The singular, Tornado.info, sounds more natural. And some people misspell the plural by forgetting the "e", which impairs the value somewhat.
Since the damage caused by tornadoes is extensive and government agencies exist to disseminate information -- safety precautions, storm warnings, etc. -- I can imagine Tornado.info (the first-choice option in .INFO) being used.
There is commercial value in tornadoes as well, since storm shelters must be built and supplies purchased. I can imagine mid $xxx for this one. But I'd be a bit surprised by more.
I hate it when domainers tell other domainers that they should develop a domain simply because it happens to be gathering dust. But Tornadoes.info might generate some referrals if someone had contacts in the Midwest.
Gravity.info ... Apart from looking up what a person weighs on Jupiter or the moon, I can't think of any information people need about gravity.
Don't see a commercial niche here. It's a good brand name, but those buyers don't gravitate to .INFO.
I'd say $xx to $250 on Gravity.info