lots of talk about the polar vortex and how it has large implecations on our potential for snow/cold here in the U.K. Can anyone explain what is mean by this term? I have a rough idea but it would be helpfull if someone could tell me how to spot it, what affects it and how to spot changes?

    Thanks in advance.

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  • Location: Ashford, Kent
  • Weather Preferences: Anything
  • Location: Ashford, Kent

    Thanks very much for that, like I said I had a rough idea what is meant by pv but that makes it clearer in my mind. I asure you I am a duffer and a beginner so your post is just perfect.

    Now I know what i'm looking for I understand better how the position affects the origins and trajectories of our weather.

    Cheers for that!

    AH :)

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  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

    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

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    • 4 weeks later...
  • Location: Bishopbriggs, near Glasgow
  • Weather Preferences: Cold snowy winters, warm dry summers
  • Location: Bishopbriggs, near Glasgow

    I know this is a silly question, but what is the polar vortex everyone talks about, is it a semi permanaent are aof low pressure over the pole, would be glad of an explanation.


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  • Location: Bishopbriggs, near Glasgow
  • Weather Preferences: Cold snowy winters, warm dry summers
  • Location: Bishopbriggs, near Glasgow
    OK, I'm not a meteorologist, but try this for a starter:

    Thank you for that it certainly explains it.


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    • 8 months later...
  • Location: Exeter, Devon, UK. alt 10m asl
  • Location: Exeter, Devon, UK. alt 10m asl


    I thought i would take the liberty of bumping this. The time of year means that references to the polar vortex are starting to creep into the model discussion threads and “what on earth is the polar vortex?” is a question that crossed my mind last year. A quick google search gives a basic definition but if someone is willing and has the time then maybe putting something in the guides section with regards to its position and impact on UK winter weather would be a good thing? There is an excellent post by steve murr http://www.netweather.tv/forum/index.php?showtopic=36450

    But the links to the charts in that post seem to be broken?



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    I thought id add this before it gets confined to th bin, not to much more in terms of the Polar vortex but some more winter time reading-

    An Ode to Winter- 2005/2006

    Well ive posted this over on NW & im sure the residents of TWO may find it interesting...

    what ive tried to put together is a fairly simplistic look at the factors that goven our Winter in the United Kingdom, Firstly an appreciation of the major players in control of our wintertime in terms of Pressure systems-including the factors that effect the strengths and length of occurence of these features-

    Ive then devoted a whole section specifically for 'snow' and what to look for when looking into the specific make up of delivering Snow-

    Finally forecasting techniques is covered -

    I hope you enjoy regards


    The Winter Features.....

    Icelandic Low & Azores high

    Bartlett High

    Siberian high

    Scandi High

    Greenland high

    Icelandic Low & Azores high

    Before ANY winter becomes either Cold or Signficantly Cold there is a Large Hurdle to over come- and this hurdle has seemingly grown in stature over the last 18 years-

    The positioning of the Azores high over the last 2 decades has gradually drifted Northwards towards the Pole, Reasons for this can be attributed to the +VE NAO phase, A stronger Ferrell Cell,Warmer location SST's, A Northerly tracking jet stream and 'possibly' a Northerly Sink zone for the NAD-

    The problem with its northwards penetration is that its influences over the UK are never to far away-

    Even in Colder conditions where a trough has kicked East across the Country the warmer maritime Air is often flooding back in OR the AH is ridging back eastwards to kill any convection by neutralizing the Divergence Aloft-

    To compound this Problem, the Icelandic Low has almost become Omnipresent, -What Happens is with plenty of Gas in the PFJ pushing the atlantic storms along North-Eastwards towards iceland where they 'generally' Slow down and Stall as they come up against the buffers from the Greenland & artic High pressure zones-

    This often allows secondary low pressure waves to develop on the Southern flank of the main low pressure swing eastwards across the UK,

    The net result has often been a succession of Mild westerlies with rain, followed by the 'odd' Showery day in a north-westerly flow-

    This is then often replaced again by another atlantic surge-

    The pattern can replicate over and over again-

    Take 09th Jan THIS year for starters the icelandic Low and Azores high working together like a well oiled machine-

    4th Jan 2005

    Then below the 17th of january-

    17th Jan 2005

    Spot the Difference-!!!

    This pattern lasted from Dec 29th to the 21st of Jan- 23 Days- Thats wiped out 3 weeks of Winter!!

    The problem with this 'rut' is that the Polar front and consequent boundry between Very Cold air is ALWAYS held well North of the Uk-

    On the Link the cold air is firmly held along the 60 Degree North line which is far out of reach for the UK-...............

