High Pressure over the UK

High Pressure, Poor TV Reception and Viking Colonisation

Despite the current quiet weather conditions, people have been complaining about their TV reception. The internet is awash with people grumbling that their Freeview picture is freezing, breaking up, or in some cases, not working at all. People can understand when a severe storm interrupts their viewing pleasure, but the recent benign weather is causing a little more confusion. 
Our settled weather has been caused by an area of extremely high air pressure. Measurement have reached record levels in some places, with Brussels reporting a maximum of 1048.1 hPa, the highest pressure ever reported in Belgium; and Zagreb, recording 1047.8 hPa, which was a new record for the Croatian capital.
In the UK, the pressure was even higher. 1050.5 hPa was reported at Mumbles Head in South Wales, but it wasn’t quite a UK record. It was the third highest, behind 1050.9 hPa reported in January 1957 and 1053.6 hPa in January 1902. 

First, a reminder that air pressure is simply a measure of the quantity or mass of air over a particular place at a particular time. High surface pressure indicates more air, and low pressure less air. High pressure tends to be associated with converging air at upper levels, whereas low pressure indicates diverging air at upper levels. 

In general, we tend to see lower pressure values in the winter - so how come all these high pressure records have occurred in January? The reason for both is that the pole-equator temperature gradient is biggest in the winter hemisphere, which in turn leads to the polar front being more unstable - intense mid-latitude weather systems can form.  For more on this, see Pete Inness' series of weather lectures. If you’ve got extreme Low pressure somewhere in the northern hemisphere then the air that’s been removed from the column above it has to go somewhere else. Often it’ll be spread fairly widely but sometimes it concentrates into more confined regions of extreme High pressure. The High over the weekend formed at the same time as a deep Low to the south of Greenland – you can almost imagine the air diverging out of the top of the low being transported eastwards in the jet stream and descending over the UK. Such very high pressure over the UK tends to be very transient, unlike the big continental winter highs like over Siberia which may last for weeks. 
Within an area of high pressure, the air is sinking. As it sinks, it compresses and its internal energy rises - this causes it to warm. Cloud is very unlikely to form in warming air. However, at the surface, the clear skies of an area of high pressure normally make things very cold in winter as the ground loses heat to space, cooling the air immediately above the ground. Where the descending, warming air hits the cold air near the ground, they don’t mix. One just lies on top of the other, and there’s an abrupt change in temperature and humidity.  Whereas normally you would expect air temperatures to get colder with height, you now see warmer air lying above colder air. This line of discontinuity is called an inversion. Sometimes you can tell where the inversion is if you look out of the window of an aeroplane, because all the dust and pollution will remain trapped in the bottom layer of air. However, most of the time the inversion is completely invisible.
Despite being invisible, it is very much a physical barrier to a radio or TV signal which is transmitted through the air. To a signal, the inversion is a mirror, so it bounces off it, reflecting back towards the ground, and this is what causes the problems. Generally a home would need to be in a direct line from a television transmitter in order to receive the Freeview signal, but with so many signals bouncing off the inversion, the signals from different transmitters overlap and cause interference.
Signals can travel a surprisingly long way if there is also a reflective surface on the ground for the returning signal to bounce off, such as a flat, calm sea. The signal can simply keep reflecting between the inversion and the sea until it eventually fades away. In Doha, for example, it is fairly common to hear radio from Dubai, about 235 miles over the Arabian Gulf, and an old colleague of mine swears that with the older analogue signal, which had a wider bandwidth, he could pick up TV from Russia in Hull if the weather conditions were just right.
It’s not only TV and radio signals which are affected by an inversion, visible wavelengths can be affected as well. In April 1977, residents of Grand Haven in the USA looked across Lake Michigan and saw city lights and a flashing red beacon. This was strange given that Milwaukee, the nearest city in that direction, was over 80 miles away. In order to see around the curvature of the Earth, they would have had to be around 1300 metres above ground level, not on the shore of the lake. However, their sightings were later confirmed to have been Milwaukee; the lights of the city had been bent, or refracted, by the temperature inversion allowing it to become visible on the other side of the Great Lake. 
Refraction happens because light naturally bends towards the denser air. Therefore in winter, when there is an inversion and warmer air lying on top of colder, denser air, the light bends towards the Earth. It is known as an Arctic superior mirage, and may also have made it possible for the Vikings to see Shetland from Norway and East Greenland from Iceland
Of course a sight like this is extremely rare, but it does go to show how physically real an inversion can be and there isn’t really anything that can be done to change it. The only real solution is to wait for the weather conditions to change.
Fortunately for those currently suffering from TV withdrawal symptoms, the current area of high pressure over the UK has reached its peak. Slowly, over the coming days, TV and radio signals should slowly return to normal.



This article was contributed to theWeather Club by our guest author, Steff Gaulter.

Steff Gaulter

Categories: Weather
Tags: Atmospheric Dynamics Weather WorldWeather

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