Live Fuel Moisture
Live fuel moisture is the water content of living vegetation, and it is the single most important factor determining how much fuel is available to burn. When fire agencies talk about fuels "curing" through the summer, this is the number they are watching.
What It Measures
Fuel moisture content is the weight of water in a fuel as a percentage of the fuel's oven-dry weight. Because the water in living tissue can weigh more than the dry material itself, values above 100 percent are normal: a fresh green leaf can hold three times its dry weight in water, a moisture content of 300 percent.
Moisture governs fire behavior because evaporating water absorbs heat. High fuel moisture slows energy release and keeps heat from preheating nearby fuel to ignition temperature (Burgan and Rothermel 1984). If the heat needed to drive off a fuel's moisture exceeds what a firebrand can deliver, ignition simply does not occur (Simard 1968). As vegetation dries through the season, less heat goes into evaporation and more into flame.
Reading the Numbers
| Moisture content | Stage of vegetative development |
|---|---|
| 300% | Fresh foliage; annuals developing early in the growing cycle |
| 200% | Maturing foliage, still developing, with full turgor |
| 100% | Mature foliage; new growth complete and comparable to older perennial foliage |
| 50% | Entering dormancy; coloration starting; some leaves dropped. Also indicates drought stress |
| Below 30% | Completely cured; treat as dead fuel |
Why It Matters in San Mateo County
Local fire agencies sample live fuel moisture through the season, and the numbers shape when our fire danger peaks. Coastal fog keeps our fuels wetter longer than inland counties, but by late summer and fall, chamise, coyote brush, and other chaparral fuels routinely cure toward critical thresholds, and offshore wind events arrive exactly when moisture bottoms out. This is also the science behind your own landscape: an irrigated, healthy plant is a high-moisture fuel, and a drought-stressed or dead one burns like cured chaparral. See fire-safe landscaping.
View current California live fuel moisture data (National Fuel Moisture Database)