Radiation-temperature dependence of water at microwaves

Abstract

Abstract Pure water radiation-temperature dependence at wavelength 8 mm in the temperature range of 30-50 °C has been experimentally investigated. Measurements were made with the microwave radiometer with central frequency 37.5 GHz (wavelength 8 mm). The absolute method of measuring the intensities of distributed radiation was used. A horn with a 30x30 mm opening was used as an antenna. Calibration of the intensity of the received radiation was carried out by its own radiation of a black body at different temperatures. The equipment had a sensitivity (fluctuation threshold) 0.2 K at a time constant τ = 1 sec. An increment of the radio-brightness contrast is about 3 K in this temperature range. The water radio-emission depends non-linearly on the kinetic temperature. It was measured, that effective temperature of the skin-depth layer increases from 0.28 up to 0.4 °C per degree of water heating. It was shown, that the radiation temperature of the water surface for the 8 mm waves and temperature range 30-50 °C is determined not by the bulk kinetic water temperature, but by the effective temperature of the skin-depth layer formed under the influence of heat exchange with the atmosphere.