Transpiration is the process by which plants lose water in the form of water vapour into the atmosphere.Water is lost through stomata, cuticle and lenticels. Among the environmental factors affect the rate of transpiration are;-
1.Temperature;High temperature increases the internal temperature of the leaf ，which in turn increases kinetic energy of water molecules which increases evaporation.
High temperatures dry the air around the leaf surface maintaining a high concentration gradient.More water vapour is therefore lost from the leaf to the air.
2.Humidity:The higher the humidity of the air around the leaf, the lower the rate of transpiration. The humidity difference between the inside of the leaf and the outside is called the saturation deficit.
In dry atmosphere, the saturation deficit is high.At such times, transpiration rate is high.
3.Wind; Wind carries away water vapour as fast as it diffuses out of the leaves.This prevents the air around the leaves from becoming saturated with vapour. On a windy day, the rate of transpiration is high.
4.Light Intensity: When light intensity is high; more stomata open hence high rate of transpiration.
5.Atmospheric Pressure: The lower the atmospheric pressure the higher the kinetic energy of water molecules hence more evaporation.
Most of the plants at higher altitudes where atmospheric pressure is very low have adaptations to prevent excessive water-loss.
6.Availability of Water: The more water there is in the soil, the more is absorbed by the plant and hence a lot of water is lost by transpiration.