  In $N+1$ dimensions, false vacuum decay at zero temperature is dominated by
the $O(N+1)$ symmetric instanton, a sphere of radius $R_0$, whereas at
temperatures $T>>R_0^{-1}$, the decay is dominated by a `cylindrical' (static)
$O(N)$ symmetric instanton. We study the transition between these two regimes
in the thin wall approximation. Taking an $O(N)$ symmetric ansatz for the
instantons, we show that for $N=2$ and $N=3$ new periodic solutions exist in a
finite temperature range in the neighborhood of $T\sim R_0^{-1}$. However,
these solutions have higher action than the spherical or the cylindrical one.
This suggests that there is a sudden change (a first order transition) in the
derivative of the nucleation rate at a certain temperature $T_*$, when the
static instanton starts dominating. For $N=1$, on the other hand, the new
solutions are dominant and they smoothly interpolate between the zero
temperature instanton and the high temperature one, so the transition is of
second order. The determinantal prefactors corresponding to the `cylindrical'
instantons are discussed, and it is pointed out that the entropic contributions
from massless excitations corresponding to deformations of the domain wall give
rise to an exponential enhancement of the nucleation rate for $T>>R_0^{-1}$.
