Latest snow depths and wind speeds – 24 March

As forecast, we have seen some very wintry weather over the last few days. Here are some figures showing where the most snow has fallen.

Wittering in Cambridgeshire saw the deepest snow, with 24 cm recorded at 9 am this morning.

Many areas also saw strong winds, with a gust of 61 mph recorded at Shap, Cumbria and 48 mph recorded at Machrihanish, Argyll . These winds have caused even deeper drifts of snow in some areas.

Snow depths at 9 am 24 March

TIME	SITE NAME	AREA	ELEVATION (m)	SNOW DEPTH (cm)
09:00	WITTERING	CAMBRIDGESHIRE	73	24
09:00	WATTISHAM	SUFFOLK	89	20
09:00	NOTTINGHAM, WATNALL	NOTTINGHAMSHIRE	117	19
09:00	SHAWBURY	SHROPSHIRE	72	14
09:00	ANDREWSFIELD	ESSEX	87	12
09:00	CRANWELL	LINCOLNSHIRE	63	10
09:00	LEEK, THORNCLIFFE	STAFFORDSHIRE	298	7
09:00	ABOYNE	ABERDEENSHIRE	140	7
09:00	HIGH WYCOMBE	BUCKINGHAMSHIRE	204	7
09:00	SENNYBRIDGE	POWYS	307	7
09:00	SHAP	CUMBRIA	252	7
09:00	COLESHILL	WARWICKSHIRE	96	6
09:00	HAWARDEN AIRPORT	CLYWD	11	6

March – a month of weather contrasts

Winter seems to have hung on for quite some time this year with low temperatures, frost, ice and snow affecting many areas into late March. This isn’t altogether unusual as we are more likely to see snow at Easter than at Christmas. However, March 2012 was very different with plenty of sunshine and temperatures into the low 20s Celsius. How come?

Well, this time last year the UK was under the influence of high pressure. This gave us clear skies, plenty of sunshine and with a light southerly breeze, temperatures that were well above average. In fact, Scotland set an all time record maximum temperature with 22.8 °C at Fyvie Castle in Aberdeenshire.

Visible satellite image from March 2012

This year, with a strong easterly wind bringing cold air from Scandinavia and Eastern Europe, we have quite the opposite with eastern parts of the UK in particular seeing snow, ice and temperatures around 20 degrees Celsius lower.

Visibile satellite image from March 2013

The direction of the wind therefore plays a major part in what type of weather you and I will see, especially as we have the Atlantic Ocean to our west and continental Europe to our south and east. Different wind directions bring air with different temperature and moisture contents. Meteorologically, they are termed air masses and in March 2012 we saw a Tropical Continental air mass bringing dry and warm air from the Mediterranean. This year we have been affected by a Polar Continental air mass, bringing cold air from the east. The following video explains exactly what we mean by air masses.

With different air masses constantly affecting the UK, the weather is a particularly challenging thing to forecast, especially so in March. This is because in early spring the sun is starting to rise higher in the sky and the amount of daylight hours start to increase. This means we get more heat building up in the lower part of our atmosphere. The result is slightly more energy, which in turn can lead to heavier showers. We can also see more unstable air and more active fronts as a result of greater heating. With more moisture available in the atmosphere, we also tend to see heavier or more prolonged rainfall and if this mixes with cold air, more snowfall. It makes forecasting more complicated because the extra heat and moisture adds another aspect to the weather, which tends amplify the effects of different air masses.

You can find out more about forecasting snow on our website or on the following video:

Spring swing brings colder weather and snow

We’ve had some very mild conditions this week with welcome sunshine pushing temperatures into the high teens. However, in a classic spring swing, colder weather is on the way as we head into the weekend.

By Saturday, we will see a return of easterly winds which will bring in much colder air from Scandinavia and Eastern Europe. Snow is expected across some eastern parts of the country over the weekend. By the start of next week, most of the UK will see daytime highs in low single figures with some frosty and icy nights.

So how unusual is it to see cold weather and snow in March?

The UK’s weather is very much at the mercy of where our winds come from, and throughout spring we can see sudden swings in the weather conditions. If we look back to last year we had very high temperatures at the end of March as the UK was under the influence of high pressure and light south-easterly winds. This year, this week’s south-easterly winds are now giving way to colder easterlies.

What about snow?

Statistics show that snow is more likely in March than around Christmas. As we know, heat from the sun increases as we head towards summer and this can lead to some interesting weather in March. With more heat from the sun the ground warms up more quickly and gives very unstable air, which can lead to a greater number of showers. Warmer air also holds more moisture so showers can give heavier rainfall. If this combines with cold air we can potentially see some heavy snowfall. However, easterly winds tend to be dry and so substantial snow fall is not expected over the next week.

