Fuel moisture content is defined as the mass of water in a fuel divided by the mass of that same fuel when oven dry, expressed as a percentage: \( M = 100 \times \frac{m_{wet} - m_{dry}}{m_{dry}} \). The dry-mass denominator is the whole trick, and it is where nearly everyone stumbles: because the denominator excludes the water, values above \( 100\% \) are perfectly ordinary — a living shrub at \( 150\% \) simply carries one and a half times its dry weight in water. Fuel moisture is the single most powerful lever on whether a fire spreads at all, because every gram of water must be evaporated, at roughly \( 2.26 \) megajoules per kilogram, using heat that would otherwise preheat the next stem. The confusion this resolves is reading fuel moisture as a fraction of total weight, which caps it at \( 100\% \) and makes live-fuel numbers look like errors. After this Concept you can compute a fuel moisture from a wet and a dry weight and say what it means for ignition.
Dead fuel moisture content, the percentage of water in dead wildland vegetation, governs wildfire ignition and spread potential because dead fuel (unlike live fuel) responds purely to ambient atmosph…