Why does it feel so cold? A guide to ‘feels like’ temperatures

24 02 2013

With bitterly cold conditions across parts of the UK just now, have you been asking yourself why it can feel so much colder in the wind?

The temperatures that you normally see on our website represent the temperature of the air, but this takes no account of how we actually experience the temperature. It is our ‘feels like’ temperature that gives you a better idea about how the weather will actually feel when you step outdoors.

Our ‘feels like’ temperature takes into account wind speeds and humidity to assess how the human body actually feels temperature.  For example in winter a strong wind can feel much colder than the measured temperature would indicate. The impacts of the temperature and wind combined can be much greater  than each on their own and the feels like temperature allows users to make a better assessment of conditions outdoors.

But how do you actually calculate the ‘feels like’ temperature?

We calculate a ‘feels like temperature by taking into account the expected air temperature, relative humidity and the strength of the wind at around 5 feet (the typical height of an human face) combined with our understanding of how heat is lost from the human body during cold and windy days.

On windy days the speed of moisture evaporation from your skin increases and serves to move heat away from your body making it feel colder than it actually is. The exception to this rule, however, is when higher temperatures are concerned. At higher temperatures, wind chill is considered far less significant. Instead humidity plays a greater role. When a human being perspires, the water in his or her sweat evaporates. This results in the cooling of the body as heat is carried away from it. When humidity is high, the rate of evaporation and cooling is reduced, resulting in it feeling hotter than it actually is.

Using these facts we use a formula to adjust the air temperature based on our understanding of wind chill at lower temperatures, heat index at higher temperatures and a combination of the two in between.

You can get ‘feels like’ temperatures on our five day forecast and on our Android, iPhone and Windows Phone apps.

The meteorology behind the ‘Beast from the East’

7 12 2012

Update: See how the weather situation for the coming week has developed over the weekend: How the ‘pest from the west’ will beat the ‘Beast from the East’

The ‘Beast from the East’ – not the Grammy nominated live album of 1988 from heavy rock band Dokken – but the phrase being used to describe what looks set to be some cold and wintry conditions to the UK next week has been mentioned in many newspapers today.

But what actually is the meteorological situation and what is the outlook for next week?

Strictly speaking our weather is not expected to be coming directly from the east next week, but more from the northeast, tracking across the North Sea from Scandinavia, bringing cold north to northeastly winds to our shores. As this cold air moves over the relatively mild North Sea, the air will pick up moisture and become increasing unstable, bringing scattered showers to eastern parts of the country as shown on the chart below.

Forecast chart 1200 Monday 10 December 2012

The showers are most likely across eastern parts of Scotland and northern England on Monday.

Although next week will certainly be very cold, especially compared to a brief respite from the cold conditions over the coming weekend, at the moment it is not expected to be as cold as the bitter conditions we saw back in December 2010 when temperatures fell to -21.3 deg C on 2nd December at Altnaharra in Scotland, which was the lowest December temperature recorded in the UK since 13th December 1995.

Met Office forecasters will be monitoring this developing weather situation throughout the weekend and will update forecasts and warnings. This will ensure the public has access to the latest weather forecasts that will help them prepare, plan and protect themselves from the impacts of the snow and icy conditions expected next week.

Met Office Cold Weather Alerts have been issued for the whole of England as the forecast weather could increase the health risks to vulnerable patients.

The latest forecasts and warnings can be found on the Met Office website, on our mobile apps and through TV and radio broadcasts on the BBC and ITV.

Cold weather clue in upper atmosphere

4 12 2012

During a period of colder than average weather across northern Europe in February this year we issued a blog discussing potential causes – including changes in circulation high up in the atmosphere.

We also discussed these disruptions, known as Sudden Stratospheric Warmings (SSWs), in a news release about advances in predicting these events in our long-range guidance for winters.

SSWs happen when the usual westerly winds in the stratosphere, between 10km and 50km up, are disrupted, break down and even reverse.

This signal can then burrow down to the lower atmosphere over the course of a few days to a few weeks, thereby reducing the westerly winds at lower levels.

This illustrates how the reversing of winds in the stratosphere 'burrows' down to surface level.

Figure illustrates how reversal of winds in the stratosphere ‘burrows’ down to surface level.

For the UK in winter, that means a disruption to the westerly flow that usually brings mild air from the Atlantic and there is a potential to allow easterly winds to take hold, bringing in cold air from the continent.

So SSWs can herald cold weather ahead. However, there are variations in their magnitude – sometimes they are fairly minor, just a ‘wobble’ in the flow of the stratospheric winds, but sometimes they are more pronounced, with a complete reversal of the pattern of winds.

