# How are global cloud cover estimates generated and how is the data accessed?

The website ventusky.com offers three models to produce cloud estimate: ICON, GFS and GEM. They extract a fractional cloud number, shown in percentage.

From the ERDDAP portal to the GFS model (NCEP_Global_Best), the only relevant data regarding clouds is:

• dlwrfsfc (net downward longwave radiation flux, $Wm^{-2}$)
• dswrfsfc (net downward shortwave radiation flux, $Wm^{-2}$)

Is there a way to estimate fractional cloud coverage from these two parameters or does GFS offer some other product for global cloud coverage?

Is there a GRIB or other type of portal with an API to access cloud coverage estimates?

Edit: I tried openDap as suggested, but the help pages are not clear. And I don't understand the ranges on the time specifications. Let's say I want to look at 22N, 155W (just 1 point) and look at a 24-hour time frame. I tried this.

Lat 88, lon 820, time 301950-301970 (I just assumed, probably incorrectly that it ends at the current offset from 2015-01-15T00:00:00Z). But the results are not what I would expect.

Any help would be appreciated.

• @gansub that something is on topic elsewhere on the internet does not make it off topic here. – Semidiurnal Simon Apr 8 '17 at 8:19
• @gansub that something is on topic elsewhere in Stackexchange does not make it off topic here ;-) More seriously, I would argue that while providing detailed support on OpenDAP in a way that was not earth science specific would be off-topic here, simply suggesting that OpenDAP might be a solution to an earth science problem is not. – Semidiurnal Simon Apr 8 '17 at 11:46
• @gansub This forum seemed a little slow, so I thought it would be ok to discuss the mechanics of how to access the data which was part of my original question. While OpenData might help with the technical bits I think there are weather specific questions about what OPeNDAP settings and results are. – user6972 Apr 8 '17 at 19:46
• @gansub ah, apologies - I hadn't spotted the sequence of edits. I still probably wouldn't complain about off-topicness in this case, but I'm not any sort of authority :-) – Semidiurnal Simon Apr 8 '17 at 23:22
• @user6972 - are you comfortable programming in python ? If so forget the technical nuances of OPeNDAP and just use Siphon - unidata.github.io/siphon and that will help you download GFS data in a netCDF format – gansub Apr 9 '17 at 3:26