Biology
Plant Temperature Compared to Surroundings
Quick fact
A well-watered leaf in full sun can be several degrees cooler than the surrounding air, thanks to evaporative cooling from transpiration; conversely, a drought-stressed plant can become several degrees warmer than the air.
Why this is interesting
Have you ever touched a leaf on a hot day and felt it surprisingly cool? Or noticed a plant in full sun feeling almost hot? Plants aren't just passive thermometers—they have a special relationship with temperature.
Read the full explanation
Understanding Plant Temperature Compared to Surroundings
Think of a leaf as a tiny balance sheet. It receives energy from sunlight, which warms it. But it also loses energy by giving off water vapour through its pores (stomata), which cools it down. When the energy in equals the energy out, the leaf reaches a steady temperature. If transpiration is strong, the cooling effect can exceed the warming, making the leaf cooler than the air. If the plant is short on water, stomata close to save water, but then cooling stops, and the leaf warms above air temperature.
A deeper explanation
The leaf's temperature is governed by its energy balance: absorbed sunlight (solar radiation) heats the leaf, while it loses heat by emitting longwave radiation, through convection with the air, and by transpiration (evaporation of water). Transpiration cools the leaf because water molecules evaporating carry away latent heat. The rate of transpiration depends on stomatal opening, humidity, and wind. When stomata close to prevent water loss, the cooling component diminishes, causing leaf temperature to rise. This is why the difference between leaf and air temperature is a sensitive indicator of plant water stress. In nature, many plants have adaptations, such as reflective hairs or orientation, to manage heat load, but the core principle remains the same—plants actively regulate their temperature through water use. Understanding this balance also explains why canopy temperatures can be used in agriculture to detect irrigation needs.