adj. Daily; pertaining especially to actions that are completed within 24 hours and are repeated every 24 hours (Whiteman 2000).
The diurnal variability of nearly all of the meteorological elements is one of the most striking and consistent features of the study of weather.
The diurnal variations of important elements at the earth's surface can be summarized as follows: 1) temperature maximum occurs after local noon and minimum near sunrise; 2) relative humidity and fog are the reverse of temperature; 3) wind generally increases and veers by day and decreases and backs by night; 4) cloudiness and precipitation over a land surface increase by day and decrease at night; over water, the reverse is true but to a lesser extent; 5) evaporation is markedly greater by day; 6) condensation is much greater at night; and 7) atmospheric pressure varies diurnally or semi-diurnally according to the effects of atmospheric tides. (AMS 2000)