What causes tropospheric ducting?

Normally, the higher you go into the atmosphere, the colder it is. However, high pressure can cause a temperature inversion. This is where a layer of air in the troposphere will be at a higher temperature than the layer below it. Some of the radio signals from a transmitter bounce back towards the ground and potentially up again. The inversion effectively creates a duct for signals to travel along. 

When does it happen?

Tropospheric ducting is relatively common during summer months and especially during evenings in periods of stable and fine weather. It tends to happen when we have very hot days followed by rapid cooling at night. This is why it is common near the coast. 

When we are aware of this happening, we will put up a news item on Works and Warnings.

What effect does it have?

As the signals can travel further afield it can become possible to receive services you can’t normally get. Your aerial can pick up the signals from distant transmitters as well as signals from your local one or, if the signal is very strong, you will pick up the more distant radio service.

If they are on the same frequency, it can cause co-channel interference. This will cause a temporary loss of reception or two radio services to be heard at the same time. These additional signals can sometimes cause problems with relay transmitters in our network. 

How long does it last?

This is hard to estimate as there are many variables that cause tropospheric ducting. It often appears at the same time on consecutive days and then can disappear for several months at a time.

What should you do?

When this happens there is nothing you can do but to wait until the weather changes. You should not re-tune your radio.