1999 Volume 55 Issue 2 Pages 137-143
The radiative flux in the lowest three meters above a bare-soil surface was directly measured on calm nights with little cloud cover. Although divergence of upward radiative flux occurred above 1m, convergence was often observed between 0.2m and 1m all through the night. Almost the same results were obtained for the net flux except that the transitional height between divergence and convergence was some tens of centimeters, which means that radiative warming occurred just above the bare-soil surface during the night. This phenomenon can be explained by postulating that cold air is produced by conduction at the surface of small heat-insulated projections (HIPs) such as soil grains on the ground surface, while the ground releases the heat stored during the day by radiation through the pores between HIPs and warms the air immediately above the surface at night. This “HIP hypothesis” can also account for the so-called “raised minimum (RM)” phenomenon.
