Relative humidity: It is the ratio of actual water vapour content to the saturated water vapour content at a given temperature and pressure expressed in percentage (%).
Diurnal variation in relative humidity:
- Mean maximum relative humidity occurs in the early morning.
- Mean minimum, relative humidity occurs in the early afternoon.
- Low RH in the afternoon is due to expansion of air and thus increases the total water vapour capacity
Distribution of RH
- Maximum RH is in the equatorial region due to high evaporation.
- Decreases towards poles upto 30° N and S due to subsiding air mass.
- RH increases in poles due to low temperature.
Effect of Relative Humidity on Crop Production
Relative humidity (RH) directly influences the water relations of plant and indirectly affects leaf growth, photosynthesis, pollination, occurrence of diseases and finally economic yield.
The dryness of the atmosphere as represented by saturation deficit (100-RH) reduces dry matter production through stomatal control and leaf water potential.
- Leaf growth not only depends on synthetic activities resulting from biochemical process but also upon the physical process of cell enlargement.
- Cell enlargement occurs as a result of turgor pressure developed within the cells.
- Turgor pressure is high under RH due to less transpiration. Thus leaf enlargement is high in humid areas
- Photosynthesis is indirectly affected by RH. When RH is low, transpiration increases causing water deficits in the plant.
- Water deficits cause partial or full closure of stomata and increase mesophyll resistance blocking entry of carbon dioxide.
- Moderately low air humidity is favourable for seed set in many crops, provided soil moisture supply is adequate.
For example, seed set in wheat was high at 60 per cent RH compared to
80 per cent when water availability in the soil was not limiting.
- At high RH pollen may not be dispersed from the anthers
- The incidence of insect pests and diseases is high under high humidity conditions.
- High RH favours easy germination of fungal spores on plant leaves.
The blight diseases of potato and tea spread more rapidly under humid conditions. Several insects such as aphids and jassids thrive better under moist conditions.
Very high or very low RH is not conducive for high grain yield. Under high humidity, RH is negatively correlated with grain yield of maize. The yield reduction was 144 kg/ha with an increase in one per cent of mean monthly RH. Similarly, wheat grain yield is reduced in high RH. It can be attributed to adverse effect of RH on pollination and high incidence of pests. On the contrary, increase in RH during panicle initiation to maturity increased grain yield of sorghum under low humidity conditions due to favourable influence of RH on water relations of plants and photosynthesis. With similar amount of solar radiation, crops that are grown with irrigation gives less yield compared to those grown with equal amount of 'water as rainfall. This is because the dry atmosphere, which is little affected by irrigation, independently suppresses the growth of crops.
Very High Relative humidity:
Increases heat load of plants
Reduced CO2 uptake
Reduced transpiration influences translocation of food materials and nutrients.
Moderately high RH of 60-70% is beneficial.
Low RH increases the evapotranspiration