Vapour pressure deficit is the difference between the vapour pressure air could hold at its temperature and the vapour pressure it actually holds, \( \mathrm{VPD} = e_s(T) - e \), usually reported in kilopascals. It is the physical gradient that drives water out of a leaf or a dead twig, which makes it a better predictor of drying than relative humidity alone: because the saturation curve is steep, \( 20\% \) humidity on a hot day carries a far larger deficit, and dries fuels far faster, than \( 20\% \) humidity on a cold one. Fire climatology increasingly reports VPD for exactly this reason, and large-fire seasons correlate with it more tightly than with humidity. The confusion this resolves is treating two equally humid days as equally dangerous when their temperatures differ. After this Concept you can rank two days by how hard the atmosphere is pulling water out of fuels, even when their relative humidities are identical.
Wildfire risk and behavior emerge from the interaction of fuel, weather, and terrain acting on a landscape already conditioned by climate; ignition source alone does not determine fire size or severi…