Laboratory and field measurements of the spectral, directional emissivity of surfaces were compared to emissivities calculated from directional hemispherical reflectances to establish the apparent validity of Kirchhoff's law under terrestrial field conditions. The laboratory measurements confirmed results of an earlier experiment that showed that rocks and coarse particulate surfaces follow Kirchhoff's law in the 8 to 14 μm atmospheric window, whereas very fine particulates may not. However, laboratory measurements of samples heated from below are not an adequate simulation of field conditions. Nonetheless, field measurements demonstrated similar results for materials heated by the Sun. Apparent deviations from Kirchhoff's law of more than 0.04 were measured for fine-particulate (quartz) surfaces, dependent on the intensity of downwelling radiance from atmospheric water vapor and clouds. Examples of natural surfaces that may deviate from Kirchhoff's law include wind-blown dust deposits, fresh snow, frosts, and fine volcanic ash in the vicinity of explosive volcanism. Apparent deviations from Kirchhoff's law are caused by steep thermal gradients in the infrared skin depth. A new technique probed this vertical temperature structure, with a sensitivity of 0.2 ¿C, revealing details of a 2 ¿C temperature difference within the infrared-emitting region (the top ~150 μm) of a fine-particulate surface. ¿ 1999 American Geophysical Union