Synthetic spectra have been calculated from model hydrostatic stellar atmospheres using physical parameters (effective temperature, abundances, etc.) appropriate for Cepheids. Velocity gradients have been kinematically introduced in order to study their effect on line profile asymmetries and radial velocity curves. The results are compared to high resolution observations of $\eta$ Aql.
It is shown that a significant velocity gradient is needed near the phase of maximum infall velocity to account for observed line profile asymmetries and velocity differences of absorption lines from atoms of different ionization and excitation. The effect of this velocity gradient is to reduce the amplitude of the pulsation velocity curve at optical depth $\tau = 2/3$ by 20\% and to decrease the $\gamma$ velocity by 2 km/sec relative to the standard Barnes-Evans or Baade-Wesselink assumption of a co-moving atmosphere.
Barnes-Evans calculations of $\eta$ Aql have been made taking velocity gradients into account. The resulting size and distance of $\eta$ Aql is reduced by about 17\%.