CONDITIONS FAVORABLE FOR UPSLOPE FOG AND STRATUS

Figure 5-17.-Saturation time chart. (0’s indicate actual temperature and dewpoint observations. Straight line is forecast temperature-dewpoint trends.) In late fall and winter, when continental temperature gradients have intensified, and the land temperature has become colder than the adjacent water, poleward moving air is cooled by advection over colder ground, as well as by radiation. If the air is sufficiently moist, fog or stratus may form. During daytime, heating may dissipate the fog or stratus entirely. If not, the heating, together with the wind, which is advecting the air, sets up a turbulence inversion and stratus or stratocumulus layers form at the base of the inversion. At night if the air is cooled again and the surface pressure gradient is weak, a surface inversion may replace the turbulence inversion, and fog again occurs at the surface. However, if the pressure gradient is strong, cooling will intensify the inversion. Under these conditions stratus or stratocumulus clouds occur just as in the daytime, except with lower cloud bases. Late fall and winter advection-radiation fogs cart occur any place over the continent that can be reached by maritime air or modified returning continental air. Mainly, this occurs over the eastern half of the United States. However, since tropical air masses do not reach as high a latitude in winter as in summer, the frequency of such fogs are much less in the northern regions of the country. With large, slow-moving, continental warm highs over the eastern half of the country, however, the fogs may extend all the way from the Gulf of Mexico to Canada. CONDITIONS FAVORABLE FOR UPSLOPE FOG AND STRATUS Upslope fog and stratus occur in those regions in which the land slopes gradually upward, and those areas accessible to humid, stable air masses. In North America, the areas best meeting these conditions are the Great Plains of the United States and Canada and the Piedmont region east of the Appalachians. The synoptic conditions necessary for formation of this type of fog or stratus are the presence of humid air and a wind with an upslope component. The stratus is not advected over the station as a solid sheet. It forms gradually overhead. The length of time between the first signs of stratus and a ceiling usually ranges from 1/2 hour to 2 hours; although at times, the stratus may not form a ceiling at all. A useful procedure is to check the 5-23