Tecnobosque

Equilibrium Moisture Content Calculator

FIRE WEATHER · DEAD FUEL MOISTURE

Equilibrium Moisture Content Calculator

Translate air temperature and relative humidity into the moisture level dead wildland fuel tends toward. Then explore an idealized response over a selected fuel timelag.

Simard equations°C / °FDynamic estimate
EMC is a boundary condition—not measured fuel moisture.Rain, solar heating, shading, fuel size, species and weather history can make actual fuel moisture differ substantially.

Atmosphere and fuel

Optional response estimate

Uses a simple first-order exponential approach to EMC.

CALCULATED EQUILIBRIUM

5.8% moisture content

Simard middle-RH equation · 10–50% RH

After selected time8.1%
Change from start−3.9 points
Response completed63.2%
Remaining difference2.3 points
Drying toward equilibrium

At constant conditions, the idealized 10-hour fuel estimate moves from 12.0% toward 5.8%.

PIECEWISE MODEL

One atmosphere.
Three humidity ranges.

The calculator uses the Simard (1968) equilibrium-moisture functions reproduced by the USDA Forest Service. Temperature is evaluated in degrees Fahrenheit and RH as percent.

RH < 10%E = 0.03229 + 0.281073H − 0.000578HT
10% ≤ RH ≤ 50%E = 2.22749 + 0.160107H − 0.014784T
RH > 50%E = 21.0606 + 0.005565H² − 0.00035HT − 0.483199H
m(t) = E + [m(0) − E]e−t/τ

The optional time response is an idealized exponential approximation. E is equilibrium moisture, m(0) is initial moisture and τ is the selected timelag.

INTERPRETATION

Dead fuels follow weather—with delay.

Relative humidity is the dominant atmospheric driver of EMC, while temperature has a smaller effect. Fine fuels respond faster than larger woody fuels. A current temperature/RH pair cannot reconstruct the fuel’s previous weather exposure.

1 hFine dead fuels
10 hSmall woody fuels
100 hMedium woody fuels
1,000 hLarge woody fuels

Timelag classes are modelling categories, not exact drying times for every particle.

Sources and limits

This tool estimates EMC under constant ambient conditions. It does not estimate live fuel moisture, rain wetting, solar-heated fuel temperature, local fire danger or probability of ignition.

USDA Forest Service: moisture calculations and Simard equations · NWCG: Weather and Fuel Moisture · NWCG: fuel timelag training

Quick answers

Is EMC the same as actual fuel moisture?

No. EMC is the level approached if atmospheric conditions remain constant long enough.

Can this predict fire behaviour?

No. Fuel moisture is only one input among fuels, wind, slope and other factors.

Does it apply to live vegetation?

No. Live fuel moisture is governed by plant physiology as well as weather.