observational day

Any 24-hour period selected as the basis for climatologic or hydrologic observations.

Where observations are recorded automatically, the observational day is commonly taken as the calendar day. Where only one observation is made in 24 hours, the observational day is assumed to be the 24 hours ending at the time of observation.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.