In weather forecasts in the US, the phrase "a mix of sun and clouds" seems to be common whenever the forecast does not clearly predict rain or shine. In my recollection, forecasts used to have phrases like "partly sunny", "mostly sunny", and related variations "mostly cloudy", "partly cloudy".
My main question is, when did this transition occur? Google trends and n-grams don't tell.
I should probably stop there, but a second question nags me, which is, why? Granted, weather prediction is not an exact science, but the "mix" forecast seems like a cop-out.
P.s., if this question seems more suited to a more scientific channel I'm fine with that, but please consider I am firstly wondering about when the phraseology changed.
P.p.s., I am mostly thinking of media forecasts (particularly radio and TV). NOAA, from whom I suppose many of these outlets derive their forecasts, maintains the "partly sunny" etc. phrases.