High pressure often brings settled conditions, but can also create notable temperature differences across regions with differing topography. This is largely due to temperature inversions - thin layers in the lower atmosphere where the temperature increases sharply with height, rather than the normal decrease as you move upwards. It forms a barrier to vertical motion, and traps cool moist air below the inversion with warm, dry air just above. These inversions can also trap layers of low cloud, mist and fog close to the ground, sometimes producing several days of "anticyclonic gloom" - they can also affect your TV and radio signals, as Jim Bacon explains. Over several days, inversions can also lead to a build-up of dust and pollutants, because often there is little wind to disperse these particles trapped near the ground.
Under a large area of high pressure, air gradually sinks and therefore warms, creating an inversion. If the height of an inversion is low enough, sometimes the tops of hills or mountains will be above it, making them much warmer than the valleys or lowland areas within the cold, moist air that has been trapped beneath these inversions.
Case Study: Saturday 26 November 2016
After a long night of radiative cooling, many lowland sites across northwest England fell widely below freezing by morning. However, some high ground stations were reporting temperatures much warmer than their lower-elevation neighbours. This was due to a temperature inversion, which is clearly depicted on the midnight radiosonde ascent below (a plot of temperature and dewpoint change with height) from Castor Bay in Northern Ireland (fairly representative of the type of vertical profile that was over northwest England that morning). The temperature inversion is found where the temperature trace (red line) becomes flat / horizontal.
Castor Bay radiosonde ascent annotated to highlight inversion, altitude of each weather station and corresponding temperatures
Observed temperatures at 8am
The closer the temperature and dewpoint lines are on a radiosonde plot, the moister the air is - below the inversion both temperature and dewpoint traces are more-or-less identical, implying very moist (and cold) air trapped beneath the inversion. Aloft, there is a large gap between the temperature and dewpoint traces, depicting the much drier air found above the inversion.
At 08:00 UTC, the temperature at Great Dun Fell (847m above sea level) was 5.7C. At the same time, Blencathra (249m above sea level, in Cumbria) reported a temperature of 1.6C. Yet just 5 miles away it was -4.5C at Keswick - why such a large difference over a short distance? It's altitude, since the station at Keswick is only 81m above sea level. Usually the air gets colder as you go up through the atmosphere - but on this occasion it is clear that the reverse is true, highlighting the existence of a temperature inversion.
Dan Holley & Adam Dury 26th November 2016
Storm Angus and his friend
The first named storm of the season brought a period of strong winds and heavy rain to southern parts of Britain. Some of the worst affected areas included the southwest where Exeter picked up 60mm of rain on Saturday (the average rainfall is 83mm for the month). This rainfall triggered eight flood warnings and 49 flood alerts by Sunday across England and Wales. A wind gust of 84mph was recorded at Guernsey on Saturday night and 81mph at Langdon Bay in east Kent. Strong winds caused power outages in Devon in particular.
Following a brief spell of quieter weather on Sunday another area of low pressure approached southwestern Britain later in the day bringing further strong winds and heavy rain to southern counties of England. This system initially concentrated rainfall in southwestern Britain with pulses of heavy rain then spreading northwards into Wales, the west Midlands and northern England. Exeter picked up an additional 45mm of rain on Monday. By the end of Monday the Environment agency had issued 126 flood alerts and 40 warnings in England and Wales. Areas particularly affected were Stalybridge, Tameside where numerous homes have been flooded. Some 16,000 people losing power to their home. Other areas affected include Rotherham and parts of Greater Manchester. In Devon, the train line between Tiverton and Exeter St David's has been severely damaged by floodwater. Wind gusts with this second area of low pressure were not as strong as those associated with Angus with 70 mph recorded at Needles on the south coast and 58mph at Langdon Bay overnight.
Radar animation loop
Wet and windy weather isn't uncommon at this time of year. Deep areas of low pressure frequent northwestern parts of Britain. Occasionally, Atlantic lows are steered a little further south where the impacts can be more keenly felt. Conditions have been made worse by the quantity of leaf fall in recent days with blocked drains exacerbated the problems. Low pressure systems are particularly deep at this time of year due to the large temperature contrast across the northern hemisphere. The cold Arctic air meeting tropical warmth and moisture surging northwards helping to create and sustain these deep lows.
A quieter spell of weather is expected for the next week or so as high pressure becomes broadly in control of the weather across the British Isles.
James Wilby 22nd November 2016
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