UK under snow as seen from space

The images were captured by the Advanced Along-Track Scanning Radiometer (AATSR) instrument on board ESA's ENVISAT satellite on 7th and 8th January. AATSR is a dual-view, multi-channel, imaging radiometer with 1 km spatial resolution and a 512 km swath width.

The AATSR is funded by the Department for Energy and Climate Change (and previously funded by the Department for Food, Rural Affairs and Environment) with the platform and data processing provided by the European Space Agency.

In both images, snow can be seen lying in most UK locations, with major cities standing out as grey areas against the white snowy background. On January 7th, western Ireland was snow-free, which reflects the position of warmer air to the west over the Atlantic -- this air was blocked by cold air over the UK. On 8th January, clouds bringing further snow showers to many eastern areas, including Leicester, can be seen -- the shadows cast by these clouds are also visible. Unaffected areas and regions where thawing has occurred are indicated by red shades which are especially visible in coastal locations in the south and west, due to the warming influence of the Atlantic ocean current. These powerful images clearly demonstrate the utility of Earth observation satellites for monitoring such events.

Dr John Remedios, Head of the EOS group at the University of Leicester, said:

"These are wonderful images produced by a first class UK instrument. They illustrate how the landscape of the entire country can be dramatically altered by changes in the circulation of the atmosphere. One is particularly struck by the contrast between the UK and the western edge of Ireland where the air was warmer. Shifts in the boundaries between warm Atlantic air and cold polar air have a strong impact on the practical and economic life of the UK, as we have seen over the last few weeks. We will be using the AATSR images to examine the pattern of cold temperatures across the UK so that we can investigate in more detail which regions were worst affected by the cold spell."

According to Philip Eden, Vice President of the Royal Meteorological Society, it has been the most prolonged spell of freezing conditions across the UK since the mid 80s. The immediate drivers for this extreme weather are:

• Prevailing weather patterns (synoptic situation): The cold weather was a result of a 'blocking' high-pressure system which prevented milder Atlantic air from moving across the UK causing cold air to be fed down from the Arctic, whilst weather systems moved in from the north-east bringing moist air to the UK.
• Strength and position of the Polar Front Jet Stream: This is the narrow, fast-flowing, conveyor-belt of winds in the upper atmosphere which drive weather systems across the Atlantic -- these systems deliver weather to the UK. They are formed by temperature differences between the cold polar air and the warm tropical air, and are usually strong at this time of the year, forcing a series of depressions (systems that bring unsettled weather) over the UK. However, the jet stream is relatively weak at present, meandering widely as it flows from west to east (from the tropics to the poles and back to the tropics) -- during early January, the UK just happened to be on the 'cold side' of this conveyor-belt. Consequently, while Altantic depressions were being forced down over Portugal and into the Mediterranean, cold north-easterly winds were feeding down to the UK from the Arctic and Russia.

Fluctuating global climate anomalies which describe variations in atmospheric pressure patterns may provide further explanation:

• Arctic Oscillation (AO): The AO refers to the changes in the strength of the atmospheric pressure (measured at sea level) in the Arctic and the mid-latitudes. It is currently in an extreme negative phase -- above-normal pressure over the polar region and below-normal pressure over the mid-latitudes. This results in smaller pressure differences, and these weaker systems allow cold air to flow south. This cold blast resulted in wintery weather in Northern Europe.
• North Atlantic Oscillation (NAO): The NAO is the difference in pressure between Icelandic low and the Azores high, and is closely linked to the AO. The NAO also currently in a negative phase (above-normal pressure over Iceland; below-normal pressure over the Azores). In Northern Europe this is usually associated with below-normal temperatures, in addition to below-normal precipitation as westerlies are suppressed.
• El Nino Southern Oscillation (ENSO): This is the cyclic (3-7 years) Pacific Ocean phenomenon in which trade winds diminish and sometimes reverse in direction, causing warm air to rises in the eastern Pacific, altering global weather patterns. This can change the strength and position of the jet stream in the Northern Hemisphere, shifting it southward over the UK, favouring an increase in precipitation; however this usually brings milder, wet weather to the UK.

'Teleconnections' -- relationships between large scale climate anomalies in different parts of the world and regional weather patterns -- are naturally-occurring aspects of the chaotic atmosphere. They are well documented in many parts of the world, and are continually monitored in order to better understand them and improve climate predictions.

In reality, the recent extreme weather is likely to be a result of a combination of factors, and is certainly not evidence that 'climate change' has ceased -- such year-to-year, regional anomalies are intrinsic; it is the long-term, global trend that is important when assessing climate change. This recent extreme weather does, however, serve as a clear reminder that a warming planet does not mean the end of cold weather events!