A Simple Thermodynamic Model of Radiation Fog Formation

A simple thermodynamic model for predicting fog at 0600 h local time at Don Muang Airport in the winter from surface observations at 0000 h (midnight) is presented. The basic parameters used are temperature and humidity of the surface air at 0000 h, and mean cloud amount for 0000 h to 0600 h. It is assumed that a layer of air with constant thickness at the surface loses heat by long-wave radiation, thereby falling in temperature and increasing its relative humidity. If the dew point is reached, water vapor then condenses, and the number of drops per unit volume with an assumed constant diameter (10 pm or 20 pm) is calculated. Estimates of the visibility are then calculated from the number of drops per unit volume. Different values of the basic parameters are used to investigate how they determine the number of drops formed. The results, found by numerical integration of the differential equations representing the thermal processes, are in reasonable order of magnitude agreement with actual meteorological observations. They give an indication of the likely depth of the surface layer under a temperature inversion formed by nocturnal radiation.