Ground-based measurements on the downward thermal emission spectra from the clear sky have been made in the spring and autumn seasons. These measurements clearly show the presence of the carbon monoxide emission band between 2000--2200 cm-1. A simulation of the background emission has been performed using the FASCD3P line-by-line radiation code. The simulation has been used to isolate the downward thermal emission that is attributed uniquely to carbon monoxide. From these measurements the actual downward greenhouse radiation flux from carbon monoxide has been determined to vary between 0.06 W/m2¿16% and 0.11 W/m2¿10%, depending on the time of year and other meteorological factors. A comparison to the longwave fluxes associated with other important greenhouse gases suggests that carbon monoxide itself may be a radiatively important trace gas in the atmosphere, and may make a significant contribution to the radiation imbalance of the atmosphere. ¿ American Geophysical Union 1995