The dew point is the temperature to which air must be cooled, at constant water content, to reach 100% relative humidity. Any surface at or below the dew point collects condensation. Because it depends only on how much water the air holds, it is a more stable measure of moisture than RH.
In practice
Air at 26 °C (79 °F) and 60% RH has a dew point of about 17.6 °C (64 °F). If lights go off and the room or the inside of a dense cola drops to that temperature, water condenses on tissue. Leaves radiating heat to a cool ceiling or wall can sit a degree or two below air temperature, so trouble can start even when the air itself is above the dew point.
Rule of thumb for flower: keep the coldest surface in the canopy at least 3–5 °C (5–9 °F) above the dew point, especially in the first hour after lights-off. Run dehumidification and airflow through the dark period and ramp temperature down slowly. Free water on buds for a few hours is enough for botrytis spores to germinate.
Magnus approximation (°C): Td = 243.04 × γ ÷ (17.625 − γ), where γ = ln(RH ÷ 100) + 17.625 × T ÷ (243.04 + T).
Don't confuse with
- Relative humidity: changes with temperature; dew point does not.
- Frost point: the equivalent below 0 °C, where ice forms.
The amount of water in a material as a percentage of its weight; cured cannabis flower usually sits around 10-12% w/w, though measurement method matters.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.The dense, elongated cluster of flowers that forms at the end of a stem or branch; the main cola tops the leader.A dehumidifier removes water vapour from grow-room air, usually by condensing it on a cold coil; it is the main tool for controlling humidity in flower.Botrytis cinerea, the fungus behind bud rot or grey mould, which rots dense colas from the inside in cool, humid conditions late in flower.Germination is the stage where a viable seed absorbs water, splits its shell and pushes out a root (radicle), usually within 1-7 days at 20-30 °C.Absolute humidity is the actual mass of water vapour in a volume of air, in grams per cubic metre, independent of temperature.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.DIF is day temperature minus night temperature; a larger positive DIF lengthens stems, while zero or negative DIF tends to keep plants shorter.