    4th Jan 2005

    The Bartlett High

    Following 'hot' on the heels in our current Winter calender is the good old 'Bartlett' High- ( named I think after the famous Forecaster Paul Bartlett)

    Those in the trade have named this the 'Slug' as it just sits there moving away only very slowly-

    With this being a Winter topic I dont want to prolong the agony of thinking about those endless days of Mild, Moist Southerlies being pumped around the BH-

    For Valentine Snowlovers this chart is TOTALLY forgettable....

    14th Feb 1998

    Siberian & Scandi high-

    The mere mention of these 2 beasts often brings shivers to the spine-

    However these beasts if 'totally' in control of our weather can provide intense cold spells, but NOT necessarily buckets of Snow- ( with the exception of the far SE and Eastern coastal regions-

    Its hard to have a preference- given the fact were in the Uk and we cant be choosers-however pressed i would plump for the Scandi High any time-

    The reason is I think we often do better out of the Scandi- with a better likleyhood of sustaining Easterlies- The ridging Siberian will always want to 'retreat' back towards its home in Siberia 'usually allowing the UK to end up in the at best in the CUL or worst Southerly winds and the atlantic begins to push back in-

    The Scandi high has no core ridge pulling it back either way and often ridges westwards to greet the Greenland high- we must NOT confuse the 2- so illustrated below we have both starting with the

    Siberian High- ( these babies just build and build...)

    So then we have day 1 and an ordinary chart-

    Jan20th 1972

    Day 2 & 3 sees the Siberian High Lurking to the North-east and just beginning to develop- (Notice the CUL on day 3 over Central Europe-)

    21st Jan 1972


    Day 4 & 5 Pressure reaches 1040mb and the pressure just stalls for a day or 2-

    <a href='http://www.wetterzentrale.de/archive/ra/1972/Rrea00119720123.gif' target='_blank'>23rd jan 1972

    24th Jan 1972

    Day 6 Starting to organise now-

    25th Jan 1972

    Day 7- You would think that the cold hasnt penetrated that far west across to us- HOWEVER the SE is already under -5C 850 HPA air!!!

    26th Jan 1972

    Day 8- here comes the trigger- you ALWAYS need Lp close by to advect the cold air-

    27th Jan 1972

    Day 9- Here she comes westwards...

    28th Jan 1972

    Day 10/11- Shes here- a 1050 Mb Siberian high on holiday firing the Cold air over the Uk-

    29th Jan 1972

    30th Jan 1972

    Whilst this is a classic example follow the scenario through on the archives and we end up to FAR west to be really effected...

    Archives here...

    Moving on we show how the

    Scandi high can develop-

    Firstly a fairly normal looking chart- However pressure is on the rise over Scandi-

    27th Jan 1956

    The Second day and the atlantic is beginning to stall out to the west- look at the strong Southerly wind pushing up towards iceland

    28th Jan 1956

    The Third day sees a Core 1040 Mb High centred over Scandi- the Cold air is piling up on its Eastern flank- its just waiting to be advected westwards

    29th Jan 1956

    Day 4 & 5 its coming..... ;)

    30th Jan 1956

    31st Jan 1956

    Day 6 A full blown Easterly-

    01st Feb 1956

    The Greenland high

    How can I put this- If i had a shopping list for producing snowfall this would be in CAPITALS at the top-!!!

    The daddy, The boss,- The Single Most importartant factor in determining a decent Artic outbreak across the UK-

    ALL SUSTAINED cold periods across the UK have involved a strong Core pressure across Greenland 'Steering' the Northern Arm of the jet stream up the western side-

    An almost rarity in recent years- perhaps a forgotton entity- They are so rare ive trawled the archives looking for that 'Classic' Greenland high that ridges South, Holds then doesnt retreat or topple but goes onto repetativley force cold Northerlies & Easterlies over the Uk-

    Ive found it- ( the reason that this is pertinent to our current Winters is the starting day is almost a 'modern' winter-)

    So Day 1- Low pressure in the atlantic, Azores High in play and the polar front holding across Iceland- However the key here is the track of the second low pressure coming off the Eastern seaboard- Often people comment on the procession of low pressure systems running across the Atlantic in quick succession-Our Cold spell is on the cards already as the jet Stream is Bifercated across the South-western coast of Greenland-

    7th Feb 1955

    Day 2- That Low pressure is being forced south and the core pressure over Greenland drops to 1040 mb-

    8th feb 1955

    Day 3- And were under starters orders- Cold just starting to stream into Scotland-

    9th Feb 1955

    Day 4- Look at that lovely long fetch of Cold air all the way from the Fram straight/Svalbard-

    10thFeb 1955

    We then enter into a succession of Northerly & Easterly dominated days-

    ( PLUS LOOK HOW -VE the AO is on the chart below)

    Day 5 Northerly-

    11th Feb 1955

    Day 6 North-Easterly

    12th feb 1955

    Day 7 North-Easterly

    13th Feb 1955

    Day 8 Northerly ( You may think this is the beginning of the end...)

