Twice-daily mixing heights, one morning and one afternoon were calculated by using the computational program MIXHTS. MIXHTS utilizes the meteorological data collected from the King Khalid International Airport (KKIA) surface and upper air stations in Riyadh city during the year 2002. Climatic data shows that the city of Riyadh has in general, a hot and dry weather in summer and cold and dry in wintertime, whereas strong insolation is dominant all over the year. Monthly average afternoon mixing heights are ranged from 1629 m to 3971 m, whereas the morning mixing heights are ranged from as low as 935 m to 2920 m. Estimation of mixing heights obtained by this work were found in a good agreement with the daily maximum value obtained from the dry adiabatic temperature method
