Raining cats and dogs wouldn't be as heavy as sheet rain.
My favorite expression of rain--and definitely in the hard rain category--is the
gullywasher. There is a bright day, and suddenly the rain comes down in
torrents. The gullies and gutters fill quickly, and then, just as suddenly, the gullywasher has bellied up. I like seeing gullywashers just to have an excuse to say
gullywasher. Maybe I should put this term on Raven's Tripping thread!
But how do I define
rain? Sure, the misty rain qualifies as rain, but when the atmosphere is simply heavy with humidity and even mist, I wouldn't call that rain. I like my rain, misty or otherwise, to perceptively fall. However, that's just my take.
Raining pitchforks sounds like painful rain--rain falling so hard, it causes a stinging sensation in the flesh. Definitely not a "Singin' in the Rain" kind of rain.
The coolest name for fog I know is
pogonip, the fog that contains frozen moisture that can actually kill. People who live in the pogonip regions of the US know to run inside when a pogonip arrives. Deadly.
