We discuss errors and uncertainties in calibrating infrared radiometers to measure the temperature of clouds in the earth's atmosphere. Many of the points we make apply to any radiometric
temperature-sensing application where the target temperature is less than the ambient temperature. Dominant uncertainties and errors are due to ambient radiance reflected from the blackbody-simulator source, thermal fluctuations in the radiometer, and imprecise voltage measurements. Our improved technique removes, reduces, or accounts for these errors and uncertainties. The resulting calibration uncertainty is ± 0.8°C for a radiometer with a 10-v output range. We verified this accuracy by comparing cloud-base temperatures measured by ground-based IR radiometers, in situ (radiosonde) sensors, and other remote sensors on the ground and on satellites. We made these comparisons for spatially uniform blackbody clouds that filled the field of view of our ground-based radiometers.