Cold Snap is longest for a decade
9th January 2009
Lyneham entered one of the longest cold snaps for more than 10 years. Cold, mainly dry and frosty conditions, which set in on Boxing Day 2008, continued to last for the next fourteen days as the weather systems were dominated by a large region of high pressure from the continent. This below normal period is the longest prolonged spell of cold weather for this time of year since 1996, with temperatures barely rising above zero centigrade (32°F).

Usually, prolonged cold spells happen in late January and February because the weather in early winter comes from the warm Atlantic Ocean rather than the cold continent. Over the last 20 years, winter temperatures have risen quite substantially; so we have perhaps forgotten what it is like to have this sort of spell of weather.

Not only has the weather been cold, but also for large parts of the country, it has been dry. Over a large part of Britain it hasn't actually rained since 13th December 2008, Three weeks without rain at this time is very unusual and again has not happened since around 1996. The lowest temperature for December 2008 was on New Years Eve as thermometers dropped to -4.6°C while many people decided to celebrate the hogmanay in sub zero temperatures. There was 15 days during the month with air frosts.

The temperatures reached their coldest, -7°C (19.4°F) on Sunday night when there was a slight fluffy of snow during the early morning making local driving conditions hazardous.  With sub-zero temperatures for a long period, even during daytimes, ground temperatures were just right for the snowfall to settle and remain. The lowest temperature of this cold snap was -12°C (10°F), which was recorded in the north of Scotland.

Manchester was the coldest city, with daytime temperatures staying close to -3°C (27°F) for much of the day. London and the South East were less cold.

The low temperatures do not threaten long-term records. Just over a decade ago, in the last days of 1995, new records were set in Scotland, putting the present cold snap into perspective.

On Dec 30 1995, the British national low temperature record of -27.2°C (17°F) was equalled in the small Sutherland village of Altnaharra. The same temperature was previously recorded in 1895 and 1982 in eastern Scotland.

Remarkably, 1995 saw record low temperatures set each day from Dee 27 to 30. Such "date-records" are not that difficult to achieve but this spell of intense cold, just over a decade ago, really shows how much colder the turn of the year can be, and how far from a record temperatures are this year.

Hoar and Rime Frosts
The cold weather of the past week has provided some excellent opportunities to observe the development of white frost.

It is important to distinguish between the two main types of white frost: hoar and rime. Hoar frost comprises ice crystals, often in the form of feathers or needles, which are seen on surfaces cooled by outgoing radiation - that is, the loss of heat energy to outer space which has its greatest effect at night. The frost may be composed in part of drops of dew frozen after formation, and by ice crystals deposited from water vapour in the atmosphere when the temperature is below freezing

Some materials cool more quickly than others while objects close to the ground cool more quickly than those higher up. The metal and glass of your car are very efficient "radiators", so they are among the first things to develop a coating of frost. Blades of grass become frosted more quickly than concrete or asphalt because they are not significantly affected by the conduction of heat from below ground, whereas hard surfaces are.

Outgoing radiation is inhibited by cloud and fog, while the collection of cold air in the lowest few feet of the atmosphere is easily dispersed. By contrast, the higher the air humidity, the more water vapour is available for sublimation into ice crystals.

Thus the heaviness of the frost is not an indication of how cold it has been. Quite the contrary, in fact. The most copious deposits of hoarfrost occur under cloudless skies with no wind when the air temperature is close to 0°C (32°F) and the relative humidity near 100 per cent.

Rime is deposited by fog. Even when the temperature is several degrees below freezing, fog is comprised of tiny water droplets. But these droplets freeze immediately when they come into contact with other objects. Such a freezing fog will deposit rime on everything - trees, fencing, aerials, telephone lines. Again the abundance of the deposit is not related to the temperature.

These weather conditions make life for aviators very hazardous. Flying through snow crystals or snowflakes will not form ice but may form a line of heavy frosting on the wing leading edge at the point of stagnation, which could increase stalling speed on landing. Flying through wet mushy snow, which is a mixture of snow crystals and supercooled raindrops, will form pack snow on the aircraft. With these extreme weather conditions we have experienced on the ground, the maintenace crews at RAF Lyneham have additional pre-flight tasks ensuring all the frost is removed from the aircraft wing edges and flying controls to make the aircraft safer for the aircraft flight. Ice accretion on the wing leading edge is a major concern for aircraft for take off. Airflow disruption will reduce the maximum lift coefficient attainable thus raising the stalling speed considerably. Crews are meticulous to ensure the wings are frost free for the task. Because the aircraft has to fly at a greater angle of attack to maintain lift, the induced drag also increases and the aircraft continues to lose airspeed, making it impossible to sustain altitude if the stall is to be avoided. Fuel consumption will also increase considerably.