The rate of transpiration depends on how quickly water vapour can evaporate inside the leaf and diffuse out through the stomata. Four main environmental factors affect it.Light intensity. Stomata open in the light so that carbon dioxide can enter for photosynthesis. As light intensity increases, more stomata open and the rate of transpiration increases. In the dark, most stomata close and transpiration is very low.Temperature. A higher temperature gives water molecules more kinetic energy, so they evaporate and diffuse faster. Warm air can also hold more water vapour, so the air outside the leaf stays drier and the diffusion gradient is steeper. The rate of transpiration increases.Humidity. Humid air already contains a lot of water vapour. This reduces the water vapour concentration gradient between the inside and the outside of the leaf, so the rate of transpiration decreases.Air movement (wind). Still air allows a layer of humid air to build up around the leaf. Wind removes this layer, keeping the water vapour concentration gradient steep, so the rate of transpiration increases.Water availability. If the soil is dry, the plant loses turgor and the stomata close to prevent wilting, so the rate of transpiration falls.These factors can be investigated with a potometer by changing one factor at a time, for example using a fan for air movement or a lamp at different distances for light intensity.