Dew

Abstract Dew formation on short grass has been studied with a balance, recording weight of condensation, and with filter papers to absorb moisture. Three regimes are distinguished: in the first, daytime evaporation continues and the grass remains dry; in the second, the surface continues to lose weight but the grass becomes wet owing to the partial condensation of water vapour evaporating from the soil; in the third, this loss of weight ceases or there is an increase in weight attributed to condensation of atmospheric water vapour. A distinction is therefore made between ‘distillation’ of water vapour from soil to grass (1–2 mg cm −2 hr −1 ) occurring on very calm nights through a laminar layer with a transfer coefficient approaching the molecular value; and ‘dewfall,’ the turbulent transfer of water vapour from the atmosphere, negligible when the wind at 2 m falls below 0.5 m/sec but reaching 3–4 mg cm −2 hr −1 with stronger winds. This distinction is supported by analysis of the surface heat‐budget. On windless nights, since heat flux from the soil and net radiative loss were almost equal, the transfers of sensible and latent heat from the atmosphere were negligible and distillation was of much greater importance than dewfall. Implications for taller crops and warmer climates are briefly considered.