DISSIPATION To determine the surface temperature necessary for the  dissipation  of  fog  by  using  the  Skew  T  Log  P Diagram, trace dry adiabatically from the intersection of the average mixing ratio line and the temperature curve to the surface level. The temperature of the dry adiabat at the surface level is the temperature necessary for dissipation. This temperature is known as the CRITICAL  TEMPERATURE.  This  temperature  is  an approximation,  since  it  assumes  no  changes  will  take place in the stratum from the time of observation to the time  of  dissipation. This  temperature  should  be modified on the basis of local conditions. See figure 5-18. In considering the dissipation of fog and low clouds, you  should  consider  the  rate  at  which  the  surface temperature  will  increase  after  sunrise.  Vertically  thick fog, or multiple cloud layers, will slow up the morning heating at the surface. If advection fog is present, the fog may be lifted off the ground to a height where it is classified  as  stratus.  If  ground  fog  is  present,  the increase  in  surface  air  temperature  will  cause  the  fog particles  to  evaporate,  thus  dissipating  the  fog.  Further heating may evaporate advection fog and low clouds. FORECASTING  ADVECTION  FOG OVER THE OCEANS In  the  absence  of  actual  temperature  and  dewpoint data and with a stationary high (a southerly flow is assumed),   use   the   following   method   to   forecast advection fog over the ocean. 1. Pick out the point on an isobar at which the highest sea temperature is present (either from the surface chart or a mean monthly sea temperature chart). Assume that at this point, the air temperature is equal to that of the water and has a dewpoint 2 degrees lower. 2. Find the point on the isobar northward where the water is 2 degrees colder. From this point on, patchy light fog should occur. 3. From a saturation curve chart (fig. 5-14), find how much further cooling would have to occur to give an excess over saturation of 0.4 GM/KG, and also 2.0 GM/KG.  The  first  represents  the  beginning  of  moderate fog  and  the  second  represents  drizzle. 4. As the air continues around the northern ridge of the high, it will reach its lowest temperature, and from then on will be subject to warming. The pattern will then be drizzle until the excess is reduced to 2.0 GM/KG, and moderate 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 high is moving, trajectories will have to be calculated. The fog is usually less widespread than calculated, and drizzle is less extensive. Also, clearing and lifting on the east side of the high is slightly faster. This method appears to work well in the summer over the Aleutian areas where such fog is frequent. FORECASTING UPSLOPE FOG Orographic lifting of the air will cause adiabatic cooling at the dry adiabatic rate of 5.5°F per 1,000 feet. If  an  adequate  amount  of  lifting  occurs,  fog  or  low clouds will form. This process can create challenges for the forecaster. The  procedures  for  determining  the  probability  of fog or low clouds during nighttime hours at stations having upslope winds are as follows: 1. Forecast the amount of nocturnal cooling, 2.  Determine  the  expected  amount  of  upslope cooling by using the following steps: a. Determine the approximate number of hours between sunset and sunrise. b.  Estimate  the  expected  wind  velocity  during the nighttime hours. c. Multiply a by b. This will give the distance the upslope wind will move during the period of the day when  daylight  heating  cannot  counteract  upslope cooling. d.  Determine  the  approximate  terrain  elevation difference  between  the  station  and  the  distance computed  in  c.  Elevation  difference  should  be  in  feet. (Example,  2.5  thousand  feet.) e.  Multiply  the  elevation  difference  by  the  dry adiabatic rate of cooling. (Example, 2.5 times 5.5 = 13.75°F  of  upslope  cooling.) 3.  Add  the  expected  amount  of  upslope  cooling  to the expected nocturnal cooling to arrive at the total amount  of  cooling. 4.  Determine  the  late  afternoon  temperature dewpoint spread at the station under consideration. If 5-26