Many natural media are layered at the scale of microwaves. Examples include frozen soil over moist soil, ice over water, and snow over soil. The microwave brightness spectra of such media may exhibit interference patterns. Such patterns have been observed for emission from fresh-water ice but not for emission from snow or from seasonally frozen soil. Three factors which determine whether interference is detectable are the spectral resolution of the radiometer, the uniformity of layer thickness, and the distinctness of interfaces between layers. Analyses of these factors show that: (1) The radiobrightnesses of layered media vary sufficiently slowly with wavelength that radiometers designed for the radio-astronomy bands provide adequate spectral resolution; (2) a variability of layer thickness greater than 15 percent of the free-space wavelength in the area viewed by the radiometer will effectively eliminate an interference pattern; and (3) a diffuse interface, whose thickness is 15 percent of the free-space wavelength, is transparent to microwaves so that effects of the interface will not appear in the radiobrightness spectrum.