  We revisit the evaporation process of gas from dwarf galaxies after it has
been photo-ionized by the UV flux from the first stars and AGNs and heated to
T~10^4K or 2x10^4K respectively. Earlier estimates, based on the balance
between pressure and gravity, indicated that dark haloes of virial velocity
lower than Vevap ~ 11-13 km/s have lost most of their gas in a dynamical time.
We follow the continuous evaporation by a thermal wind during the period when
the ionizing flux was effective. We find that the critical virial velocity for
significant evaporation is significantly higher. For example, if the ionization
starts at z-ion=10 and is maintained until z=2, a mass loss of one e-fold
occurs in haloes of Vevap ~ 25 (or 35 km/s) for T ~ 10^4K (or 2x10^4K). Haloes
of Vevap ~ 21 km/s (or 29 km/s) lose one e-fold within the first Hubble time at
z=10. Any dwarf galaxies with virial velocities smaller than Vevap must have
formed their stars from a small fraction of their gas before z-ion, and then
lost the rest of the gas by photo-evaporation. This may explain the gas-poor,
low surface brightness dwarf spheroidal galaxies. By z<1, most of the IGM gas
was evaporated at least once form dwarf galaxies, thus providing a lower bound
to its metallicity.