As always, the Met Office will be working with different agencies to keep Britain on the move, and to keep people safe and well during periods of cold weather. The latest forecasts and warnings can be found online, through our mobile apps and through TV and radio broadcasts.

It’s cold but why is there no frost?

There’s no denying that we have seen some cold weather this winter with plenty of frost, ice and in many cases, snow. However, the last week has been cold – arguably perhaps feeling colder than any other time this winter – but we haven’t seen any evidence of this on the ground in the way of frost. So how is this possible?

For a classic frosty night we need a few ingredients: low temperatures, clear skies, calm winds and moisture. A clear, calm night gives excellent radiation conditions – by this we mean that the heat absorbed by the Earth’s surface during the day escapes readily back into space and allows temperatures to fall. If the temperature falls to the dew point (the temperature to which air must cool for it to become saturated with water vapour) moisture will condense and form droplets on the ground’s surface. When temperatures fall below freezing the droplets freeze and we get frost.

So what about the last few days? They have been cold but there hasn’t really been any prolonged or hard frost. How come? Well, much of Scotland and Northern Ireland has had the required ingredients and been frosty, but the rest of the UK has only had low temperatures. Much of England and Wales have seen a fair amount of cloud and some brisk winds.

Surface pressure chart from 25 February 2013

Cloud acts as a blanket and although temperatures have fallen during the night-time, cloud cover has stopped them falling well below freezing and therefore made it difficult for a thick frost to form. The wind is also important as it mixes the lower part of our atmosphere. Rather than having cold air pooling in one place and causing low temperatures, the wind can bring less cold air from another location or even bring it down from the upper atmosphere. This also helps to keep temperatures from falling too low. However, easterly winds this week have certainly made it feel very cold indeed!

Visible satellite image from 25 February 2013

Lastly, the air near the surface has been relatively dry. This is important because it means the temperature of the air must fall very low in order to reach its dew point. The cloud and wind has stopped this from happening easily and therefore reduced the risk of frost.

Cold weather, then, brings lots of different tastes of winter, especially to the UK, and we have seen nearly all of them this season. More information on all types of weather can be found here.

Latest snow depths and wind speeds – 5 February

As forecast, unsettled wintry conditions brought snow and strong winds to parts of the UK overnight and this morning.

Eskdalemuir saw the deepest snow, with 14 cm of snow recorded at 10 am this morning, while Aviemore recorded 12 cm.

Many areas also saw strong winds, with a gust of 78 mph recorded at Culdrose, Cornwall and 99 mph recorded at Cairngorm Summit.

Snow depths at 10 am 5 February

TIME	SITE NAME	AREA	ELEVATION	SNOW DEPTH ( CM)
10:00	ESKDALEMUIR	DUMFRIESSHIRE	236	14
10:00	AVIEMORE	INVERNESS-SHIRE	228	12
10:00	DRUMALBIN	LANARKSHIRE	245	10
10:00	GLENANNE	ARMAGH	161	9
10:00	TULLOCH BRIDGE	INVERNESS-SHIRE	249	7
10:00	REDESDALE CAMP	NORTHUMBERLAND	211	7
10:00	BALLYPATRICK FOREST	ANTRIM	156	5
10:00	SPADEADAM	CUMBRIA	285	5
10:00	THOMASTOWN	FERMANAGH	72	3
10:00	BINGLEY	WEST YORKSHIRE	262	2
10:00	ALBEMARLE	NORTHUMBERLAND	142	2
10:00	WADDINGTON	LINCOLNSHIRE	68	1
10:00	SHAWBURY	SHROPSHIRE	72	1

Maximum gust speeds 5 February

TIME	SITE NAME	AREA	ELEVATION	MAX GUST SPEED (mph)
00:00	CULDROSE	CORNWALL	76	78
04:00	SCILLY ST MARYS AIRPORT	ISLES OF SCILLY	31	75
00:00	CHIVENOR	DEVON	6	67
03:00	ISLE OF PORTLAND	DORSET	52	66
04:00	JERSEY AIRPORT	JERSEY	84	66
03:00	GUERNSEY AIRPORT	GUERNSEY	101	64
00:00	CAMBORNE	CORNWALL	86.85	62
02:00	SOUTHAMPTON, OCEANOGRAPHY CENTRE	HAMPSHIRE	26	62
01:00	SOUTH UIST RANGE	WESTERN ISLES	4	62
00:00	AVONMOUTH	AVON	9	62
00:00	CARDINHAM, BODMIN	CORNWALL	200	61
01:00	TIREE	ARGYLL	9	60
03:00	WIGHT: ST CATHERINES POINT	ISLE OF WIGHT	20	60
01:00	WIGHT: NEEDLES OLD BATTERY	ISLE OF WIGHT	80	60
02:00	YEOVILTON	SOMERSET	20	60
01:00	CAPEL CURIG	GWYNEDD	216	59
01:00	ABERDARON	GWYNEDD	95	59
04:00	ISLAY: PORT ELLEN	ARGYLL	17	58
01:00	LERWICK	SHETLAND	82	58
00:00	MUMBLES HEAD	WEST GLAMORGAN	43	56
01:00	ODIHAM	HAMPSHIRE	118	56

