Millimetre wave sky radiation temperature fluctuations

The movements of clouds and moist air are responsible for temporal changes in sky radiation temperatures. Detailed are measurements of the sky radiation temperatures made during periods of cloud movement and light rain. The measurements were made over the frequency bands 26 to 40 GHz (in the ka-band) and 90 to 98 GHz (in the w-band) using a direct detection and heterodyne radiometer. Temporal derivatives and the spectra of the temperature changes are examined. The measurements indicate that the most rapid temperature changes are due to cloud movements. The standard deviations of these temperature changes were measured at 0.030 K/s for the ka-band and 0.072 K/s for the w-band. The spectral analysis indicated that these changes took place at frequencies less than 10 mHz. The higher frequency fluctuations in the sky radiation temperatures were less than 20 mK and less than 30 mK at ka-band and w-band respectively. This level of fluctuation represents an upper limit set by the difficulty of achieving DC coupling over the long measurement periods of /spl sim/1000 s.