The more significant the SSW, the more likely it is to have an impact at the surface and also the greater the potential impact.

Met Office observation systems have picked up a minor SSW in the stratosphere over the past few days, suggesting that this may have an impact on the UK.

Jeff Knight, a Climate Scientist at the Met Office, said: “Satellite and other observation data show that there is a minor SSW going on and this is one factor amongst many others which could perpetuate the colder than average conditions we have seen recently.

“It could take anything from a few days to a few weeks if it is going to have an impact. However, it’s consistent with the current 30-day outlook from the Met Office which favours colder than average conditions – albeit with a fair amount of uncertainty.”

The Met Office will continue to monitor the situation and, as ever, will keep everyone up to date on any periods of cold weather through our forecasts and warnings.

Freezing conditions continue as temperatures fall to -11.8 C last night

8 02 2012

The freezing weather continued to grip parts of the UK overnight once again with the lowest temperatures across North Yorkshire and other eastern parts of the UK where cloud breaks persisted through the night. The lowest temperature was -11.8 deg C at Church Fenton in North Yorkshire, where 12 cm of snow is still lying following the snow last weekend. The coldest place in Scotland was Cromdale with -9.5 deg C, whilst in Wales, Sennybridge in Powys fell as low as -7.7 deg C

The table below shows a list of the coldest spots across the country overnight into Wednesday 8th February.

CHURCH FENTON                          -11.8 NORTH YORKSHIRE     
LINTON ON OUSE                         -10.7 NORTH YORKSHIRE     
TOPCLIFFE                              -10.3 NORTH YORKSHIRE     
CROMDALE                               -9.5 MORAY               
BUNTINGFORD                            -8.9 HERTFORDSHIRE       
DISHFORTH AIRFIELD                     -8.8 NORTH YORKSHIRE     
LEEMING                                -8.6 NORTH YORKSHIRE     
SCAMPTON                               -8.5 LINCOLNSHIRE        
MARHAM                                 -8.2 NORFOLK             
SOUTH NEWINGTON                        -8.2 OXFORDSHIRE         
BRAMHAM                                -8.2 WEST YORKSHIRE      
HIGH MOWTHORPE                         -8.1 NORTH YORKSHIRE     
LECONFIELD                             -8.1 HUMBERSIDE          
SANTON DOWNHAM                         -7.8 NORFOLK             
CRANWELL                               -7.8 LINCOLNSHIRE        
SHOBDON AIRFIELD                       -7.8 HEREFORD & WORCESTER
WOODFORD                               -7.8 CHESHIRE            
MALHAM TARN                            -7.7 NORTH YORKSHIRE     
SENNYBRIDGE                  -7.7 POWYS               
ASTWOOD BANK                           -7.7 HEREFORD & WORCESTER
NORMANBY HALL                          -7.7 HUMBERSIDE          
LITTLE RISSINGTON                      -7.6 GLOUCESTERSHIRE     
NOTTINGHAM, WATNALL                    -7.5 NOTTINGHAMSHIRE     
LEEK, THORNCLIFFE                      -7.5 STAFFORDSHIRE       
COTON-IN-THE-ELMS                      -7.4 DERBYSHIRE          
DUNKESWELL AERODROME                   -7.4 DEVON               
SHAP                                   -7.4 CUMBRIA             
BEDFORD                                -7.3 BEDFORDSHIRE        
HOUGHTON HALL                          -7.2 NORFOLK             
LISCOMBE                               -7.2 SOMERSET            
FYLINGDALES                            -7.1 NORTH YORKSHIRE     
COLESHILL                              -7.1 WARWICKSHIRE        
NEWPORT (SALOP)                        -7.1 SHROPSHIRE          
PERSHORE                               -7.1 HEREFORD & WORCESTER
MONKS WOOD                             -7.1 CAMBRIDGESHIRE      
PENNERLEY                              -7.1 SHROPSHIRE          
TREDEGAR, BRYN BACH PARK         -7 GWENT               

The cold weather persists with the brightest weather across southwest England, Wales and northern England. A few snow flurries possible across the far southeast of England. Further north, Western and northern Scotland and Northern Ireland will see outbreaks of rain, possibly falling onto frozen surfaces in northwest Scotland. Tonight, there is an increasing chance of seeing freezing rain across parts of southern and eastern Scotland, bringing a significant ice risk. England and Wales remaining generally dry with a severe frost once again. On Thursday rain and snow is forecast to edge into northern England, generating an ice risk here. It will remain very cold across England and Wales, with the best of any brightness further west, although it will be milder in the north.



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