Definitions for "Diurnal variation"
The changes of value, for example, of a meteorological element within the course of a (solar) day. More especially, it denotes the systematic changes that occur during the average day.
This term is used to indicate the changes, in the course of an average day, in the magnitude of a meteorological element. The most striking example of this is the diurnal variation of barometric pressure in the tropics, the chief component of which has a 12-hourly period. The maxima of this variation are about 10 a. m. and 10 p.m., the minima about 4 a.m. and 4 p.m. local time.
A regular swing in mood, usually starting the day in deep depression but feeling less gloomy as the day progresses. This is one characteristic sign of DEPRESSION (qv).