Relative Humidity, Dew Point, and Condensation
Use relative humidity and dew point correctly to identify condensation risk on leaves, walls, ducts, and other crop-space surfaces.
Educational reference · evidence, sources, and limits shown below
Use relative humidity and dew point correctly to identify condensation risk on leaves, walls, ducts, and other crop-space surfaces.
Terms to know
- Relative humidity
- Ratio of actual vapor pressure to saturation vapor pressure at the same air temperature, expressed as a percent.
- Dew point
- Temperature at which the existing water-vapor content would reach saturation during cooling.
- Condensation
- Conversion of water vapor to liquid water on a surface at or below the local dew point.
- Leaf wetness
- Presence of liquid water on leaf or flower surfaces, whether from condensation, irrigation, or guttation.
Core science
Relative humidity is temperature-dependent. Air can contain the same amount of water vapor while RH rises as it cools and falls as it warms. Dew point is tied more directly to vapor content: when a surface temperature drops to or below the local dew point, condensation becomes possible.
A room can average below 100% RH and still produce wet leaves. Cold exterior walls, chilled supply air, a night-time canopy, uninsulated pipes, dense flowers, and pockets with weak air mixing may have surface temperatures below the sensor-derived dew point. Condensation is therefore a surface-temperature problem as well as a room-humidity problem.
Wetness duration matters biologically. Liquid films can support spore germination and infection for some pathogens, while persistent high humidity can slow drying and create concealed microclimates. Disease still requires a susceptible host and capable pathogen; dew is a risk factor, not proof that disease will occur.
Why this matters in cultivation
- Calculate dew point from a verified temperature and RH pair, then compare it with the coldest relevant surfaces. Map lights-off transitions, exterior corners, supply-air paths, and the interior of dense canopies rather than trusting one wall sensor.
- Correct the moisture source and temperature pathway: remove irrigation and standing-water sources where appropriate, improve drainage and mixing, coordinate heating with ventilation or dehumidification, and avoid directing cold or very dry jets at plant tissue.
Measure and record
Air state
Temperature, RH, calculated dew point, sensor ID, location, and timestamp.
Surface state
Leaf, flower, wall, duct, pipe, or glazing temperature and measurement method.
Transition
Lights, heating, cooling, irrigation, ventilation, and dehumidification status before the event.
Wetness
Location, start, end, suspected source, and photograph or leaf-wetness reading.
Response
Control change, confirmation measurement, and disease-scouting follow-up.
Common misconceptions
Correction: A colder surface can reach the dew point while warmer room air remains unsaturated.
Correction: Both derive from vapor pressure, but dew point describes condensation temperature and VPD describes a vapor-pressure difference.
Correction: Distribution, surface temperature, infiltration, and moisture generation can preserve local risk.
Evidence limits
Dew-point formulas are approximations and sensor error propagates into the result. Use calibrated measurements and a safety margin when surfaces are close to the calculated dew point.
Related encyclopedia topics
- THC-ENC-084-089, THC-ENC-218-220, THC-ENC-282-300, and dry-room condensation control.
Source notes
- Shamshiri R.R. et al. (2018). Review of optimum temperature, humidity and vapour pressure deficit for greenhouse microclimate evaluation and control. International Agrophysics 32:287-302.
- University of Massachusetts Extension. Reducing Humidity in the Greenhouse.
- THC Cultivation SOP Source Materials Packet v1.0 (project source, May 2026).
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.