Quick answer: Relative humidity (RH) describes how much water vapor is in the air relative to the maximum possible at that temperature. Dew point is the temperature at which condensation begins. Vapor pressure deficit (VPD) describes the difference between saturation vapor pressure and actual vapor pressure.
Why RH changes with temperature
Relative humidity can change when air temperature changes even if the actual amount of water vapor does not. This is why an RH number should always be recorded with air temperature.
Dew point and condensation
When a surface cools to or below the dew point, condensation can form. In grow spaces, storage rooms, greenhouses, and enclosures, condensation risk depends on surface temperature as well as room RH.
VPD and plants
VPD is used in controlled-environment growing because it relates to the drying demand around leaves and influences transpiration. The suitable target varies by crop, growth stage, leaf temperature, lighting, airflow, and root-zone conditions. A chart based only on room temperature and RH is an estimate if leaf temperature is not considered.
Read the trend together
- Record air temperature and RH from the same location and time.
- Compare day, night, lighting, irrigation, humidification, dehumidification, and ventilation cycles.
- Watch duration outside the desired range, not just momentary peaks.
- Investigate condensation, leaf response, and mold risk rather than using one RH number for every space.
Sources
- University of Missouri Extension: Vertical Farming Project Report
- University of Minnesota Extension: Controlling Moisture Problems
Reviewed August 1, 2026. Educational content; application-specific targets should be confirmed for the actual system, crop, or aquatic species.