DISSIPATIONTo determine the surface temperature necessary forthe dissipation of fog by using the Skew T Log PDiagram, trace dry adiabatically from the intersectionof the average mixing ratio line and the temperaturecurve to the surface level. The temperature of the dryadiabat at the surface level is the temperature necessaryfor dissipation.This temperature is known as theCRITICAL TEMPERATURE. This temperature is anapproximation, since it assumes no changes will takeplace in the stratum from the time of observation to thetime of dissipation.This temperature should bemodified on the basis of local conditions. See figure5-18.In considering the dissipation of fog and low clouds,you should consider the rate at which the surfacetemperature will increase after sunrise. Vertically thickfog, or multiple cloud layers, will slow up the morningheating at the surface. If advection fog is present, thefog may be lifted off the ground to a height where it isclassified as stratus. If ground fog is present, theincrease in surface air temperature will cause the fogparticles to evaporate, thus dissipating the fog. Furtherheating may evaporate advection fog and low clouds.FORECASTING ADVECTION FOGOVER THE OCEANSIn the absence of actual temperature and dewpointdata and with a stationary high (a southerly flow isassumed), use the following method to forecastadvection fog over the ocean.1. Pick out the point on an isobar at which thehighest sea temperature is present (either from thesurface chart or a mean monthly sea temperature chart).Assume that at this point, the air temperature is equal tothat of the water and has a dewpoint 2 degrees lower.2. Find the point on the isobar northward where thewater is 2 degrees colder. From this point on, patchylight fog should occur.3. From a saturation curve chart (fig. 5-14), findhow much further cooling would have to occur to givean excess over saturation of 0.4 GM/KG, and also 2.0GM/KG. The first represents the beginning of moderatefog and the second represents drizzle.4. As the air continues around the northern ridgeof the high, it will reach its lowest temperature, and fromthen on will be subject to warming. The pattern will thenbe drizzle until the excess is reduced to 2.0 GM/KG, andmoderate fog until 0.4 GM/KG is reached.If actual water and temperature data are available,use these in preference to climatic mean data. If the highis moving, trajectories will have to be calculated.The fog is usually less widespread than calculated,and drizzle is less extensive. Also, clearing and liftingon the east side of the high is slightly faster. This methodappears to work well in the summer over the Aleutianareas where such fog is frequent.FORECASTING UPSLOPE FOGOrographic lifting of the air will cause adiabaticcooling at the dry adiabatic rate of 5.5°F per 1,000 feet.If an adequate amount of lifting occurs, fog or lowclouds will form. This process can create challenges forthe forecaster.The procedures for determining the probability offog or low clouds during nighttime hours at stationshaving upslope winds are as follows:1. Forecast the amount of nocturnal cooling,2. Determine the expected amount of upslopecooling by using the following steps:a. Determine the approximate number of hoursbetween sunset and sunrise.b. Estimate the expected wind velocity duringthe nighttime hours.c. Multiply a by b. This will give the distancethe upslope wind will move during the period of the daywhen daylight heating cannot counteract upslopecooling.d. Determine the approximate terrain elevationdifference between the station and the distancecomputed in c. Elevation difference should be in feet.(Example, 2.5 thousand feet.)e. Multiply the elevation difference by the dryadiabatic rate of cooling. (Example, 2.5 times 5.5 =13.75°F of upslope cooling.)3. Add the expected amount of upslope cooling tothe expected nocturnal cooling to arrive at the totalamount of cooling.4. Determine the late afternoon temperaturedewpoint spread at the station under consideration. If5-26

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