the expected cooling is greater than the late afternoonspread, either fog or low clouds should be expected.Wind velocity will determine which of the twoconditions will form.FORECASTING STRATUS FORMATIONAND DISSIPATIONFog and stratus forecasting are so closely tiedtogether that many of the fog forecasting rules andconditions previously mentioned also apply to theforecasting of stratus clouds.Determining the Base and Topof a Stratus LayerOne of the first steps in forecasting the dissipationof stratus is to determine the thickness of the stratuslayer. The procedure is an follows:1. Determine a representative mixing ratiobetween the surface and the base of the inversion.2. Project this mixing ratio line upward through thesounding.3. The intersection of the average mixing ratio linewith the temperature curve gives the approximate baseand maximum top of the stratus. Point A in figure 5-19is the base of the stratus layer, and point B is themaximum top of the layer. Point A is the initial base ofthe layer; but as heating occurs during the morning, thebase will lift. Point B represents the maximum top ofthe stratus layer; although in the very early morning, itmight lie closer to the base of the inversion. However,as heating occurs during the day, the top of the stratuslayer will also rise and will be approximated by point B.If the temperature and the dewpoint are the same at thetop of the inversion, the stratus will extend to this level.To determine the height of the base and the top ofthe stratus layer, use either the method previouslyoutlined for fog, or the pressure altitude scale.Determining Dissipation TemperaturesTo determine the temperature necessary for thedissipation of a stratus layer, the following steps areprovided:1. From point A in figure 5-19, follow the dryadiabat to the surface level. The temperature of the dryadiabat at the surface level is the temperature requiredto be reached for stratus dissipation to begin. This ispoint C.Figure 5-19.-Sounding showing the base and the top of stratus layers. Also note temperature at which dissipation begins andtemperature when dissipation is complete.5-27