it's not the humidity .. moisture in the air and/or hotter air makes the mixture richer (there are less air molecules per volume of wet, hot air than there are in cool, dry air.)
Your bike seems to be running lean. Choking it cuts off some air and makes the mixture more correct.. makes it richer.
A dirty carb is very likely .. fuel flow is restricted, leaning the mix.
The weird thing is you say the lean-problem disappears when you get past 20 mph.
So it sounds like the carb is not metering fuel correctly at mid-throttle.. this could be a mechanical problem, like throttle cable adjustment or something to do with the slide-needle position.. it could be float level.. maybe a vacuum leak from loosening bolts around the intake or a fuel leak..
The point is it may be something other than just dirt.