From AMS Glossary
As typically defined in meteorology: Daily, especially pertaining to actions that are completed within 24 hours and that recur every 24 hours; thus, most reference is made to diurnal cycles, variations, ranges, maxima, etc.
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 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 (see heliotropic wind, land and sea breeze, mountain and valley wind); 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; 7) atmospheric pressure varies diurnally or semidiurnally according to the effects of atmospheric tides.
Term updated 4 April 2017.