No, its not about the impacts of global warming.
And what the probability of precipitation really means.
The most frequent question is:
What is the difference between partly cloudy and partly sunny?
Really, folks ask that all the time. Sometimes it seems like a subtle put-down about meteorologists, but sometimes it reflects real curiosity.
Well, let's give the answer based on the official National Weather Service definitions.
There are two things you have to know about this. First, different terms are used during the day and the night. The National Weather Service (NWS) talks about clouds at night and sun during the day. Why? Because there is no sun at night ! Second, the definitions are based on the percent coverage of the sky. So let's examine what the NWS uses!
At night, the sky goes from clear, to mostly clear, to partly cloudy, to mostly cloudy, and finally to cloudy as the sky coverage increases from 0 to 100%. During the day, the descriptions go from sunny to mostly sunny to partly sunny to mostly cloudy (since you can't see the sun then!) to cloudy. Here is a table with the exact definitions.
NIGHTTIME DAYTIME PERCENT OF SKY COVER CLEAR SUNNY 0 TO 5 MOSTLY CLEAR SUNNY 6 TO 25 PARTLY CLOUDY MOSTLY SUNNY 26 TO 50 MOSTLY CLOUDY PARTLY SUNNY 51 TO 69 MOSTLY CLOUDY MOSTLY CLOUDY 70 TO 87 CLOUDY CLOUDY 88 TO 100