The most asked weather question, answered.

I get asked a lot of weather questions.  What do you think is the most frequent?  

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

It all makes sense I guess.  And Pixar actually made a short movie in which the star is a lovable cloud called partly cloudy.