(a) Intensity of light:
During the day, the stomata is open to facilitate the internal CO2 distribution of photosynthesis. At night it closes. Therefore, most transpiration occurs during the day. When it is cloudy during the day, the stomata are slowly closed and transpiration is reduced.
(b) Humidity of the atmosphere
Transpiration is reduced when the air outside is moist. High humidity in the air reduces the level of external distribution of internal water vapor across the stomata, thereby reducing transpiration rate.