There are different sources for weather information in the internet. They usually will provide you with some weathercode, which you like to display as an Icon. With the help of this small tool you can convert the retrieved code into some suitable icon name, which can be converted into the real icon with the Name to Icon converter.
As of now the converter supports weather codes from OpenWeather and from the Finnish Metereological Institute. The conversion is based on the information found at OpenWeather and github.
If the entered code can't be converted to any Icon name, the name 'cloud-question' is returned and after converting to some Icon you migth see on your display.
Openhab Blockly Nspanel - Library Documentation