According to Greg Pease (http://pipesmagazine.com/blog/out-of-the-ashes/dust-in-the-wind-a-primer-on-tobacco-moisture/) pipe tobacco is packaged a bit moist (for easier handling) at 15-18%, while 12-14% is his preferred moisture content for smoking. Of course, others may like their tobacco even drier than that.
According to Agriculture and Food Chemistry (vol 5, no. 4, pg 294 (1957)), 12% moisture corresponds to 65% relative humidity and 16% moisture corresponds to 70% relative humidity at 80 F. What this means is that if the average RH is 65% your tobacco would, if left long enough, equilibrate to 12% moisture content. And, for long-term storage this would be very good.
However, the rate at which tobacco dries depends on the difference. So, if your tobacco is at 15%, and you prefer 12%, at 60% rel humidity it will be painfully slow. My guess, based on what I know of drying processes (which is not a great deal) is that you would need something more on the order of a 10% difference for your tobacco to dry noticeably in a reasonable time (like an hour, but I'm guessing here, I've not done any controlled experiments). This would correspond to a relative humidity around 40% (5.5% moisture content at equilibrium). The amount of sugar and other humectants added to the tobacco will alter these numbers a bit, as will the type of tobacco, but the principle remains the same.
In the winter, with the heat on in the house and the relative humidity at 40% or so, you can set your tobacco out for a bit and expect that it will dry. In humid weather you are going to have to resort to other means to dry your tobacco. Put a dish of water on the table and see how long it takes for the level to go down by 15 or 20% and you'll get the idea.
Lamp, microwave, toaster oven? Somehow I think that the boss would not be too happy if her next bagel came out of the toaster oven tasting like 1Q or Haddo's so I would stay away from the toaster oven. I'm sure it would work well, except for the bagel thing. But that's more of a question for the marriage counselor and I'll steer clear.
For those who smoke a lot of flake tobacco -- can you use a regular "pop-up" style toaster for flake tobacco? How cool would that be?
Heat works because, all other things being equal, relative humidity decreases as the temperature increases. This is why it gets unbearably humid in the evening in the summer. The temperature drops and the relative humidity goes up. Of course, when you heat your tobacco to dry it, you don't want to over-heat your tobacco. Overheating will very likely affect the taste. Unless, of course, you overheat your tobacco and like the result.
Many here seem to like putting their tobacco in a shallow dish under a desk lamp for a while (15 minutes?) and claim that this works well for them. Seems easy enough to do, though I wonder about the future when there are no more incandescent bulbs available. Compact fluorescent and LED bulbs don't generate much heat. Still, for the time being this seems a good, easy, inexpensive solution. Others have recommended a hair dryer. That should work very well also, if one can figure out a means to keep the tobacco from blowing all over the place.
Edited to fix typo.
