  We estimate the amount of vorticity generated at second order in cosmological
perturbation theory from the coupling between first order energy density and
non-adiabatic pressure, or entropy, perturbations. Assuming power law input
spectra for the source terms, and working in a radiation background, we
calculate the wave number dependence of the vorticity power spectrum and its
amplitude. We show that the vorticity generated by this mechanism is
non-negligible on small scales, and hence should be taken into consideration in
current and future CMB experiments.
