The AAR, sometimes denoted AAR0, of a glacier with a mass balance equal to zero. Glaciers do not in general have mass balances equal to zero. The Balanced-budget AAR is usually estimated as the AAR at
which a curve (often linear) fitted to a relation between AAR and the Annual surface mass balance Bsfc, observed over a number of years, crosses the axis Bsfc = 0. The AAR0 of non-calving glaciers has been found to vary roughly between 0.5 and 0.6 on average, although the range of variation is substantial. On calving glaciers it is typically larger, approaching 1.0 on the Antarctic Ice Sheet. AAR0 can exceed 0.8 on tropical glaciers of year-round ablation type. The Balanced-budget AAR may differ from the steady-state AAR because it summarizes observations made in conditions that may not approximate to steady state.
The ELA, sometimes denoted ELA0, of a glacier with a climatic mass balance equal to zero on average over a number of years. The Balanced-budget ELA is usually estimated as the altitude at which a curv
e fitted to an observed relation between Annual ELA and Annual mass balance Ba crosses the axis Ba = 0. The uncertainty in such estimates can be substantial, especially when mass-balance sampling is sparse or the equilibrium zone occupies a large fraction of the glacier surface. The Balanced-budget ELA may differ from the steady-state ELA because it is estimated from observations made in conditions that may not approximate to steady state. In particular, most measurements of mass balance published over the past several decades have been negative.
The hypothetical horizontal mass flux (dimension [M T-1]) through a vertical cross section that would be equal to the mass balance (usually the climatic mass balance) over the region upglacier from th
e cross section. Comparison of balance flux and actual mass flux at the same cross section gives an indication of the health of the glacier. If the mass balance of the glacier is zero it follows that at the terminus the balance flux and mass flux are equal, and if there is also no calving that they are equal to zero. If the two are equal at all cross sections the glacier is in steady state.
The volumetric balance flux divided by the area of the vertical cross section through which it passes. Comparison of balance velocity to actual velocity, that is, to the actual volumetric flux (mass f
lux divided by average density) divided by the area of the vertical cross section, gives an indication of the health of the glacier.