Maximum gust speeds – mountain sites

TIME	SITE NAME	AREA	ELEVATION	MAX GUST SPEED (mph)
09:00	CAIRNGORM SUMMIT	INVERNESS-SHIRE	1237	99
07:00	CAIRNWELL	ABERDEENSHIRE	928	86
08:00	AONACH MOR	INVERNESS-SHIRE	1130	75
09:00	BEALACH NA BA	ROSS & CROMARTY	773	67
04:00	GREAT DUN FELL	CUMBRIA	847	56
10:00	GLEN OGLE	PERTHSHIRE	564	54

Warnings for ice, snow and wind remain in place in some areas.

Overnight wind speeds – Wednesday 30 January

Following Met Office amber weather warnings for wind in some parts of the UK yesterday, some high gust speeds were recorded late last night and into the early hours of the morning. The highest gust speed at lower levels was recorded at Fair Isle, Shetland which saw speeds of 86 mph this morning, while the highest gust recorded at a mountain site was 135 mph at Cairngorm Summit.

Max gust speed at lower level sites

Date/time	Site name	Area	Elevation	Max Gust Speed (mph)
30/01/2013 06:00	Fair Isle	Shetland	57	86
30/01/2013 05:00	Lerwick	Shetland	82	84
30/01/2013 07:00	Kirkwall	Orkney	26	82
30/01/2013 01:00	Loch Glascarnoch	Ross & Comarty	269	79
30/01/2013 01:00	Stornoway Airport	Western Isles	15	78
30/01/2013 04:00	Wick Airport	Caithness	36	75
30/01/2013 00:00	South Uist Range	Western Isles	4	75
30/01/2013 00:00	Tain Range	Ross & Cromarty	4	70
30/01/2013 05:00	Altnaharra	Sutherland	81	68
30/01/2013 03:00	Wight: Needles Old Battery	Isle Of Wight	80	68

Max gust speed recorded at mountain sites

Date/time	Site name	Area	Elevation	Max gust speed (mph)
30/01/2013 00:00	Cairngorm Summit	Invernessshire	1237	135
30/01/2013 01:00	Aonach Mor	Invernessshire	1130	110
30/01/2013 07:00	Cairnwell	Aberdeenshire	928	105
30/01/2013 00:00	Bealach Na Ba	Ross & Cromarty	773	101
30/01/2013 02:00	Glen Ogle	Perthshire	564	77

Warnings remain in place for wind in some areas, you can keep up to date with the latest severe weather warnings on our website.

The worst storm in years?

Various articles in the news today said that the weather over the weekend was the worst storm to hit the UK in years, and that there is more to come this week. There was indeed a very deep area of low pressure in the Atlantic over the weekend. At its deepest, on Saturday 26 January, the central pressure of the depression was 932 millibars and it was sitting some 1,800 nautical miles west of the UK. It came closest to the UK during the day yesterday with a central pressure of 950 millibars but was still around 600 nautical miles to the north west of Scotland.

Satellite image from 26 January 2013

To put this into context, the storm that affected the UK on 3 January 2012 had a central pressure of 953 millibars but was centred right on the west coast of Scotland and brought winds in excess of 80 mph to the Central Belt and a gust of over 100 mph in Edinburgh. Property was damaged, as well as trees, and there was disruption on the road network and with ferry crossings. Power supplies were also affected significantly.

The storm in January 2012 was therefore much more disruptive and severe than any wet and windy weather we have seen so far this year.

Much of the recent severe weather has been attributed to the phrase “Weather Bomb”, which is not a perfect meteorological term but is defined as an intense low pressure system with a central pressure that falls 24 millibars in a 24-hour period. This happened to the depression over the Atlantic during the weekend but as it was miles away from the UK its impacts were minimal. A better description can be more directly linked to the meteorological phenomena known as rapid cyclogenesis. This is where dry air from the stratosphere flows into an area of low pressure. This causes air within the depression to rise very quickly and increases its rotation, which in turn deepens the pressure and creates a more vigorous storm.

