It's raining if it is actually doing so; it is rainy if it is that sort of weather, but is not raining at the moment or if it is just sort of drizzling along. That's how I use them, I think. As for other weathers, just tell it like it is: It's sunny It's cloudy It's hot and humid It's really cold and miserable It's overcast It's clear and bright It's changeable It's scorching It's been sleeting since dawn It's a gorgeous day etc.
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