The rate of transpiration will be maximum when the soil is humid and atmosphere is dry because of following reasons.
(i) Availability of more water in the soil is necessary for the absorption of water by the roots in order to carry it up to the xylem. Thus, lack of water decreases transpiration and over all health of the plants. Whereas, more water in soil increases transpiration rate.
(ii) In dry climates, transpiration is increased. Water is forced to diffuse more rapidly into the air due to the concentration difference between the environment outside and inside the plant. Low humidity creates a vapour gradient between the plant and the air. In dry air, there is a lack of water, forcing water to be pulled from the plant to the atmosphere increasing transpiration. Therefore, in humid climates, transpiration is less effected by diffusion.