Forest and agricultural burning release chemical compounds and particulate matter into the atmosphere. Although most of this material contributes to visibility reductions through haze and provides chemical constituents available for reactions with other atmospheric pollutants, there are occasions when smoke is entrapped locally and combines with water vapor to produce zero visibility smoke/fog or superfog. Superfog threatens transportation when it drifts over roadways – a problem in the South, especially at night. In 1997, a project to measure smoke moving along the ground at night included a small prescribed burn that produced superfog. Then, in 2001, ground photographs of superfog were taken at the edge of a prescribed burn during daytime. These observations made possible the development of two new hypotheses for superfog formation. One of these hypotheses, the moisture excess hypothesis, is the subject of this paper. Measurements of relative humidity and temperature in smoke from smoldering fuels support the hypothesis that moisture boiled from smoldering fuels may be a greater contributor to superfog than hygroscopic nuclei in the smoke.