    14th Feb 1955

    Day 9 North-Easterly

    15th feb 1955

    Day 10 Northerly- This chart shows a renewed burst around the GH- more is commented apon this scenario in the Freezing month section-

    16th Feb 1955

    Day 11-Northerly- Back to a great square one again-

    17th Feb 1955

    Day 13- Easterly

    19th feb 1955

    The cold weather lasted untill the 27th of feb thats 2 and a half weeks of cold from one strong greenland high ( backed up by a neg AO) the true Daddy-

    Whats it All about-

    DAM lines/Thickness- Whats that all about-?

    The Jet stream-

    The polar front and Zonal/Meridional flows

    850 temps-


    Dewpoints SALR/DALR

    Evaporational cooling


    A Freezing Month-

    DAM lines/Thickness- Whats that all about-?

    DAM/Thickness is Measured in dekametres- it is the measure of how warm or cold a layer of the atmosphere is ( in the Lowest part of the troposphere circa 5Km)

    Higher vales indicate warmer air, lower values indicate colder air-

    The most common layers wherein thickness values are analysed and forecast are as follows-

    Because 'most model watchers are insistent on viewing the 500 HPA charts ( 500 Mb) then we do loose some of the accuracy when trying to forecast snow- this is because the 500 millibar layer is very deep - some 18000 feet above ground level and not as accurate as the 850Mb charts at determining how PPN will reach the ground-

    However as we constantly insist on using the 500's then there is an approximate guide-

    The link provided is the one to use when just looking at thickness-

    Go to the section that says 'Mittl. Wolken'

    Indicated on the 500 HPA charts are isopleths at 18 dam intervals

    The ones you will commonly see in winter are-

    The 492 DAM line which is Purple

    The 510 DAM Line which is dark blue

    The 528 DAM Line which is Light blue

    The 546 Dam line which is Green

    The 564 line which is yellow (ish)

    We Often hear as the 528 Line being the Snow line - However thats probably a misrepresention of the truth-

    The 528 DAM line coming south usually represents the bulk air mass change to a Polar type-

    Values of around 522 DAM on the 500's give about a 50/50 chance of snow- with 519 DAM usually the guaranteed value for snow- ( APPROX)

    With using such small variations in numbers to predict snow then the problem is hard using the 500's- However the METO chart on the GFS NEVER gives an 850 equivalent- so to calcualte surface thickness we have to use the equation-

    500 height-((SLP-1000)*0. ;)

    So for instance using the chart for 12 th Jan 1987- for EAST ANGLIA

    12 jan 1987The Holy Grail popesmiley.pngpopesmiley.png

    the calculation would be-

    520- (( 1030-1000)X0. ;) = 520- 24 = 496 DAM

    Actual records & values that day un-officially got to 491 DAM-

    This Calculation is good when the isobars are tightly packed or 'Baroclinic zone'- you MUST ensure you get the correct SLP closest to your region and follow the colour charts correctly to get the correct DAM amount....

    At least now you will be able to get thickness for your area from the UKMO charts....

    The Jet stream-

    The jet stream is generally defined to be a current of fast-flowing air at high altitudes, somewhere between 8-15 km (25,000 to 48,000 ft or about the 400-200 mb pressure level) above the Earth's surface. Jet stream wind speeds blow, by definition, in excess of 94 km/h (57 mph) and can reach nearly 500 km/h (300 mph). The jets are quite variable in their properties but a typical jet stream is hundreds to thousands of kilometres (miles) in length, about 160 to 500 km (100-300 miles) wide and about a kilometre deep (5/8 mile). (Though the polar jet streams form along the polar front in the Prevailing Westerlies belt that circles the planet, they do not always circumnavigate the globe as a continual stream.) The jet winds usually have a west to east direction, though they may loop with large north-south deflections.

    The northern polar jet stream (also called the polar jet, or the mid-latitude jet stream, or just the jet stream) is the one having the most influence on weather across much of the United States,Canada- and across the Atlantic Basin.The polar jet stream can usually be found somewhere in the latitude belt from 40-60 degrees at an altitude from 7600 metres to 10,600 m (25,000-35,000 ft).

    The highest wind velocity is found in the jet stream core where speeds can be as high as 460 km/h (290 mph) in the winter. The jet stream core region averages 160 km/h (100 mph) in winter and 80 km/h (50 mph) in summer. Those segments within the jet stream where winds attain their highest speeds are commonly known as jet streaks.

    The polar jet stream position marks the location of the strongest temperature contrasts between polar and subtropical latitudes on the Earth surface. Therefore, the strongest polar jet stream velocities usually occur during the winter months. During the summer months, when the equator-pole surface temperature differences are less dramatic, the jet winds blow slower and are usually found at higher latitudes.

