Question

What happens to transpiration on a hot windy day

Answer 1

Transpiration usually increases on a hot, windy day — but with important caveats.

Why it tends to increase
- Higher temperature raises the saturation vapor pressure of air, so for the same relative humidity the vapor pressure deficit (VPD = driving force for evaporation) is larger. Larger VPD → more water vapor wants to leave the leaf.
- Wind reduces the leaf’s boundary layer (the still air layer next to the leaf), lowering boundary-layer resistance and allowing water vapor to diffuse away faster.
- Together, higher VPD and lower boundary-layer resistance increase total conductance to water vapor, so transpiration rate rises.

Simple relationship (conceptual)
- Transpiration E ≈ g_total × VPD, where g_total is total conductance (function of boundary-layer and stomatal resistances). Wind decreases boundary-layer resistance (↑g_total); heat increases VPD — both tend to raise E.

When transpiration can decrease instead
- If soil water is limited or the plant senses excessive water loss, the plant will close stomata (increase stomatal resistance) to conserve water. Stomatal closure can reduce transpiration despite heat and wind.
- Prolonged stress can lead to wilting, reduced photosynthesis, cavitation in xylem, or leaf scorching.

Practical consequences
- Faster water loss and increased irrigation demand in crops.
- Higher risk of dehydration for plants if water supply is inadequate.

So: initially and all else equal, transpiration increases on a hot, windy day; if the plant responds by closing stomata (or if soil water is limiting), transpiration can be reduced.