n. The amount of time necessary for a dead fuel particle to lose or gain 63 percent of the difference between its initial moisture content and its equilibrium moisture content at a constant temperature and relative humidity.
The length of time for the moisture content of fuel to equilibrate to changing atmospheric moisture levels is highly dependent upon the surface-area-to-volume ratio. Fuel particles with small diameters have a larger surface-area-to-volume ratio and are thus able to equilibrate more rapidly. The timelag concept applies to dead fuel particles only, not to live fuel.
For example, if a fuel particle less than one-quarter inch in diameter (1-hour timelag) has a moisture content of 20 percent while the equilibrium moisture content equals 10 percent, it will take one hour for that fuel particle to lose 63 percent of the difference between 20 percent and 10 percent, or 6.3 percent. Thus, after one hour, the moisture content will be 20 minus 6.3, or 13.7 percent. After another hour under constant conditions, the fuel particle will lose 63 percent of the difference between 13.7 and 10, or 2.3 percent, and then will have a moisture content of 11.4 percent. If temperature and relative humidity does not change, a fuel particle will reach 95 percent of its equilibrium moisture content after four timelag periods.
The 63 percent used in this timelag definition comes from the solution of a step response function written as 1-1/e, where e is the base of the natural logarithm (Byram 1963).