When does it stop being partly cloudy and start being partly sunny?

Well when it's slightly more sunny than it is cloudy it is considered partly sunny

And if it's slightly more cloudy than it is sunny it is considered partly cloudy

...I guess that's how it works I dunno I'm not a meteorologist XO

When I want it to be.

when does it start to be dark and stormy?

oh ..oh i know everytime uaan shows up

Wait what the hell?
They changed it?
THEY CHANGED IT?!?!

*flip*

I don't think meteorologists define weather as 'partially sunny' or 'partially cloudy' as often as they would use different terms for it. In saying that, that's just because I've never heard them use those terms here... who knows?

To actually answer your question, aliens.