Humidity

Relative humidity is the ratio of the water vapour actually present in the air to the maximum it could hold at that temperature, expressed as a percentage. Because warmer air can hold more water vapour, relative humidity changes through the day even when the absolute moisture content of the air stays roughly constant — it typically falls as air warms and rises as air cools, all else being equal.

Plants lose water through their leaves as vapour, and the rate of that loss depends heavily on how much "drying power" the surrounding air has — a relationship often summarised as vapour pressure deficit, which combines temperature and humidity. Very low humidity can push transpiration demand beyond what roots can supply, contributing to water stress even when soil moisture is adequate, while sustained high humidity can affect pollen behaviour and fruit-surface drying in some crops.

Many important fungal and bacterial pathogens, including those causing downy mildew and gray mold, spread more readily under prolonged high humidity or free moisture on plant surfaces. Powdery mildew is a partial exception, often favoured by high humidity but able to establish without free water on the leaf. Low humidity, by contrast, is less associated with disease but increases evaporative water loss and can compound drought stress.

• Ventilation and, in some systems, dehumidification in greenhouses and other protected cultivation
• Plant spacing and pruning to improve airflow through the canopy and speed leaf-surface drying
• Irrigation timing that avoids prolonged leaf wetness, such as watering early in the day rather than in the evening
• Selecting disease-resistant cultivars in regions or seasons with persistently high humidity

Humidity is measured with hygrometers and, increasingly, networked sensors used in greenhouses and precision-agriculture systems to track microclimate conditions in near real time. Many crop-disease forecasting models use humidity and leaf-wetness data alongside temperature to estimate infection risk and inform the timing of preventive management.