    The polar jet stream is formed in the region of greatest contrast between polar and subtropical air known as the polar front where the cold, dry polar air meets the warm, moist air from the subtropical regions. From a climatological viewpoint, the position of the polar front forms a more or less even band around the globe, slipping north and south with the seasonal changes.

    In the vicinity of the polar front, the air pressure drops more rapidly with increasing altitude in the denser cold air than in the less dense warm air. This temperature effect on air density results in air pressure at any given altitude being higher on the warm (equatorward) side of the polar front than on the cold (poleward) side. When cold air and warm air masses sit side by side, the higher the altitude, the greater the pressure difference between the cold and warm air. Thus, across the polar front, the horizontal pressure differential (or gradient) causes air to flow from the warm side of the front towards the cold side.

    Once the air begins to flow, it is deflected by the Earth's rotation (called the Coriolis effect) and prevented from flowing directly from high to low pressure. Air flowing from higher pressure towards lower pressure is deflected to the right in the Northern Hemisphere (or to the left in the Southern Hemisphere). The resulting flow produces a westerly wind current generally flowing towards the east, parallel to and above the polar front. Very strong temperature and pressure gradients in the polar frontal zone can intensify these wind speeds to over 94 km/h, thus forming a jet stream.

    The polar front and Zonal/Meridional flows

    Defined it simply means-

    'The boundary between warm air from the subtropics and cold air from polar regions'

    From a climatological viewpoint, the position of the polar front forms a more-or-less even band around the globe at mid-latitudes in both hemispheres. The average polar-front position slips north and south with the seasons in response to the annual hemispheric heating cycle-

    As seen by this image the polar front 'generally' runs to the North of the British Aisles over the Winter period, with Low Pressure systems running along the Northern Side of the Jet stream- hence the location of the common Low pressure- The icelandic Low

    Seasonal location of the polar front

    Of particular interest to meteorologists and weather forecasters is the ever-changing pattern of long-waves that form around the polar front. These long-waves, very visible on polar projection maps, undulate around the hemisphere with three to six cycles-

    At times, that global belt fits tight, having three or four, small-amplitude undulations (little north-south latitude variation) around the hemisphere. At other times, it has as many as six large-amplitude loops (great north-south variation). How those wave loops sit over the hemisphere, or portion thereof, determines what temperature regimes are experienced on the surface below.

    When a long section of the polar front, as seen on weather maps, is smooth like the surface of a calm sea, the upper-level winds, including associated jet streams, run generally parallel to the latitude lines, This is know as a zonal flow. Under zonal-flow, north-south undulations of the frontal boundary are small, and the surface temperatures across the country, as seen in the isotherm (lines of equal temperature) pattern on the weather map, layer in zonal (east-west) bands with warm air to the south and cold air to the north.

    when a zonal flow pattern cuts off the exchange of heat between the polar region and the tropics, great thermal contrasts develop across the polar front. And this becomes the zonal pattern's downfall. Eventually, some chink, some small perturbation, develops in the zonal pattern, and a short wave, with initially small north-south extent, arises upon the polar front. If the short wave amplifies (grows in size), it can distort the flow into a new pattern that crosses the latitude lines, a pattern termed meridional. In meridional flow, cold air rushes southward while warm air streams northward.

    As the polar-front meridional pattern rises into Rossby waves of deep north-south extent there becomes massive Warm and Cold advection processes Northwards & Southwards.... Once the pattern becomes locked we are subjected to either Masses of Warm moist air adevection and were are in the 'upper-level ridge' (Image A)or Strong Cold air advection in the upper-level trough- (Image B )

    Image A

    Image B

    Image B (2) showing advection process

    To try to illustrate the location of the polar front in terms of the UK its easy to pick 3 images from the models- Firstly the 500mb charts which illustrate upper level disturbances, secondly the theta charts which are great at showing the division of the polar front and contrasting Air masses and 3 the location of the Jet stream which can show in what direction or flow the polar front is going to move in- (As described above as zonal or Meridional).

    So then below we have a 'typical' UK set up- A baroclinic low pressure system just to the east of Iceland-

    looking at the 500mb chart - on the South-Eastern side there will be a warm sector ( The warm air mass in the Warm sector) and the surrounding Northern and North Western side the colder polar air)

    This is further illustrated by the theta chart- with the Theta E temp of between 2 & 6 Showing the jet stream & Polar Front location- Notice the Britsh Aisles on the Southern Sice in the Warm Sector-

    Finally looking at the Jet stream there is a pretty continuous stream coming across the Atlantic showing that any colder air pushing into towards the uk will be short lived- ( This would only become Meridional if the jet stream gets cut off-)

    500 mb chart showing upper lever low-

    Theta chart showing the division of the air masses-

    PF jet stream

    The typical conditions experienced here will be Mild rainy one day, then cooler showery the next-

    850's Temps-

    The 850 Millibar charts are often used as a 'quick' reference guide to the threat of snow falling, and whether it will make it to ground/sea level.

