Abstract
A long aircraft flight in fog at altitude 60 m was conducted on 25 September 1981, due west from San Francisco, California, until clear air was reached. The fog began near the coast and the air remained foggy until the boundary with the warmer seawater. This sea surface temperature transition provides a sudden 3° to 4°C temperature increase between two regions of relative uniform surface temperature water.
In fog the random vertical motion (turbulence) is extremely low, the foggy air essentially being still, but on encountering warmer water convection starts, the air becomes turbulent, and rapid drying results due to the now warmer cloud parcels penetrating the inversion so the drier overlying air is mixed down.
The fog average drop diameters are close to 11 μm, but the concentration ranges from 10 to 250 drops mg−1. This corresponds to slow entity-type entrainment mixing. It seems that the fog formed near the coast and then drifted westward, but whether any entrainment is still active at the fog top when the samples were taken is uncertain. The lack of random vertical motion shows that cloud-top cooling is negligible in the fog because there can be no heat transport when there axe no fluctuations in the vertical velocity to transport cooler parcels downward (and create eddy fluxes). Thermal surface convection is negligible when the air is almost the same temperature as the water, and the air in fog is saturated, so mixing ratio differences supported by evaporation cannot provide a source of buoyancy. Because the turbulence (vertical velocity fluctuations) is so low within the fog, no source of convection can be present, and there is little alternative to accepting that there is no radiative cooling at its top.
