icon: A machine-readable text summary of this data point, suitable for selecting an icon for display. If defined, this property will have one of the following values: clear-day, clear-night, rain, snow, sleet, wind, fog, cloudy, partly-cloudy-day, or partly-cloudy-night.
FYI. You'd only need to add/alias 5 class names to have full compatibilty with the icon names used in the Forecast.io API.
via this page: https://developer.forecast.io/docs/v2