1. Complete list of descriptions:
Weather.Com themselves do not provide any list of valid descriptions. BUT to be able to do some localization I collected an almost complete list of descriptions. This list can be found in the L10N files (e.g. 'Weather::Com::L10N::de').
2. Mapping icon to description:
As Weather.Com does not provide a list of valid descriptions, they do not provide a mapping, too.
For what purpose could one need such a mapping? For each weather condition you always get an 'icon()' method and a 'description()' method...
There would be sensible possibilities to implement such a mapping - if there is a real need for it!