Why does it work that way?
AFAIK, weather icons that contain nighttime elements (like cresent) are supposed to be used by entries that show "Current Weather" during night hours only. Entries, which are set up to show weather for the entire day (like "Today's Weather", "Tomorrow's Weather", "2 Days Weather" and so on), are supposed to use generic weather icons only, independent from time of day (i.e. they shall never use nightime icons).
So, is it is bug in Zooper or is it a bug induced by a bad data from weather provider? And even if it is bad data from weather provider, why doesn't Zooper catch the error and filter it out on its side?