Weather and fire behavior: Relative humidity

The ideal relative humidity range for burning is between 30 and 50 percent. Fires are difficult to control when the relative humidity falls below 30 percent. A relative humidity higher than 50 percent may be appropriate in some circumstances but may not accomplish your objectives in other situations. Relative humidity usually declines as air temperature increases. A rule of thumb is that each 20 degree rise in temperature reduces the relative humidity by about half.