  We investigate the metastability of scalar fields in quantum field theories
at finite temperature, focusing on a detailed understanding of the bounce
solution. At finite temperature, the bounce solution depends on two variables:
the Euclidean time $\tau$ and the spatial radial distance $r$, and it is
periodic in the $\tau$ direction. We propose a novel method to determine the
bounce that describes transitions in a thermal bath, suitable for numerical
calculations. Two types of bounces exist for transitions in the thermal bath:
$\tau$-dependent and $\tau$-independent bounces. We apply our method to compute
these bounces in several models, including both thin-wall and thick-wall
scenarios, to examine their properties. Specifically, we evaluate the critical
temperature below which the $\tau$-independent bounce becomes destabilized due
to fluctuations, rendering it irrelevant. We demonstrate that in the thick-wall
case, the $\tau$-dependent bounce smoothly transitions into the
$\tau$-independent one as temperature increases, whereas in the thin-wall case,
the transition between the two types of bounces is discontinuous.
