I was visiting bbc.co.uk and was amused by their weather forecast. Rather than saying, "partly cloudy", they say "sunny intervals". I much prefer "sunny intervals". It's a glass half full kinda thing. So proper. I love London.
On this side of the pond though I often wonder what the difference between mostly sunny and partly cloudy is. Could it simply be whether the forecaster is a "glass is half empty" or "glass is half full" type?
I bet the weather guys just mix it up intentionally between Mostly Sunny, Partly Cloudy, Scattered Clouds, Intermittent Clouds, etc...
I mean, there are only so many ways to say the same thing day after day, year after year. One day, they will use Partly Cloudy, and the next day they will use Mostly Sunny, just to make it different. You don't want the viewer to become bored or think of the weather as predictable.
It's the same reason sports guys try to use different words when recapping scores from the day. i.e. The Cubs downed the Cardinals 4-2 today. Or The Cubs hammered the White Sox 12-1 this afternoon. Or the Cubs outlasted the Yankees 10-9. You just gotta use a different verb or it gets boring.