Grass Surface Research Articles

A study of the transfer of heat and water vapour above a surface of short grass

AbstractThe profiles of temperature, humidity and wind have been determined by conventional methods in the lowest 2 m of the atmosphere above a close‐cut grass surface, with simultaneous observations of the heat absorbed in the soil and of the short‐wave and long‐wave radiative flux of heat. Twentynine sets of observations were made; the details are given in Appendix I.The ratio of the gradients of temperature and humidity is found to be independent of height, and changes with height of the vertical convective flux of heat and of water vapour are relatively small; the ratio of the coefficients of eddy diffusion of heat and water vapour is therefore independent of height. These coefficients are then assumed equal and the convective fluxes of heat and of water vapour determined (Bowen's method). The fluxes are also determined by an aerodynamic method which assumes equality of the eddy viscosity and the appropriate diffusion coefficient. The results of the two methods agree in most cases, in both stable and unstable stratifications, but there are some exceptions which cannot be explained.Absorption of radiation by the air is investigated. It is shown that this is comparable in effect with convection in the control of temperature and relative humidity. Particular attention is paid to four occasions with very light wind (including two with fog formation) when horizontal mixing appeared to be of major importance.Some examples are presented of wind profiles which do not conform to the law proposed by Deacon. A value between 0.5 and 1.5 is suggested for the critical Richardson number.

Notes on the measurement and estimation of atmospheric radiation –2

AbstractAn account is given of further measurements of atmospheric radiation which confirm former conclusions regarding the errors of two types of radiation chart when used to compute total radiation at the surface. The greater part of the difference between computed and observed radiation is attributed to two causes – emission from particulate matter and the variation of emissivity of water vapour with temperature. The results suggest a form for this variation between −5°C and +25°C.Some measurements and computation of the change of radiative flux in the lowest metre at night, i.e., of the direct radiative cooling of the air, are reported. Miscellaneous subjects discussed include the radiation from atmospheric dust, the radiative temperature of a short grass surface and its measurement by simple means in all conditions, and an anomalous occasion with very high incoming radiation for which no explanation could be found. Some comments are offered on the experimental justification of two possible uses of the radiation charts.