  Yellow Hypergiants (YHGs) are massive stars that are commonly interpreted to
be in a post-red supergiant evolutionary state. These objects can undergo
outbursts on timescales of decades, which are suspected to be due to
instabilities in the envelope. To test this conjecture, the stability of
envelope models for YHGs with respect to infinitesimal, radial perturbations is
investigated. Violent strange mode instabilities with growth rates in the
dynamical regime are identified if the luminosity to mass ratio exceeds
$\approx 10^4$ in solar units. For the observed parameters of YHGs we thus
predict instability. The strange mode instabilities persist over the entire
range of effective temperatures from red to blue supergiants. Due to short
thermal timescales and dominant radiation pressure in the envelopes of YHGs, a
nonadiabatic stability analysis is mandatory and an adiabatic analysis being
the basis of the common perception is irrelevant. Contrary to the prevailing
opinion, the models considered here do not exhibit any adiabatic instability.
