length of record

The period during which observations have been maintained at a meteorological station, and which serves as the frame of reference for climatic data at that station.
The standard length of record for the purpose of a normal has been fixed by the World Meteorological Organization as 30 years (i.e., three consecutive 10-year periods), which is a reasonable average for the length of a homogeneous record desirable for most of the meteorological elements. Homogeneous records as long as 50 years are rare due to breaks or gradual changes being introduced by changes in the hours of observation, in the observational practices, in the site or instruments used, or by a gradual change in the character of the surrounding country, such as the growth of a city. It is often possible, however, to account for these changes and to construct a composite record that may cover a century or more.

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.