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I notice this radially symmetric weather radar pattern over London, UK today.

Is this a real weather pattern, or a measurement or model artefact? If it is a real weather pattern, what causes it?

The image is from https://www.netweather.tv/live-weather/radar, which shows the following legend under the chart:

Weather Type Radar The weather type detection uses a combination of hi-resolution model data, along with ground observations to show whether rain, sleet, snow, hail or freezing rain is falling. It's an excellent guide, but bear in mind that in very localised situations some variation may occur.

The pattern seems to have started developing at around 21:35 BST today 2023-07-19.

Edit: I've added an additional image that shows a more dramatic pattern from today 2023-07-22 superimposed on a real rainfall pattern, together with a red dot at postcode HP3 0EJ in the village of Bovingdon (relevant because of the Bovingdon arrival stack for Heathrow airport).

enter image description here enter image description here

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    $\begingroup$ Since I wrote this question, I think I've figured out this must be associated with aircraft in the "Bovingdon stack". Presumably the water content of the engine exhausts is hard to distinguish from low levels of rainfall? $\endgroup$
    – Croad Langshan
    Jul 20 at 8:18

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This is a measurement artifact, perhaps due to aeroplanes or misconfigured Wi-Fi.

Weather radar detects not only precipitation, but also aeroplanes, birds, insects, wind turbines, mountains, and high buildings, among other things. Additionally, active sources of interference can be ships or home Wi-Fi. Met services, or people working at their request, routinely tell people to change their Wi-Fi settings. Finding them in a city can be a challenge.

Software that processes weather radar data for the purposes of precipitation monitoring needs to filter out those other sources; such filtering is always imperfect. If the filter is set too strictly, light precipitation will be filtered out. If it's set not strictly enough, artefacts such as the one in your example will remain in the data.

Some artifacts show up as arcs, whereas others show up as rays.

The figure below shows an example of what data may look like if they're not corrected at all. The figure on the left is without a correction applied. The figure on the right is corrected.

radar data with or without correction
Source: Deutscher Wetterdienst (DWD))

For the location in your image, I would suspect either aeroplanes or misconfigured Wi-Fi.

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    $\begingroup$ How would misconfigured wifi (on the ground) account for the radial pattern that correlates with the location and orientation of the Bovingdon stack? I guess planes emit radio signals of various kinds, perhaps including misconfigured wifi - but at any given moment only a few planes are present along these radial lines, which makes me suspect water vapour in contrails as the cause rather than the planes themselves? $\endgroup$
    – Croad Langshan
    Jul 22 at 11:39
  • $\begingroup$ Ah I see there is a radar station at Bovingdon - perhaps this is the source of some of the data used here? geograph.org.uk/photo/3389301 That would account for the pattern, which after all does look both suspiciously dramatic and neat to be associated with contrails! $\endgroup$
    – Croad Langshan
    Jul 22 at 11:45
  • $\begingroup$ @CroadLangshan Radar emits a signal and measures its reflection. From the time of reflection, the radial distance is estimated. If a constantly emitting source (wifi) is mistaken for a reflection, the system estimates there is a reflection at every radial distance, so you get a reflection at all distances (up to the maximum configured radius for the radar). $\endgroup$
    – gerrit ♦
    Jul 24 at 7:04
  • $\begingroup$ \@gerrit right! (but you're replying to a comment where I was still talking about planes in the stack as the hypothesis, not radar at Bovingdon!) $\endgroup$
    – Croad Langshan
    Jul 24 at 9:37
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As @gerrit points out, this is a measurement artifact.

The artifact is associated with the location of ground weather rather used by the site (see link in my edited question), rather than with planes in the Heathrow Bovingdon arrival stack, their contrails, or misconfigured WiFi (see the second image I've added to my question - in fact if I correct for the precise location of the radar in Bovingdon, the alignment with the radial pattern gets even better).

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  • $\begingroup$ Now that I write this, I realize that part of the radial artefacts may well be caused by misconfigured wifi that have to be subtracted out as gerrit suggests. In fact, though I haven't aligned the images carefully to compare, I think the second image I've uploaded shows that as gerrit described, the attempt to subtract interfering signal on top of rainfall leads to lines of underestimated rainfall, while my first image shows the same subtraction causing lines of overestimated rainfall on a day with low or no rainfall. $\endgroup$
    – Croad Langshan
    Jul 22 at 12:04

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