Through   the   use   of   upper   air   soundings, measurements can be made of temperature and relative humidity, from which stability characteristics can be determined.  Refer  to  the  publication,  Use of the Skew T, Log P in Analysis and Forecasting,  NAVAIR  50-IP-5, for  complete  information  on  analyzing  upper  air soundings. GENERAL PROCEDURE FOR FORECASTING  FOG The synoptic situation, time of year, climatology of the  station,  air-mass  stability,  amount  of  cooling expected,  strength  of  the  wind,  dewpoint-temperature spread, and trajectory of the air over favorable types of underlying  surfaces  are  basic  considerations  you  should take into account when forecasting fog. Consideration  of  Geography  and Climatology Certain  areas  are  more  favorable  climatologically for fog formation during certain periods of the year than others.  All  available  information  pertaining  to climatology  should  be  compiled  for  your  station  or operating area to determine the times and periods most favorable  for  fog  formation. You should also determine the location of the station with respect to air drainage or upslope conditions. Next, determine the type of fog to which your location would be  exposed.  For  example,  inland  stations  would  be more likely to have radiation fog and shore coastal stations advection fog. A determination should then be made  of  air  trajectories  favorable  for  fog  formation  at your station. Frontal Fog Frontal fogs are associated with the forecasting of the movement of fronts and their attendant precipitation areas. For example, fogs can form in advance of a warm front, in the warm air section behind the warm front (when the warm air dewpoint is higher than the cold air temperature), or behind a slow moving cold front when the air becomes saturated. Air-mass Fog The first step in the forecasting of air-mass fogs is  to  determine  the  trajectory  of  the  air  mass  and estimate the changes that are expected to occur during the night. If the air mass has been heated during the day and there was no fog the preceding morning, no marked   cloud   cover   during   the   day,   and   no overwater  trajectory,  fog  will  not  form  during  the night.  However,  during  fall  and  winter,  nights  are long  and  days  are  short,  and  conditions  are  generally stable. When a fog situation has been in existence, the  same  conditions  tend  to  remain  night  after  night, and  the  heating  during  the  day  is  insufficient  to effectively  raise  the  temperature  above  saturation. Also, determine if the air has had a path over extensive bodies  of  water,  and  whether  this  path  was  sufficient to  raise  the  humidity  or  lower  the  temperature sufficiently to form fog. Then construct nomograms, tables,  etc.,  by  using  dewpoint  depression  against time of fog formation for various seasons and winds; modify these in the light of each particular synoptic situation. FACTORS TO BE CONSIDERED IN FOG AND STRATUS FORMATION Wind, saturation of the air mass, nocturnal cooling, and air-mass trajectories have a role in the formation of fog or stratus clouds. Wind Wind velocity is an important consideration in the formation  of  fog  and/or  low  ceiling  clouds. When the temperature and dewpoint are near one another at the surface and eddy currents are 100 feet or more in vertical thickness, adiabatic cooling in the upward portion of the eddy could give the additional cooling   needed   to   bring   about   saturation. Any additional  cooling  would  place  the  air  in  a  temporary supersaturated   state. The extra moisture will then condense out of the air, producing a low ceiling cloud. Adiabatic heating in the downward portion of the eddy will  usually  evaporate  the  cloud  particles.  If  all  cloud particles evaporate before reaching the ground, the horizontal visibility should be good. However, if many particles  reach  the  ground  before  evaporation,  the horizontal visibility will be restricted by moderate fog. Clouds that form in eddy areas may at first be patchy and then become identified as ragged stratus. If the cloud  forms  into  a  solid  layer,  it  will  be  a  layer  of stratus. When conditionally unstable air is present in the eddy, or if the frictional eddy currents are severe enough, 5-19