Please do not adjust your set...
Those of you who remember the days of black and white TV may well recall the occasional messages about interference to television pictures being due to the weather. Well, that same physics still happens today, but modern digital TV signals are affected in a different way and it may not be so obvious...
This is not the place for details of the physics, save to say that the idea that radio waves can be affected by the atmosphere is no different to that of the mirage seen on a shimmering road in the heat of summer. I'm sure you will have seen this effect, where light from the sky (being electromagnetic waves) is bent by changes in the refractive index of the air close to the hot road surface. Another example would be how a stick appears to bend when the end is placed into water.
Back to the atmosphere, it happens because the refractive index of air is affected by changes in temperature and/or moisture. These contrasts found near large areas of high pressure where changes between cool, moist, foggy air near the surface and warmer, drier air above the fog top are also capable of altering how radio waves (also an electromagnetic wave, but a bit farther down the spectrum) propagate.
On Wednesday 11th Dec 2013 we were in the middle of one of these special periods of weather, where the state of the atmosphere affected how radio signals propagate. It was most noticeable at the higher VHF and UHF frequencies, but this also happens to be where amateur radio operators have frequency allocations and, for them, this anomalous propagation becomes a good thing and allows signal to be received over much greater ranges than usual. Instead of the usual 50-100km range, distances over a 1,000km are possible, usually around the edge of the High.
Surface analysis chart for 12:00 on Thu 12th Dec 2013, red shading depicting zone of enhanced radio signals
The weather chart shows the High over Europe and the zone where the radio signals were enhanced (red shading). You probably remember hearing in the news about the disruption caused to air travel by the widespread fog, another good clue that there may be some enhanced radio propagation taking place.
In meteorology we use weather balloons, called radiosondes, to measure the temperature and moisture changes in the atmosphere. What we are looking for is the two main plotted lines of temperature and dew point being close together in cool moist air near the surface, and very widely spaced in the warmer dry air just above the top of the fog. The resulting change this produces in the refractive index can bend the radio wave, so that it is trapped in a duct and can travel great distances without much attenuation. The balloon plots from Bordeaux into England and across the North Sea to the Baltic all exhibited the required pattern, and radio amateurs were very busy with stations from northern Spain to the Baltic coming through.
Nottingham radiosonde ascent at 00:00 on Wed 11th Dec 2013
This excitement is set to decline for us in southern Britain during Thursday 12th Dec as the winds pick up and gradually break down the temperature contrast, called an inversion, and ultimately disperse the foggy air near the surface. Good news for TV viewers and bad news for the morse keyers... but it was a very fine example while it lasted!
For more information on amateur radio visit the RSGB website.
Jim Bacon (G3YLA) 12th December 2013