Factors affecting the Transpiration Rate
The list below gives the factors which affect the transpiration rate. Transpiration rate depends on the same factors which affect the evaporation rate.
An increase in the air temperature warms the water inside the leaves more quickly causing it to evaporate quicker. It also increases the capacity of the air to absorb more water.
Direct sunlight has the same effect as temperature as it warms the leaves up quicker. It also cause the stomata to open, thus more transpiration occurs during the day and the rate is higher on a sunny day compared to a cold dull day.
Transpiration relies on diffusion. Windy conditions cause the air molecules to be blown away from the leaves preventing the air around the leaves becoming saturated with water molecules.
Humidity is the concentration if water vapour in the air. At low humidity there is a lower concentration of water molecules in the air around the leaves. This concentration gradient helps the transport of water molecules from the leaves by diffusion. High humidity means the air around the leaves is already saturated and has a higher concentration of water molecules than inside the leaves.