A type of ultra-cool brown dwarf with an effective temperature lower than 500 K. Near infrared spectra of these objects show deep absorption bands of H_{2}O and CH_4. So far only seven brown dwarf can
didates belonging to this class have been found, all Y0 subtypes. These objects are very dim, with H magnitudes 19-23. The precise definition of the Y class requires new findings in the future about these objects.
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. 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. 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 Syst
A type of glacier on which ablation by melting or sublimation occurs throughout the year. Year-round ablation is typical of glaciers in the inner tropics, where there are two seasonal temperature maxi
ma each year, and seasonal temperature variations are smaller than diurnal temperature variations. The seasonal variation of mass balance is affected more by variation of accumulation rates between wet and dry seasons than by variation of ablation rates between winter and summer. Year-round ablation is also observed at low altitude on glaciers in some warm maritime climates, as in Norway, and on high-latitude glaciers where ablation is predominantly by sublimation, as in the Dry Valleys of Antarctica.