It's not an idiom. It means just what it says -- it's either going to rain that day, or it will be sunny.
When a cloud is white that means it will be sunny and it must rain.
No, the idiom is only used to refer to rain.
None, it was sunny all day.
because of the climate. Instead of having somedays rain, and some days sunny, they have sunny periods and rain periods (weeks of rain).
in sunny days
No it will not rain Thursday, August 16.It will be sunny, in the 80's.
In the UK: if you do the rain dance then everyday you do the rain dance if you want it to be sunny do the sunshine dance
A rain shower while the sun is shining will only last a few minutes.
If the probability that it will rain is 0.25, then the probability that it is sunny is 0.75. The probability of it being sunny for five days, then is 0.755, or about 0.2373.
mostly sunny and sometimes rain
Rainbow
It's not really an idiom. It means "what are you thinking about."