@takumhara
Windy changed it from cloud base to ceiling, but it is running under the same name on Windy. So the ceiling means > 50% of sky covered by clouds, in aviation it is higher than 4 octas, so it can be BKN = Broken or OVC = Overcast, where BKN = 5-7 octas, OVC = 8/8 octas fully covered.

And to answer your qestion, this parameter gives the height of ceiling in meters (m) or feet (ft) above the level of the model (ECMWF) orography, so it is AGL.

Read the corresponding article of Ceiling:
https://confluence.ecmwf.int//display/FCST/43r1+new+parameters%3A+Ceiling

But be aware of it, because of the ECMWF model has a 9km resolution, so it's sampling the terrain with 9 km steps. If you are planning around mountainous areas, you can't rely on it, do not use it for planning in such areas.

@egruebel
There was a general bug in currents feature, but it was fixed.
Read the according thread about that bug:
https://community.windy.com/topic/7772/current-unavailable

Does your reported bug still exists?

@jsdhornby
I answered you here for the same question:
https://community.windy.com/topic/7582/windy-launches-observation-vs-forecast/36

@jsdhornby
That coloured area represents the observed temperature on the upper side and the dew point on the lower side measured by the weather station. And its colour is corresponding to its colour scale depending on its value.

You can note that the more closer these two lines (values) to each other, the more saturated is the air. Obviously when the two values are equal --> the air is 100% saturated in this case (it's depicted on windy as a very little gap between the two lines, but above the chart at the value indicator you can read 2 equal values). In this 100% saturated air, the air contains a lot of moisture and there exist a low visibility condition, due to the very high probability of fog creation, which can transform into a low level stratus cloud with the help of a little wind. So it can influences the developing weather phenomenas.

Anyway, the other 2 dashed lines represents the forecasted TEMP and DEW POINT, for that weather model, which you select at the bottom of the chart.

Or if you are thought about the starting pop up charts and not the weather stations' "observation vs forecast" charts, there the coloured area represents only the forecasted temperature, as you can see it below.

@PhileasFogg1905
Tomas corrected it. So it will appear on Windy soon (I think in the next update).

@dzuremar
I found some precipitation out of the radar ranges. Maybe does it the same projection error, or does it a range problem?

Europe High Pressure - Warm Front

https://www.windy.com/annotation/5c63c17dc4cbfc0014c6710c

I just tried it, It's cool!:)
(Btw I depicted the warm front by triangles by mistake, imagine it correctly with semi-circles..)

This image is licenced under Creative Commons licence and can be used/modified freely in any possible way

Create your own annotation at https://www.windy.com/annotate

@justinlimbo
Such things that are specified for example on the Wunderground website here in the "Personal Weather Station Installation Guide", such as: where to place it depending on the environment, at what height...etc. All of the criterias that are given standing for keep a standard predetermined accuracy. In case of these conditions are not met with the given criterias, it will lead to inaccuracy, and obviously these weather stations will not be reliable at all. (as @Tomber42 said in his reply)
Anyway I absolutely agree with @Tomber42 because it's trivial that most of people looking at weather stations to get precise and reliable observation informations.. such as for example the Airports' weather stations are mainly the most reliable stations in my opinion.. and I can totally trust in it (because of it has regular maintenances, and operates under strictly specified standards).

And it would be nice if the same trust could be presented in the PWS's in the future, but imho that will need some installation criterias in the side of Windy.

If Windy would like to make it reliable and a good information source (such as now the most of weather stations), I think they should make a "Personal Weather Station Installation Guide" too, or something similar to that (as Wunderground did..), so those people who reported their PWS's should make those configurations that are laid down in the "Installation Guide", and their PWS's will appear just after that on Windy site.

To my mind, it would be a good solution to the all "PWS's unreliability and real data measuring" questions.

Btw, I don't say that this is a bad thing, I like it really much, and this "Observation vs forecast" feature is really unique and useful. But I'm just wondering what will happen with the newly coming PWS feature on the map.. And I just got this idea that I wrote above.

This is not an offense from my side, these are only some thoughts of the questioned area and potential ideas for the predictable problems.

Anyway I trust in Windy team, you guys always make really invaluable and unique features with a nice and clean interface. Just do it guys, as you always do!:))

@ecavaleiro
I asked exactly the same recently from @dzuremar and he said it's because of the source decreased its range due to some reason. For Madeira they not get the official data, but they get it from Rainviewer.com. Anyway as you can see recently there are a lot of newly added radar coverages, these big and fast extensions are thanks to Rainviewer (such as: Portugal, Turkey, W part of Russia, Australia).