Yeah, and there is a lot of variability within even one of those too. Here in north central florida, we're probably in the semi-tropical region with wet prairie and swamps around. Right now the humidity outside is 80%, and 45% in the house. I've seen the humidity in the house get down into the 30%, like after a cold front comes through (e.g depending on how far south that midwest storm reaches in a couple of days).
If you're really concerned, you can buy small cool-air humidifier for about $30 at a big box store. I bought it not for my corns though...mostly for my sinus issues, but the BP and corns aren't complaining. It took the 30% humidity we had a month ago and bumped it to closer to 50% overnight.
See link here
Though after reading the reviews you might want a different model. No problems with mine so far.