  Hiscock and Weems showed that there is an attractor behavior in the evolution
of asymptotically flat Reissner-Nordstr\"om black hole under Hawking
evaporation. If the initial charge-to-mass ratio $Q/M$ of the black hole is
relatively small, then the ratio first increases until the black hole hits the
attractor, and then starts to discharge towards the Schwarzschild limit.
Sufficiently charged black holes, on the other hand, simply discharge steadily
towards the Schwarzschild limit. In this work we further investigate the nature
of the attractor, and found that it is characterized by the mass loss rate
being equal to the charge loss rate. The attractor is not necessarily related
to the specific heat in a general evaporating black hole spacetime, but for the
Reissner-Nordstr\"om case part of the attractor lies very close to the boundary
of the region where specific heat changes sign.