Infographic: 2012 weather review of the year

Hover over the image to link through to more detail on the UK weather in 2012.

How did the ‘Great Storm’ of 1987 develop?

From the trail of devastation left by the ‘Great Storm’ of 1987, it’s clear that it was an unusual event.

Analysis of the storm suggests there had been nothing like it since 1703 and that it was an event so rare you would only expect a storm of that magnitude once every 200 years.

That does need clarifying, however, as we have seen storms as powerful as that before and since then – but they have affected areas which are more used to stormy weather such as the far north of the UK (like the north coast of Scotland) and far South West (like the Isles of Scilly).

So what was truly unusual about this storm was that it affected the South and East of England – which had an important bearing on the impact of the storm.

But how did the storm develop?

Initial phases

Most Autumnal storms head in from the Atlantic to the west of the UK, but this storm developed over the Bay of Biscay to the south.

It started as particularly warm tropical air and very cold polar air collided, forcing the warm air to rise and creating an area of low pressure.

The big difference in temperature between the warm and cold air helped to cause rapid ascent and therefore particularly low pressure – at one point it measured 951mb over the English Channel.

Crucially, just to the west of the low, pressure rose rapidly (due to descending air), to leave a big differential in pressure. You can see the difference in pressure in the tightly packed isobars in the (hand-drawn) chart from the early hours of 16 October 1987, below.

Surface pressure chart for the morning of 16 October 1987

The atmosphere naturally tries to even out this pressure imbalance with the air flowing from the high pressure towards the low pressure – what we feel as wind

Much like water flowing down a plughole, that air doesn’t rush in straight lines but spins around the centre of a low pressure until it reaches the middle due to the Coriolis effect.

The bigger the difference in pressure between the high and the low pressure, the faster the flow of air is – and in this case that big differential led to hurricane force winds.

A sting in the tail

We now know that the strength of the storm was boosted by a phenomenon known as the ‘Sting Jet’, where cold dry air descends into storms high in the atmosphere.

Rain or snow falling into this jet of air evaporates and cools the air further, adding more energy which translates into stronger winds. By the time this ‘sting in the tail’ reaches the ground it can produce winds of 100mph which are concentrated over a small area.

In 1987, no-one knew sting jets even existed, but now they are well understood and included in forecast models. The storm which affected Scotland in December 2011 was boosted by a sting jet, explaining the maximum gust speed of 164mph recorded on top of Cairngorm.

The combined impact

It’s clear that several factors came together to make this storm particularly ferocious, but it was the track of the storm which was perhaps most significant.

Arriving on the south coast of the UK, it tracked north and east over the course of several hours before reaching the Humber estuary at about 5.30am.

This path took in a large, built-up and very populated part of the UK which exacerbated the damage caused.

Storm caused by most intense low to cross UK in September in 30 years

The low pressure system that has brought heavy rain, strong winds and flooding to the UK is the most intense to cross the UK in September for more than 30 years, with the lowest air pressure of 973mb being recorded on Tuesday morning.

Pressure chart at 6am on 25 September 2012

To find a similarly intense low pressure system that affected a wide part of the UK in September you need to go back to 1981, when pressures below 970mb were reported across central parts of the UK.

Like this week, this low pressure system brought unsettled weather as it crossed the British Isles – tracking east over the Isle of Man before heading north to Cumbria, Northumberland, eastern Scotland, Orkney and Shetland.

But what do we mean by ‘the most intense’? The intensity of a low pressure system is measured as the lowest pressure recorded at the centre of the system, as this gives an indication of how active it may be. This will relate to the rainfall amounts and wind strengths associated with it.

However, pressure is only one indicator of how much wind and rain there will be, so it is possible that other systems have resulted in stronger winds or heavier rain in some places than we have seen over the last few days.

Although the storm we have seen this week is certainly unusual in that it crossed central parts of the UK, some parts of the UK have seen pressure systems of this kind of intensity many times before at this time of year. In fact, Met Office records show some 31 occurrences of pressure below 975mb being observed in the UK in September, but the vast majority of these were confined to north and west Scotland, Northern Ireland or the far west of England.

For example a deep low affected the northwest of Scotland with pressure as low as 972mb as recently as 12 September 2011, whilst the Isles of Scilly and part of Cornwall saw pressure as low as 966mb on 7 September 1995. So, with regard to the system which has recently affected the UK, the key to what makes it remarkable is that it has tracked over a wide area of the UK rather than those areas which are more used to storms of this intensity.