Clear Outside cloud amounts

[almcl]

Bit of a head scratcher.

The other night the forecast on Clear Outside looked fairly depressing but when I looked at the cloud amounts, they were really confusing.

At 8 o'clock with just 25% low cloud and 2% medium cloud it predicted that 93% of the sky would be obscured. An even more impenetrable prediction was for 10 o'clock where 1% low cloud, 2% medium and 4% upper cloud was apparently going to give 79% cover.

Does anyone know how CO arrives at these figures? Clearly straightforwards addition (which might well not work anyway) isn't involved.

The sky, in the event, stayed clear enough for me to carry out the evening's project (some Bahtinov mask testing) so once again an overly pessimistic forecast was best ignored.

[Mr Spock]

It never adding up doesn't sense. Easier for me where I am though - 100% is 100% 

[Mr Spock]

I should add I tend to look at the 'Radar and Satellite observations' from Sat24.

[Chaz2b]

I use Windy.com, a free app which can show you actual cloud cover, prediction of cloud direction and if they grow or dissipate, it also shows satellite images as well as rain, hail, snow, wind etc.

Most apps get there info from the met office so how accurate is the met office! Look outside and see!

Another app for those who like to get technical is R.A.S.P, designed by paraglider enthusiasts for flyers, very accurate at time of use.

chaz

[PeterStudz]

Yes, I use and like Windy.com. Loads of details and if you look you’ll notice that you can select different models. As an example I’m interested in Tuesday/Wednesday night as it might be clear. I’ve shown a screen shot of the first 3 - ECMWF, UKV & GFS - for 1am on Wednesday morning. As you can see they are quite different, they can’t all be right!

Different apps with use different models. The models are updated a few times a day. Some apps will, for example, only update once per day, creating further discrepancies. 

The Met Office model is UKV. Personally I find it quite reliable, although I don’t like the way cloud cover is displayed and indicated within the Met Office app. 

One of the most reliable models is ECMWF. But, as Windy.com says, most apps use GFS because it’s free. It’s no surprise that anything “free” isn’t  that accurate. 
 

ECMWF

 

UKV

 

GFS

 

[jjohnson3803]

Windy, Ventusky, and near realtime satellite images.  None of the astronomy forecasting sites / apps have been any better than 50% accurate in my experience at my location.