Water activity (aw) is the ratio of the vapour pressure of water in a material to that of pure water at the same temperature. It runs from 0 (bone dry) to 1.0 (pure water). Unlike moisture content, it measures how available the water is to microbes and chemical reactions, which is what decides mould risk and shelf stability.
In practice
ASTM D8197 specifies 0.55-0.65 aw as the acceptable range for dried cannabis flower: below it buds get brittle and harsh, above it mould risk rises. Many moulds can't grow below about 0.70 aw.
House position: what matters during the cure is the bud's moisture content and water activity, not the jar RH as such. Ways to check: - a calibrated hygrometer in a sealed jar, read only after it has equilibrated (several hours or overnight); then aw ≈ RH / 100; - a bench water-activity meter, the lab method (ASTM D8196); - the stem snap and weight loss as cross-checks.
Units and formula
aw = p / p₀ (vapour pressure over sample / over pure water, same temperature). At equilibrium, aw = ERH / 100. So 0.62 aw ≈ 62% equilibrated jar RH.
Don't confuse with
- Moisture content: how much water there is, in % by weight.
- Jar RH: only equals aw × 100 once equilibrated.
- Relative humidity of the room: air, not bud.
