Even in high humidity times and areas there is plenty of bow work you can do without stressing the bow into unwanted. You can take a stave to 4" of floor tiller without stressing it. These wet times are good times to bring raw staves to shape and then to floor tiller. When the weather turns dry, take out one of your "preforms" and continue with the tillering.
I believe a "hot box" of some sort is necessary for consistent selfbows. Unless you live in an area with consistently low humidity(then you need to hydrate). Wood is hygroscopic...meaning it takes moisture from the air. I have never actually measured the loss and gain like sumpitan has but have developed a feel for the increase and decrease of M/C in staves I am working on. The sound of the tools on the wood and the resistance in the limbs will let you know if it is too wet to work on.
If your home has a heat vent, a wood stove, utility room with hot water heater or other "micro-climates" that are drier than the rest of the house, you have a "hot box"! A simple tube with a light bulb in one end, a box made from plywood or foil backed foam insulation and duct tape with light bulb heat sources are simple, inexpensive hot boxes. A car parked in the sun is another. Let your imagination be your guide...as long as safety in your number 1 concern.
Pat