Its called tropospheric ducting and happens all the time. Its caused by a
layer of cool air which is fairly stable then a layer of a different
temperature, then a cold one again. Mist is often implicated.
Back in them old days of analogue we used to be able to get Denmarks Radio
tv down here in surrey on band 3 and German tv at uhf this way. Now though
the digital systems make the whole thing a mess.

This is different to the sporadic E layer reflections of lower frequencies
like band 1 of course though it has been known in summer for both to exist
together causing mayhem on fm particularly and boiling mud on dab etc.
Brian

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"Andrew" wrote in message
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Just been watching Dragons Den on BBC2 and at 08:30 the picture
pixellated on HD then froze.

A quick check for signal strength showed either nothing or
almost nothing on 48, 36, 35, 34, and 33 but ok on 29, so
Quest is fine but everything else has gone.

Then I tried Radio 4 FM on my Phillips cassette/radio at
the back of the house (south) and that cannot get a stereo
signal on 92.9 from the IoW. Just rotating the analogue
slightly up the dial and a French voice comes through
loudly, and clearer than R4. Rowridge is about 45 miles
away to the South West, but I have never had this sort of
co-channel interference before.

How far north is this getting I wonder ?.