Steve White G3ZVW provides an overview of an atmospheric effect that can lead to enhanced propagation at VHF frequencies and above.

When a high pressure weather system settles over the British Isles, keen VHF operators head into their radio shacks and search for long distance contacts.

Before I go into the subject of tropospheric propagation itself I would like to provide some background information, to give newcomers an overview of the subject.

What is the Troposphere?

Just as there are several Regions and Layers in the ionosphere, Fig. 1, where High Frequency (HF) radio signals can be refracted back to Earth, there are several regions to the atmosphere beneath it, Fig. 2. 2m, 70cm and microwave signals are sometimes affected by it, resulting in longer distance contacts than is usually possible.

The bottom layer of the atmosphere is called the Troposphere. The troposphere isn’t the same height all across the World, being about 14km (8.7 miles) thick in tropical regions but only about 8km (5 miles) thick in Polar regions. It is the densest part of the Earth’s atmosphere and is where the weather occurs – rain bearing clouds. To relate the height of the troposphere to something you can visualise, commercial aircraft flying at cruising altitude are likely to be about 11km (37,000ft) above sea level, so they are around the upper edge of the troposphere.

As regards temperature, the normal situation in the atmosphere is that the temperature drops as you go higher. Depending on conditions the amount varies but a 2-3°C drop per 1000ft would be normal. Something else that happens as you go higher is that the air gets thinner. As it gets thinner the refractive index changes. Interesting things occur when this doesn’t happen!

Atmospheric Pressure

The instrument used for measuring atmospheric pressure is the barometer and traditionally atmospheric pressure was measured in inches of mercury.

Now, here’s an experiment you can’t try at home! If you were to fill a 3ft-long glass tube that was sealed at one end with mercury, then turn it upside down and place the open end in a bowl of mercury, the weight of the mercury would cause a vacuum to develop at the top (closed end) of the tube. The height of the mercury column would be about 30 inches but it wouldn’t remain constant. Instead, it would change as the atmospheric pressure changed. Changes would not be perceptible from moment to moment but you would certainly notice them from day to day. These days you can’t try this experiment at home because it is recognised that mercury is such a toxic substance. Also, these days, inches of mercury have been replaced with millibars. One Bar is the standard atmospheric pressure and it is subdivided into a thousand millbars (mb).

Highs and Lows

An Anticyclone is an area of high-pressure air. It is the opposite of a Cyclone, which is an area of low-pressure air.

So, what constitutes low pressure and high pressure? The first thing that needs to be said is that it never changes by a huge amount. The nominal atmospheric pressure is 1000mb, while the pressure at the heart of the kind of winter storm we experience in the UK might be as low as 950mb and the pressure at the centre of an anticyclone might be as high as 1050mb. In other words, it doesn’t deviate from the nominal by more than about five percent.

Anticyclones bring settled weather and low wind speeds, and it is these that can result in enhanced propagation at VHF, UHF and the microwave part of the frequency spectrum.

If the weather is too windy, as it is most of the time and certainly when there is a cyclone present, the atmosphere gets stirred up too much for something vital to form – a temperature inversion.

What to Look For

If you are interested in taking advantage of this type of propagation, look for visible signs of a temperature inversion. The smoke from a fire is warm, so it rises. As Fig. 3(a) shows, smoke rising from a bonfire or a chimney is normally blown to one side and disperses but if there is little or no wind, it tends to rise straight up. If it then bumps into an even warmer layer of air, the warmer air acts as a blanket and prevents the smoke from rising further. The result is that the smoke abruptly stops rising and drifts slowly outwards instead. I show this in Fig. 3 (b). Look for this when you’re out and about. You won’t see it often but if you are observant, you will notice it from time to time. Bear in mind that even in the lightest breeze more smoke is likely to drift to one side than the other.

Smog is also an indication of a temperature inversion. In this case smoke is trapped at ground level by a blanket of warmer air that is really low in the atmosphere. The Great Smogs of London were caused by temperature inversions, trapping the smoke from industry and domestic coal fires at ground level. An estimated 4000 Londoners died during the smog of December 1952 and an estimated 6000 more died afterwards, poisoned by the fumes. The subsequent Clear Air Act outlawed the burning of coal in London.

Looking at weather maps is also a good idea. If the pressure rises above 1030 millibars, some enhanced tropospheric propagation is likely to occur. Above 1040 millibars and the likelihood becomes good, especially as the pressure starts to drop. Below 1030 millibars it tends not to do so.

A third thing to look at is the website by William Hepburn (URL below). It is a really useful resource of tropo forecasts. A lot of people refer to tropospheric propagation as ducting but the website explains how there are a variety of tropo modes, only one of which is ducting.

www.dxinfocentre.com

What to Listen For

Unlike ionospheric propagation, where different HF bands open at differing times of the day, when enhanced tropospheric propagation is taking place the 2m and 70cm bands tend to be affected by similar amounts at the same time. On VHF/UHF a beam antenna is a practical proposition for many people and it is useful to know in which direction to point it. This should be towards the station you are listening to but bear in mind that during such propagation the conditions are often more favoured around the centre of an anticyclone, rather than across the middle of it. Isobars are the lines on a weather map that show where the barometric pressure is the same. On standard maps the isobars are shown at 4 millibar intervals, and sometimes the 20 millibar intervals are shown with thicker lines, Fig. 4.

When it occurs, tropo propagation can last from hours to days. It can take place at any time of the day and any time of the year but is not affected by the solar cycle.

Even if the barometric pressure is not high enough to trigger a full-blown tropospheric opening, small scale temperature inversions occur quite often around the times of dawn and dusk. These can happen in any season but the wind needs to be very light for them to occur.

Unlike ionospheric propagation, where refractions take place 100 or more kilometres above the Earth, the refractions that cause enhanced tropospheric propagation can take place quite close to sea level. If you live on a big mountain, your signals may not be refracted because you are actually above a temperature inversion.

This article was featured in the November 2018 issue of Practical wireless