Timelag fuel moisture models measure moisture in dead vegetation and downed wood of different sizes. "Timelag" means how long it takes these fuels to gain or lose two-thirds of the difference between their current moisture and equilibrium with the air.
Grass, pine needles, small twigs
Medium branches, small logs
Different fuel sizes respond to weather at different speeds. Together, they tell you what's ready to burn right now—and what will burn tomorrow.
drive fire starts and spread rate. They dry out fast, ignite easily, burn quickly.
transition fires from surface to more sustained burning. They indicate whether conditions have been dry long enough for fires to build intensity.
sustain long, intense fires. When these are dry, fires become harder to control and require more resources.
Watch daily. Changes with afternoon humidity, overnight recovery. Critical for predicting fire starts and initial spread.
Watch after 1–2 days of dry or windy weather. Indicates whether fires will sustain or fizzle out.
Watch after extended dry periods (weeks). Indicates potential for deep-burning, hard-to-control fires.
"1-hour and 10-hour fuels dried out yesterday, but 100-hour fuels are still moist from last week's rain. We'll see fire starts that spread fast through grass and brush, but they won't likely transition into the larger fuels. Plan for quick initial attack."
You know if you're facing fast surface fires that burn out quickly (only 1-hour dry) or sustained fires consuming progressively larger material (10-hour and 100-hour also dry).
Moisture content is expressed as a percentage of the fuel's dry weight.
Thresholds vary by fuel type and local conditions. What matters most is the trend and the combination—all three fuel classes dry means very different conditions than just 1-hour dry.
No manual calculations or juggling multiple weather sites. You see all timelag indices, their context, and what they mean for your area—all in one place.
- Nelson, R. M. (2000). Prediction of diurnal change in 10-h fuel stick moisture content. Canadian Journal of Forest Research, 30(7), 1071-1087.
- Viney, N. R. (1991). A review of fine fuel moisture modelling. International Journal of Wildland Fire, 1(4), 215-234.
- Fosberg, M. A., & Deeming, J. E. (1971). Derivation of the 1- and 10-hour timelag fuel moisture calculations for fire-danger rating. USDA Forest Service Research Note RM-207.
- Fire Weather Indices Wiki | 100-hour timelag dead fuel moisture model