    The 850 charts represent the 'lowest' levels in the atmosphere- it approximates the temps at around 1.5Km, this is why this depth is better served to calculate Snow risk than the 500 HPA charts-

    As a generalisation and The 'approximate' lapse rate to use in the Winter to get DAYTIME Maxima from the 850' charts is

    1.5C per 100mb this lapse rate will DECREASE with a Colder dryer air mass- such as an Easterly- Perhaps as low as 1.3C Per 100 mb and INCREASE with a moist saturated air mass perhaps as high 1.7C per 100mb- ( Again these are my own figures)

    The benchmark figure used by Most People is observing the presence of the -10C isotherm at 850HPA-

    Based on My average lapse rate you would require an 850 temp of -13C to achieve a daytime Maxima of Freezing-

    However it is of course Never that simple & with differing Air Masses/Varying Dewpoints then Snow 'Falling' can be seen falling in air from as Warm as -4/-5C and rain from air as cold as -10C!!!

    The key is the type of Air Masses ( these effecting the Dewpoints) involved in the Lower layers and to an extent the intensity of the PPN-

    This is explained in further detail below- but as a rule of Thumb to observe snow you can use the approximation table below-

    850 Temp Range-

    -8C 850C and below (Approx 519 DAM and below) - Generally a Safe bet for snow- Convective Snow would be dry- and of the powdery consistency-

    Frontal snow on both accounts ( cold & Warm) should still fall as snow unless PPN is very light and a warm front is advancing

    -8 to -6 850C ( Approx 520 DAM to 528 DAM)- A very marginal Set up for Snow- especially at Sea level- Elevation is an added bonus here-

    Best Set ups here for snow- Convective Snow events from deep shower clouds- these will ensure the 'Snow level' Makes it to the ground-

    Sleet and rain generally be observed if the PPN is light and from an approaching warm front-

    Snow would be wet and ideal for snowballs!!

    -5 to -3 850C ( Approx 529 to 535 DAM)- The only time snow is observed here ( Apart from high elevations ) is an Easterly undercut under a warmer Airmass- i 'Think' Snow has been observed falling into 540 DAM air before....!!!!!

    NB: The 528 Line appears just on the Northern side of the -5c 850 HPA isopleth on the Wetterzentrale 'Mittl. Wolken'charts which is why I have attributed it a value of approx -6C 850 HPA

    One of the coldest 850 temps observed across the UK - Sub -20C 850 HPA here >>>


    Ensemble by its meaning

    'A unit or group of complementary parts that contribute to a single effect'

    In terms of the bringing that into the forecasting models the representations seen on for example the GFS is only one tenth of the model output for that run- and i think if its still the case one fortyeth on the ECMWF forecast-

    For the GFS Each model run ( For example the 00z,06z,12z & 18Z) has 10 member runs- What you see on the modelling screen is the operational run- there are 9 others that are missing.

    To view these other 9 members you need to follow the link and click on the run your referring to and pick your area-

    This is the link-

    Weather-online ensembles

    On the weather Online site the Operational run is RED ( On the GFS site its blue)

    The other lines are the 9 members and the graph is measuring the fluctuations of temperature from day to day at 850 HPA -

    The 9 members are diferent from the Operational run because they all have the same starting point but have different perterbations steering every run towards milder, Colder, Wetter, & Dryer conditions-

    In the case of the example the Operational run tops out at 10C on the second of Feb- and using the lapse rates found in the section befor you should be able to calculate your maxima predicted recordings-

    What to look at on the Ensembles-

    You are looking at the Graph of the GFS from T-0 to T384- so halfway across the Graph to the right is effectively Fantasy iselnd and can often be discarded-

    The left half of the Graph especially the first quarter is the early parts of the run- you are looking for consistencty and a uniform run of lines without deviation at LEAST out to 4 days ahead- ( T96)

    After this point things begin to change rapidly-especially if the models expect a weather transition around that time-

    This is where you see massive differences acros the models- The operational run is said to be the 'Truth' run- so based on the information given its should turn out the most accurate- sadly that doesnt always happen-

    Whats an outlier??

    We hear the expression outlier thrown about quite often- it simply refers to the fact that the model ( usually the operational run) is very much out of sink with the general trend of the other models-

    again in the case of the example- it could be termed a warm outlier in the medium term as there is a significant spike around the second-

    As ever in that situation and past T180 more runs are needed-

    What my friend GP does is measure the spread of ensembles for North - America to see the consistency there- if this is low often as we are down stream the innacuracies will alter with every run-

    When is fantasy island NOT fantasy Island-

    Taking the models past T 180 is a risky business and should really still be taken with a pinch of salt- the situations where FI becomes believable is where there is little transition to go through in the short term- IE less scope for things to go wrong-

    Looking into FI if a trend is spotted you still need to follow it days on end to see how it develops as it always chopping & changing-

    Things that change have a drastic effect further down the line ( as many members found to their cost last winter when there was consistency in the models for around 7 days pointing to an Easterly- when it actually arrived though it was a rather damp squid-

    Solution- Wait till T96-.....



