Glaciers are classified according to size, shape, and temperature. Temperate glaciers are at the pressure melting point throughout, whereas polar glaciers are below the pressure melting point except, in some places, at the bed. Ice is basically incompressible, which simplifies many analyses. Stresses, strains, and strain rates are second rank tensor quantities, so nine quantities are needed to describe them. Ice flows in response to stresses in excess of hydrostatic, or deviatoric stresses. To a good approximation, the strain rate, “ε”̇, can be described by “ε” ̇= A“σ” ^n in which A is a temperature-dependent rate factor, “σ”isthe stress, and n is a constant, usually taken to be 3.