Factors Affecting Transpiration
Light intensity stimulates stomata opening. This is because an increase in light intensity results in an increase of rate of photosynthesis. Photosynthesis requires gaseous exchange and the stomata is the pore in which gaseous exchange takes place. Therefore an increase in photosynthesis results in an increase in stomata opening (increased gaseous exchange).
Evaporation of water from the plant occurs primarily through the stomata, so as you can understand, an increase in light intensity results in an increase in transpiration due to an increase stomata opening.
Plants transpire faster at higher temperatures. This is because the rate of evaporation increases with temperature.
Humidity can be defined as the relative amount of water in the atmosphere. If the air is dry, then the diffusion gradient would be greater from the plant to the atmosphere. There would be an increase in the diffusion of water vapour out of the stomata and into the atmosphere. If the air is humid (containing a lot of water in the atmosphere) then the opposite would be true, and very little water would diffuse out of the atmosphere.
When there is very little wind, this means that the layer of water vapour directly surrounding the leaves is not being swept away. Because of this, there is an increase in humidity of the air directly surrounding the leaves. This results in a lower rate of transpiration.
If the plant has little water available, it will close its stomata. This will of course cause a decrease in the rate of transpiration.