    How often do our 'older' friends refer back to back the saying

    "Its to Cold for Snow"- Hum NOT strictly true but the saying does hold some credibility-

    Simply put 'Dewpoint is a measure of air moisture content.' the warmer the air is the more moisture is will/can hold- & Vice Versa-

    In Winter ( For forecasting Frost/Snow) we are concerned with the relashonship between the Actual temperature observed and the recorded dewpoint-

    Firstly Frost-

    As an example-

    Suppose one afternoon you have a temperature of 50 degrees Fahrenheit and a dewpoint of 28 degrees. Assuming that mostly clear skies and light winds are expected overnight, you could safely bet that the overnight low temperature would dip below 32 degrees and at least scattered frost would form in your region. If, on the other hand, you have temperature of 50 degrees and a dewpoint of 47 degrees, the air would have a lot of difficulty cooling to near freezing since condensation would begin at 47 degrees. As a result, no frost or below freezing temperatures would be expected. If the temperature is 50 degrees with a dewpoint of 35 degrees, you have a tough call in this situation as other factors such as wind speed and cloud cover become even more important.

    Just using Dewpoints for forecasting snow is a risky business as lapse rates ( Explained below) come into play- but there are some common examples for 'General' Snowfall across the UK -

    Warmer Atlantic Air displacing Colder Artic air-

    Probably the most common winter feature Is a warm front approaching from the west with Precipitation falling into a Colder Air mass that has established across the UK before hand-

    We/forecasters usually find when we have difficulties forecasting whether rain or snow will fall because the Dewpoint will rapidly rise as the impending front arrives-

    As warm fronts generally have light PPN and move slowly the advance of the generally saturated air often raises the dewpoint to just over freezing point level and almost on top of the surface temp- this will result in drizzle falling as there is an ever increasing deep melt layer in the lower boundry layers of the atmosphere-

    This happens SO often across the UK with Snow crazy fans often being let down at the last minute-

    2 things to draw on in these events- Elevation and PPN intensity always hold the key.....

    DALR changing to SALR

    Colder Air undercutting A warmer air mass- directly under a front-

    In this instance we have a total reversal in set ups and MORE often than not rain turns to Snow-

    The key here is the reduction of the depth of the Melt layer of the boundry layers nearer to the surface- this is propergated by continual cooling of the lower layers by both the Colder air mass arriving and evaporational cooling-

    As the air dries out so the lapse rates in the atmosphere become better

    suited to the snow level rapidly falling- ( SALR Changing to DALR)

    Enviromental lapse rates-

    Lapse rates ( DALR & SALR) of temperature are CRITICAL in the forecasting of snow-

    The boundry layer of the lower atmosphere is where all the action takes place and how steep the lapse rates are is often the deciding factor on whether Snow or Rain will reach the ground-

    So what do we have - well Vertical motion is needed to "create" moisture for clouds and precipitation. Moisture is "created" or condensed into clouds/precipitation via adiabatic cooling. Adiabatic cooling is otherwise known as the "lapse rate". Temperature decreases with increasing elevation because the atmospheric pressure decreases as elevation increases-

    DALR-Dry Adiabatic Lapse Rate: The temperature of a dry atmosphere decreases at the rate of about 10 degrees C for every 1000m of elevation increase.

    SALR- Saturated Adiabatic Lapse Rate: The temperature of a moist atmosphere decreases at the rate of about 6 degrees C for every 1000m of elevation increase.

    In terms of forecasting whether Precipitation will be rain or Snow then a good way is to Start with the 850 MB temperature and have an Approximation of lapse rates for the particular Air Mass/Weather system approaching

    The warmer Air masses and often Warm Fronts will be associated with SALR ( and higher dewpoints)-

    This usually means unexciting adiabatic lapse rates below the cloud base, and weaker convection, so the snow forming in those clouds is smaller in size (and therefore easier to melt on its way down) and warmer than it would be in a cold cumuloform cloud.

    Warm fronts are NOT a good always a good way to produce snow- as they often saturate the air ahead of any PPN band, and because precipitation is often light in nature you end up with light drizzle rather than Snow-

    The Colder air masses with Cold fronts & Convective Clouds are associated with DALR ( and lower dewpoints)

    This means the air is 'dryer' and allows a steeper temperature lapse rate through the lower layers of the atmosphere- Precipitation is often heavier in this situation with Stronger Covection and Vertically heaped clouds-

    Depending on the 850 temperatures Snow is often easier to forecast as its less likely to melt in a warmer layer of atmosphere in the boundry layer-

    The 850 temps can be as high as -3/-4C and surface temps at plus 5C, but still Snow reaches the ground without melting- even if PPN starts out as rain the potential for it to turn to snow through evaporational cooling ( Explained below) is there as the ZDl level ( Zero degree level) is pulled closer to the ground in heavier showers.....

