"Anticyclonic Gloom" is not all gloom



This spell of high-pressure weather has brought expansive low cloud to most of the country, but it also extends across much of northern Europe. These persistently cloudy conditions are known as "Anticyclonic Gloom" and are generally unpopular with most most us!

Visible satellite image of Europe at 12:00 on Wed 11th Feb 2015, courtesy Sat24/EUMETSAT

However, things are rarely that simple and, in the case of Anticyclonic Gloom, it can also be a sign that radio propagation can be affected. Signals at VHF and UHF frequencies can be trapped in a duct and travel over long distances with minimal loss in strength.

The reason the cloud becomes so widespread is because of something called a temperature inversion, which can form under regions of high pressure. These are 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. An extensive layer of cloud often identifies the height of the inversion and provides the "Anticylonic Gloom".

This temperature information can be measured by using radiosonde balloon soundings and a plot of the inversion measured in Nottingham at 00utc on 11th February 2015 is shown below. The temperature inversion is the flat line near the bottom of the trace and the air below the inversion at 599m is cool and moist at -2.1C and 99% relative humidity (RH), while just above the inversion at 827m is +8.4C and only 18% RH. This contrast of temperature and moisture over 228m causes a big change in the refractive index of the air and, like a stick appearing to bend in a beaker of water, can bend radio waves so that they follow an extended path without attenuation, its known as ducting.

Nottingham radiosonde ascent at 00:00 on Wed 11th Feb 2015

Amateur radio operators have been making radio contacts across northern Europe across the edge of the high-pressure region. One of the longer paths being nearly 1600km and for VHF Dxing as its known, there is anything but gloom from this anticyclone!

Surface analysis chart for 12:00 on Tue 10th Feb 2015

"Please do not adjust your set..." gives some additional background to these events.

Jim Bacon  11th February 2015