"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


Windy weather on the way

A very windy spell of weather is forecast to affect the British Isles over the next few days, particularly on Thursday night and into Friday morning, with the risk of gusts exceeding 100mph over parts of northern and northwestern Scotland, with another bout of strong winds expected on Saturday morning.

Average wind speeds over northern mainland Scotland and the western Isles are expected to be around 50-70mph on Thursday night, with gusts potentially exceeding 100mph at times, especially over high ground. Elsewhere, mean wind speeds will be around 15-30mph, but with the risk of gusts up to 60-70mph over northern England, northern and western Ireland and along Welsh coasts.

The winds will then ease slightly during Friday day, before the next area of low pressure moves towards the British Isles, with the winds set to strengthen once again through Friday night and into Saturday morning. The main difference being that it is forecast to be windier over southern Britain on Friday night and Saturday morning compared with Thursday night's event, with the risk of gusts around 50-60mph. In northern Britain, the winds are expected to be weaker in comparison to Thursday, but despite this there is still the potential of damaging winds, with gusts of 60-70mph.

What is responsible for the current weather? Well, currently in North America there is a very sharp temperature gradient with warm, moist air over Florida and the Caribbean, while across northeastern states of America and Canada, temperatures are as low as -20 to -30C. This sharp temperature contrast causes the jet stream, a ribbon of air 30,000 feet up in the atmosphere, to strengthen, with winds in the upper atmosphere reaching speeds of up to 250mph. This then rapidly steers deep areas of low pressure across the Atlantic causing unsettled, windy and potentially disruptive weather for the British Isles, with this pattern set to continue into the start of next week as well.

Chloe Moore  8th January 2015