    Another way to consider the Lapse rates in the boundry layers is they are an indication of the Melting strength-

    When lapse rates are small, the melting layer tends to be weak, and a deeper melting layer will be required to melt the snow. When lapse rates are large, the melting layer is strong, and snow will melt within a shallower melting layer

    All in All forecasting Snow using dewpoints, DALR/SALR rates and type of Cloud/PPN intensity SHOULD be good enough to estimate whether you will see snow or Rain- Further tinkering with estimating lapse rates per 100mb ( height/elevation) can give you estimated Snow levels in marginal situations...

    I think last year in the depth of winter- to get SURFACE Daytime maxima from the 850's- I was using a lapse rate of

    Approx 1.3C per 100mb

    So for example- if an 850 temp was -7C the surface obs for sea level would be around 4C-....

    You could then also calculate ( assuming there was no inversion) the height the ZDL would be and the Snow settling level-

    In this instance Taking an average environmental lapse rate is about 6.5 C/km

    or 0.65C per 100m then the Snow level here would be 600m or approx 2000 feet.....

    Evaporational cooling / dynamic cooling

    We often hear the terms 'Evaporational cooling' bantered about the site especially in those borderline situations- so what does it actually mean??


    Two factors that commonly change the temperature of atmospheric layers in winter are evaporative cooling and dynamic cooling. These phenomena chill the atmosphere downward in a way to allow snow to get closer and closer to the surface, until finally, it gets close enough that it doesn't have time to melt through a narrow band of above-freezing air at the ground. In time, that narrow strip of air is often chilled below freezing, too.

    The arctic air masses that move in during the winter are often extremely dry. If precipitation begins falling through that air from a warmer layer aloft, the precipitation evaporates before hitting the ground. This is called virga, and it's why you often see radar echoes over us for hours before anything actually falls to the ground.

    But it takes heat to evaporate moisture, and with every snowflake or raindrop that is evaporated, a tiny amount of heat is removed from the atmosphere. Given enough time and enough precipitation, the cooling can be quite large, several degrees. As the atmosphere moistens, the precipitation falls closer and closer to the surface before evaporating, and thus, the cooling gets closer and closer. Eventually, evaporative cooling can affect the entire atmosphere between cloud and ground, and these few degrees can make a critical difference between rain and snow.

    Dynamic cooling follows a different principle. Cold air is heavier than warm air, so it tends to sink. Warmer air, however, rises, and blocks the cold air from sinking.

    If the precipitation is heavy enough, it acts to pull down the cold air more forcefully, bringing it through the warmer layers to the surface. If this happens with temperatures not much above the freezing mark, it can cause a sudden change from rain to snow in a short time.

    Dynamic cooling can also occur if a surface low-pressure area is able to pull cold air down from the upper levels of the atmosphere.

    As we get late in the winter season and into spring, dynamic and evaporative cooling become more important in wintry precipitation outbreaks. Large-scale cold outbreaks become less frequent, but the atmosphere remains cold from several months of lowered sun angle and winter cold fronts.


    A 'toppler'- many a forecasters phrase referring to how a wintry pattern is expected to develop or in this case- deteriorate.

    Topplers in winters terms are destined 99% of the time for failure and often fail in many ways-

    First of all highlighted here is one of the GFS historic faults- in as much as it often 'overcooks' the Northerly off the back of a deep low cutting East off the North east coast and the Core atlantic ridge being sucked North towards Greenland-

    The premise is a 'deep' Low will have an 'Established' High pressure close by because of the intense advection process-

    On some occasions as a Low pressure crosses out into the North Sea so the atlantic high ridges Northwards almost reaching the southern parts of Greenland- the key here is that the jet stream is still running through the GIN corridoor, but for 1 or 2 days with the absence of a secondary Low pressure a 'nose' or ridge can remain intact-

    In that short space of time depending on how steep the gradient is and how great the cold air advcetion is the UK can be smothered by a cold pool of air straight out of the artic-

    This set up is often the one that delivers a good dose of convective Snow to Northern and Western areas as thats the 'general flow of the wind-

    But very occasionally with enough CAA and a slightly more Southerly tracking low even the South/South east can become a victim-

    The instance that most springs to mind Another 'classic' toppler scenario which spanned 6 days start to finish with a twist in the tail-

    The lead up was an all to typical picure for modern times- the azores high in place and the jet stream to the North, However as this Low passes the jet stream will temporarily become very amplified-

