  We examine the radiative cooling of coronal loops and demonstrate that the
recently identified catastrophic cooling (Reale and Landi, 2012) is due to the
inability of a loop to sustain radiative / enthalpy cooling below a critical
temperature, which can be > 1 MK in flares, 0.5 - 1 MK in active regions and
0.1 MK in long tenuous loops. Catastrophic cooling is characterised by a rapid
fall in coronal temperature while the coronal density changes by a small
amount. Analytic expressions for the critical temperature are derived and show
good agreement with numerical results. This effect limits very considerably the
lifetime of coronal plasmas below the critical temperature.
