Very thick cloud at a very low level

Your affinity for XP has obviously become legendary might well last into to 22nd century …

Peter wrote:

This is what was actually there

Looks like a strong inversion indeed.

Here is a wikipedia article about weather radars which states:
Temperature inversions often form near the ground, for instance by air cooling at night while remaining warm aloft. As the index of refraction of air decreases faster than normal the radar beam bends toward the ground instead of continuing upward. Eventually, it will hit the ground and be reflected back toward the radar. The processing program will then wrongly place the return echoes at the height and distance it would have been in normal conditions.

This type of false return is relatively easy to spot on a time loop if it is due to night cooling or marine inversion as one sees very strong echoes developing over an area, spreading in size laterally but not moving and varying greatly in intensity. However, inversion of temperature exists ahead of warm fronts and the abnormal propagation echoes are then mixed with real rain.

The extreme of this problem is when the inversion is very strong and shallow, the radar beam reflects many times toward the ground as it has to follow a waveguide path. This will create multiple bands of strong echoes on the radar images.

This situation can be found with inversions of temperature aloft or rapid decrease of moisture with height. In the former case, it could be difficult to notice

Thanks for these links @DavidS and @Aviathor
Last year i was also caught out once planning to fly over Southern France (Rhone valley). Radar showed serious precip, just over the river, satellite nothing. Rare thing..
The other interesting effect that I see very regularly, almost without fail: Southerly wind, south coast of Mallorca splattered by waves. Radar return shows precip.. Radar is placed on that coast, at about 400 ft high. So I guess that when scanning in its lowest incline it could pick up waves or the moisture that these waves throw upwards?

Peter wrote:

However what amazes me is that @bookworm sounds like this is an everyday thing and everybody knows about it, but in 16 years of flying I have never met anybody who knew about it.

It’s certainly not an “everyday thing”, but it happens in the right conditions. Generally, when I can hear aircraft approaching Carlisle on the Cambridge Approach frequency, I get suspicious about big blobs on the radar claiming to be rain.

https://en.wikipedia.org/wiki/Anomalous_propagation

Beautiful example of artefacts on the wx radar today.

Every green blob in Germany surrounds a radar head: Flechtdorf, Offenthal, Neuhaus, Dresden, Isen, Turkheim, Feldberg etc. And indeed if you look at the individual pictures from local radar (needs a DFS sub), there’s a blob in the centre of each.

In France, however, the refections appear to be working differently, and I’m guessing there’s a hole at each individual head in a sea of green.

It’s whatever composite the DWD puts together (as used by the Golze boxes).