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Agree, some of the cloud data seem to contradict each other. I’m planning a flight on Monday from L.A. to the Bay Area and it gives me no low and mid cloud along the way, but then says cloud base 5000ft (btw, does anyone know if this is supposed to be AGL or MSL? Haven’t managed to find that out yet). For comparison, http://www.usairnet.com/ gives a 12k ft cloudbase which chimes with other forecasts and also with the ‘high clouds’ in Windy.

I suspect this is like so many wx websites we have seen over the years. The programmer is not a pilot and just hacks together some data sources.

It’s like those websites which show cloud tops height. It looks wonderful – until it turns out they just convert the temperature into altitude using the ISA profile, which most of the time is useless (at piston GA altitudes).

Also GFS doesn’t model the air in straight 3D. Clearly it would be computationally too hard. It uses a concept of low medium and high cloud and runs different algorithms for each one. For some reason the result is that a lot of cloud is missing totally in the forecast, especially the lower stuff at GA VFR altitudes like 2000-5000ft.

Administrator
Shoreham EGKA, United Kingdom

I suspect you’re right, Peter. Only thing is it uses the ECMWF model for that which is supposed to be more accurate. I’ll keep monitoring the forecasts, will compare to real world on Monday and report back!

Airborne_Again wrote:

There’s something really weird with some of the Windy data. E.g. it right now over large parts of Sweden it gives really low cloud tops (e.g. 2000 ft) while at the same time saying there are 95% medium and high cloud in the same location. It doesn’t make sense.

My interpretation is when the cloud is in layers or not continous cover, the tops are the first layer tops in that area

liftvectorup wrote:

My interpretation is when the cloud is in layers or not continous cover, the tops are the first layer tops in that area

That’s a reasonable interpretation, but before I would use a web site for a weather briefing, I want to know for certain what things mean.

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

That’s a reasonable interpretation, but before I would use a web site for a weather briefing, I want to know for certain what things mean.

Totally agree! I just checked Windy again for my Monday trip and – at least the way I read it – it contradicts itself. Is there any explanatory website for this thing? I can only find some sort of group with very limited explanations for their data.

OK, so I checked what Windy said against reality on a trip L.A. – S.F. – L.A. Truth be told, there wasn’t much to check, because simply put – there wasn’t any WX. Aside from some low-level haze over the Central Valley (which is pretty much a permanent feature of the area), there wasn’t anything.

However, what really got me interested was that Windy forecast widespread low cloud with a base of 5000ft. Nothing like that materialized. It correctly forecast some very high stuff, though. Interestingly, checking it again today, they seem to have changed the interface wrt the cloud cover. Seems a bit a WIP. Certainly good to have another string to the wx bow, but don’t think I would rely on Windy.com as my main source for getting wx.

@172driver, which model were you looking at on Windy? I see that for the continental USA it offers the choice between NAM 3km, ECMWF 9km or GFS 22km

ELLX

@lionel, I tried all three, although the default is the ECMWF.

Re the cloud forcasr liw, med high. When you click on an area it will display for example 65% now does this mean 65% coverage or 65% chance there will be cloud their ?

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