I need to find good and reliable data source of cloud coverage (to use GIS) with forecast for few days and be able to parse it.
Do you know good one? I know that NOAA provides weather data, but couldn't find any data of clouds.
Open GIS-ready cloud data are available at various levels of detail, both in time and space.
It is quite tempting to work with satellite photos of cloud coverage; unless you are a professional, don't do that - there are a bunch of hidden snags you have to know about.
For current data on (points) airfields and airports of the world, your best bet is METAR, where you can learn type of clouds and cloud coverage, as well as visibility figures. There are many applications (in Python and Ruby, among others) that access and decipher METARs (you can do that as well, after a bit of training).
If you need in-depth point forecasts of probability that cloud cover will be less/greater than a given threshold, you can use NOMADS (may be overkill for your purposes, though).
For wide-area coverage with forecasts up to 8 days, the ideal stuff comes from GRIB, easily downloadable and parseable with command-line utilities and GUI programs. These are forecasts of most probable weather, though, unlike the extensive data from NOMADS ensemble.
There is a comparison of different weather forecast services here: http://www.randalolson.com/2014/06/21/accuracy-of-three-major-weather-forecasting-services/.
Maybe this helps to select the best service for your requirements.