50% either way.

Is it not true that the same sky may be described as partly cloudy or partly sunny depending on the disposition of the observer? One who is focused on the clouds may see the sky as partly cloudy, while one who is focused on the sun may see it as partly sunny. The reality has not changed, only the words we use to describe it.

It is based upon time of day. The difference between "partly cloudy" and "partly sunny" is primarily based on the time of day and the amount of cloud cover. Both terms are used when 3/8 to 5/8 of the sky is covered by clouds. However, "partly sunny" is used only during daylight hours, while "partly cloudy" can be used to describe conditions at night as well.