A simple and rugged polythene shielded radiometer is described. The instrument measures the net hemispherical thermal infrared irradiance of the sky even during daylight hours, using white and black sensors to separate solar from long‐wave thermal radiation. A compensation radiometer used to calibrate our instrument is also described, the calibration procedure permits measurements of the hemispherical thermal infrared irradiance of the sky with an error of ∼5% during the night and of less than 10 W/m2during daylight hours.