Abstract
A LinkeβFeussner radiometer was used to measure the radiative temperature of natural surfaces, with small errors caused by the divergence of radiative flux and by its angular variation. On typical cloudless summer days, the maximum temperature of tall crops and of open water was close to maximum air temperature; a bare soil surface exceeded air temperature by 20Β°C; short grass was the coldest surface at night. A linear variation of net (total) radiation with net shortβwave radiation was established for several tall crops, and the increase in longβwave loss per unit increase of net radiation (heating coefficient) was 0Β·08. Daily totals of net radiation as a percentage of solar radiation income were: 37 per cent (bare soil), 41 per cent (short grass), 46 per cent (tall crop), and 53 per cent (water).
When the aerodynamic character of a crop is known, the effective resistance of the stomata to waterβvapour diffusion can be related theoretically to the difference between surface and air temperature. For rough vegetation, estimated stomatal resistance is 0Β·4 to 0Β·8 sec cm^β1^ and the estimated heating coefficient is almost independent of wind speed at about 0Β·1.