A touch of frost : that’ll be the anticyclone that’s come to stay for a bit.

Last night was the chilliest in many parts of Britain for over three years. What were the conditions that brought it about - and why could it persist for much of the rest of the week? A High Pressure airmass has settled over Britain, and it means the cold snap is set to continue for a few days - and nights - yet. http://www.bbc.co.uk/news/uk-scotland-30870799

Star-spangled, cold, frosty nights; clear, blue, sharp days with few clouds to obscure the low arc of a winter sun slewing across the southern sky – it must be a High Pressure system that has landed, and will hang around for a few days yet. The changeability of Britain’s weather is due to a combination of factors: a group of islands to the west of a continental landmass, at the interface of polar air to the north and tropical air to the south, and on the flight path of an oscillating jet-stream. Throw in the Gulf Stream approaching from the south west and you have the recipe for a mixed bag of weather. The passage of consecutive depressions – areas of Low Pressure – give rise to bands of cloud and rainfall interspersed with brighter, sunnier periods that can alternate over hours. But when high pressure moves in, it all settles down for a prolonged period of time, days or even weeks. It’s what yields those consecutive nights of frost in winter, and those longed-for wall-to-wall blue-skied sunshine days of summer.

High pressure builds from air descending from on high; literally a forcing up of the pressure at the ground as air piles down from above. Known as Anticyclones, the conditions within the airmass feature clear skies and limited moisture (air that is coming from the dry upper atmosphere and that warms as it descends does not produce much in the way of clouds), very light – if any – wind (anticylones can be thousands of miles across with relatively little pressure difference within them, so there is minimal pressure-gradient to form air movement) and relative stability over a period of time as more changeable depressions are lower-atmosphere features that are diverted around anticyclones (hence the term ‘blocking anticyclones’).

In winter the short hours of daylight and long period of night means there is a net loss of radiation from the ground so cold conditions can intensify as the anticyclone lingers. Frost can persist all day and fog can become a feature with temperature inversions creating a chilled layer of moist air next to the chilled ground (fed with moisture from the soil, vegetation or rivers) that is cooler than the descending air above. (If you’re near a power station, you may see the water vapour from the cooling towers form a chilled horizontal cloud that is unable to rise into warmer air above).

Much of Britain experienced its chilliest night for three years on the night of the 18^th/19^th January 2015 under these conditions and you can begin to see why it could be like this for the rest of the week.

This website shows the current winds anywhere on the planet. You will be able to identify depressions and anticyclones from the pattern and intensity of winds: http://earth.nullschool.net