The Life Cycle of Valley Fog. Part I: Micrometeorological Characteristics

Abstract

Extensive measurements were made of micrometeorological variables associated with eleven fogs in the Chemung River Valley near Elmira, N.Y. Temperature was measured at five levels on a 17 m tower, dew point at three levels, wind speed and direction at two levels, and net radiation and vertical wind at one level. Visibility was measured at three locations, and dew deposition and evaporation at one location near the surface. Vertical temperature distributions were also measured using an aircraft. The microphysical variables are discussed in Part II of this paper. Consistent patterns of behavior of all micrometeorological variables were observed. The formation of ground fog may be explained by radiational cooling of the surface and associated low-level heat exchange. To explain observed temperature behavior (maximum cooling rate near 100 m in the 6 h preceeding fog) and the initial formation of a thin fog layer slightly below that level, it seems necessary to invoke Defant's model of valley circulation. Radiation divergence at the fog layer aloft then produces an inversion near the fog top and unstable conditions at lower levels. The fog base therefore propagates downward. Dew deposition is responsible for formation of a low-level dew point inversion before fog forms, a necessary condition for initial fog formation aloft. The inversion breaks as fog forms and dew weight is constant from that time until sunrise. Evaporation of dew after sunrise maintains saturation throughout the fog depth as fog temperature increases, and is therefore responsible for fog persistence. Dissipation of fog occurs when evaporation rate is no longer adequate to maintain saturation.