(1) Ice that contains a liquid phase of no more than moderate salinity with which it is in thermodynamic equilibrium at the solidliquid phase boundaries. (2) Less precisely, ice that is at its pressur
e-melting point. Other factors, notably the salinity of water inclusions, can be of comparable importance to the hydrostatic pressure for determination of the melting point. Sense 2 is adequate for simple purposes, but the details illuminated by Lliboutry (1971) and Harrison (1972) are likely to be of practical importance in detailed work.
A measure of the kinetic energy of randommotion with respect to the average. Temperature is properly defined only foran equilibrium particle distribution (Maxwellian distribution).
A model of mass balance in which surface ablation is estimated as a function of temperature, usually near-surface air temperature measured either on the glacier or at the nearest weather station; temp
eratures may also be taken from upper-air soundings, meteorological reanalyses or climate models. A leading form of temperature-index model is the positive degree-day model, in which a degree-day factor represents the dependence of ablation on temperature. Temperature-index models are valuable because they require only simple input variables and perform well when suitably calibrated, for example by allowing for the differences in reflectivity between surfaces of ice and snow by choosing a smaller degree-day factor for snow than for ice.