    Jan 27th 2003 Fax chart

    The second day (28th) sees that intense 1055 mb atlantic high begin to retrogress Northwards as the Jet stream flow changes to Meridional and Cold air sweeps first into Iceland-

    Jan 28th 2003 Fax Chart

    The next day sees the first ( OF 3) proper cold day with the -5C 850 HPA covering the country and -7/-8C creeping into Scotland-

    Jan 29th 2003 850's

    This would have seen deep convection cells Pushing South across the country with a 'Good' Covering in northern Scotland from blizzard conditions-

    The 'twist' in the tail on the final day- (and one that is VERY rare as day 3 often sees the return of the atlantic-) sees a trough develop in the flow- travelling down the Eastern side of the UK-

    For this Ive used the Met-Office Bracknell original FAX chart for that day-

    It clearly shows 2 inportant things-

    1) the trough developing in the North-east and-

    2) the Block being broken and effectively the beginning of the end of the cold spell over Iceland....

    Fax chart Thurs 29th Jan 00z

    That trough travelled south and was the 2 inch snow event hat brought the M11 in cambridge to a standstill causing 15 hour traffic jams-

    The Ket to 'ANY' Snow/trough reaching Southern and Eastern areas in these st ups is for the Low pressure that has tracked to the east of us to be on a more Southerly Lattitude that the Atlantic high- If this is the case then the Windflow will be that ever so slightly North-easterly- as was the case in the M11 Fiasco

    By the 31st the atlantic air is already surging into western areas- with the last few wintry showers clearing the South-east-

    Jan 31st 2003 Fax Chart

    And by the next day- the change is complete-

    Feb 01st 2003 Fax chart

    A freezing month-

    Just as the title says a freezing month is where the average temperature for the month is below freezing-

    Since 1900 there have only been SEVEN of these recorded- So as you can imagine they are rare-

    I have been through all these months and whats obvious is the Greenland High pressure being CONSISTENT throughout each-

    What is surprising though to me is how key an oscillation between Low and High pressure over Scandi is Key to prolonging the Cold spells-

    You would have thought that extended cold spells would have the Scandi/Siberian high staying the dominent force all the way through- Not so-

    The best set-ups have always been when the Greenland high Holds strong forcing the Jet stream through the Pole and then surging back south breaking the Block between Greenland & Norway-

    This often Means for Us we remain in a pattern of Easterlies, veering Northerly then once again swinging back Easterly as troughs dive South into Europe-

    The other Caveat is the Greenland high ridges far enough south to hold the STJ south of the UK-

    An example of this is infamous Jan 1963 where the CET was -2.1C

    A few examples of what I mean is below-

    The Month opens with a bitter Easterly already-

    Jan 01 1963

    See the Greenland high is strong, The PFJ is sround and through the artic and the STC is nicely along the channel-

    As we move onto the 5 th- very little change- although the STJ has moved North a couple of Hundred miles, although the Cold is embedded across the uk which means more Snowfall- This may be an end to the cold as the STJ is moving north-

    Jan 05th 1963

    Jan 05th 1963 850's

    The 850's may not look that cold but the thickness levels are good enough to support snow-

    After this the Greenland ridges south again just keeping the gentle North-easterly going-

    10th Jan 1963

    Jan 13th 1963 ( Look at the trough moving south through Norway

    Moving onto the next day that trough is travelling south south-east and advecting Very cold air South west through Norway again-

    Jan 14th 1963

    Jan 14th 1963 850's

    See that cold pooling building up again- its only going one way....

    And onto the seventeenth- a BITTER easterly again, back to square one-

    Jan 17th 1963

    Jan 17th 1963 850's

    Now you would think that this would be the end- but..........

    Whats got to be observed from this chart isnt whats going on over the BI- as it appears that the high 'MAY' slip away SE and introduce milder winds-

    18 th Jan 1963

    Look closely at Greenland - The GH staying firm at 1050mb sending the PFJ around the Artic, this is a time bomb waiting for another southerly burst-

    2 days later we are again back to square one-

    Jan 20th 1963

    although not as cold the thickness levels support yet more snow-

    Jan 20th 1963 850's

    After a brief settled spell moving us onto the 24th to the Close of the month we see another complete repitition of the cycle again-

    Jan 24th 1963

    Jan 26th 1963

    Jan 28th 1963

    Jan 30th 1963

    Jan 31st 1963

    Jan 31st 1963 850's

    In summary- this type of pattern evolves and repeats over around 7 days- and there was 4 repititions over the month-

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  • Location: Leeds (Roundhay) 135m
  • Location: Leeds (Roundhay) 135m

    Very good post steve. Took me a while to read it. I manged to understand most of it. Must of took you ages to do this :angel:

    Look forward to your winter forecast

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