I think it should be "and snow".
A thunderhead , dense clouds that rise high into the sky in huge columns , produce hail, rain , or snow.
Either "and" or "or" will work in this sentence. Using "or" emphasizes that rain, hail, and snow do not occur at the same time/place during a thunderstorm and is probably the technically correct choice (from a science standpoint). Using "and" is appropriate if you are just listing the possible results of the thunderhead.
Your verb -- produce -- should be in the singular form -- produces -- to agree with your subject -- thunderhead.
|link comment||answered Aug 22 '12 at 13:56 Jeff Pribyl Grammarly Fellow|
I like Jeff's advice, and I also recommend separating the phrase "dense clouds that rise high into the sky in huge columns" with dashes instead of commas:
A thunderhead -- dense clouds that rise high into the sky in huge columns -- produces hail, rain, and snow.
That way, it's clear that the phrase is defining "thunderhead" rather than listing other types of clouds.
|link comment||answered Aug 22 '12 at 15:00 Actually Holly Expert|