The Life Cycle of Valley Fog. Part II: Fog Microphysics

Abstract

Extensive measurements were made of the microphysics of valley fog in the Chemung River Valley near Elmira, New York. This paper discusses data on drop size distributions, drop concentrations, liquid water contents, and haze and cloud nucleus concentrations obtained on eight fog nights. The behavior patterns of the microphysical variables were found to be extremely consistent. Shallow ground fog usually occurs prior to the formation of deep valley fog. The data show that ground fog is characterized by droplet concentrations of 100 to 200 per cubic centimeter in the 1 to 10 μm radius range with mean radii of 2 to 4 μm. As deep fog forms aloft, droplet concentration near the surface decreases to less than 2 cm−3 and the mean radius increases from 6 to 12 μm. Droplets of radii <3 μm disappear. Thereafter, droplet concentration and liquid water content increase gradually until the first visibility minimum at the surface when typical values range from 12 to 25 cm−3 and 50 to 150 mg cm−3, respectively. The small droplets reappear at first visibility minimum. Subsequently, bimodal drop size distributions occur in approximately half of the fogs with one mode at 2–3 μm radius and a second mode between 6 and 12 μm. Aloft, drop size distributions become narrower and the mean radius decreases with both increasing altitude and increasing age of the fog. The cloud nucleus concentration active at S = 3.0% is usually between 800 and 1000 cm−3 near the surface and decreases to 500–800 cm−3 at 300 m. It is argued from the data that supersaturation in the thin ground fog exceeds that in deep fog. The initial surface obscuration in deep fog appears to be due to droplets that form aloft and are transported downward into unsaturated air by turbulent diffusion. New droplets are apparently not generated near the surface until after the first visibility minimum.