Dead fuel moisture responds solely to ambient environmental conditions and is critical in determining fire potential. Dead fuel moistures are classed by timelag. A fuel's timelag is proportional to its diameter and is loosely defined as the time it takes a fuel particle to reach 2/3's of its way to equilibrium with its local environment. Dead fuels in NFDRS fall into four classes:

• 1-h, less than 1/4" diameter.

Fine flashy fuels that respond quickly to weather changes. Computed from observation time temperature, humidity, and cloudiness.

• 10-h, 1/4 to 1" diameter.

Computed from observation time temperature, humidity, and cloudiness. Or can be an observed value, from a standard set of "10-Hr Fuel Sticks" that are weighed as part of the fire weather observation.

• 100-h, 1 to 3" diameter.

Computed from 24-hour average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges.

• 1000-h, 3 to 8 " diameter.

Computed from a 7-day average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges.