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We describe a vertically scanning infrared radiometer for measuring the air–sea temperature difference without disturbing the water skin layer. The radiometer operates with a single wavelength channel that is 1.1 µm wide, centered on 14.2 µm, on the short-wavelength edge of a CO_2 atmospheric absorption band. The resulting high atmospheric absorption enables calibration of the horizontal-viewing signal with an in situ air-temperature sensor. The signal at all other scan angles is measured relative to that at the horizontal, providing a differential air–sea temperature measurement that is nearly independent of calibration offsets that can be a problem with independent air- and water-temperature sensors. We show data measured on a ship in the Tropical Western Pacific Ocean during July 1999, which exhibit important discrepancies from in situ data using bulk air- and water-temperature sensors. These discrepancies illustrate important differences between bulk versus skin water temperature.