The duration of the Earth’s revolution round the Sun, forming a natural but slightly variable unit of time.
In glaciology, as in other disciplines concerned with the natural progression of the seasons, the year may vary in length for reasons of necessity or convenience, and depending on whether the particular investigation requires precise treatment of calendar time. For the latter, see Julian date.
In mass-balance practice the year is always either exactly or approximately 365 calendar days long (the duration of a calendar year which is not a leap year; see hydrological year, mass-balance year). However the sidereal year is very nearly equal to 365.2564 mean solar days. In turn, the mean solar day is very nearly equal to 86 400 seconds, and 1 day is defined in the Système International d’Unités as an accepted non-SI unit equal to 86 400 seconds exactly.
The practice when brevity is desirable, regardless of hemisphere, is to identify the hydrological year, mass-balance year or measurement year by the calendar year in which it ends. For example the mass-balance year 2000 began in calendar year 1999 and ended in calendar year 2000.
Glossary of glacier mass balance and related terms. Working Group on Mass-balance Terminology and Methods of the International Association of Cryospheric Sciences (IACS)