Studies on the measurements of crop emissivity and sky temperature

A method is presented for measuring crop thermal emissivity in situ by changing the environmental longwave irradiance. The thermal environment of wheat canopy was modified using a bottomless hemispherical cover which can be swiftly opened and closed by hand. An extrapolation technique is used to determine the step change in the crop brightness temperature immediately after the environment is modified so that the effect of the response of the real leaf temperature to the environmental change is removed. The mean sky temperature, which determines the natural environment, was obtained from multiangular measurements of an infrared thermometer with a narrow field of view of 2.0°.