I have tested here with Zalmor settings and its working OK
(i.e I made it for testing where I could set the extra temperature 4 setting (and connected to my Davis station) and set the local time to night time
setting that extra temp sensor to close to the current outdoor temperature, the overcast icon showed OK
setting it to a value this is lower, and less than the threshold difference set to for clear sky , the clear sky icon was show ok
note that there is an average reading in use so it take a little while (minute) for the change
you can set to show debug info (in the misc setup) and then under view, debug info, the 2 values are shown (outdoor temperature and the extra temp sensor value)