  The asymmetries observed in the line profiles of solar flares can provide
important diagnostics of the properties and dynamics of the flaring atmosphere.
In this paper the evolution of the Halpha and Ca II 8542 {\AA} lines are
studied using high spatial, temporal and spectral resolution ground-based
observations of an M1.1 flare obtained with the Swedish 1-m Solar Telescope.
The temporal evolution of the Halpha line profiles from the flare kernel shows
excess emission in the red wing (red asymmetry) before flare maximum, and
excess in the blue wing (blue asymmetry) after maximum. However, the Ca II 8542
{\AA} line does not follow the same pattern, showing only a weak red asymmetry
during the flare. RADYN simulations are used to synthesise spectral line
profiles for the flaring atmosphere, and good agreement is found with the
observations. We show that the red asymmetry observed in Halpha is not
necessarily associated with plasma downflows, and the blue asymmetry may not be
related to plasma upflows. Indeed, we conclude that the steep velocity
gradients in the flaring chromosphere modifies the wavelength of the central
reversal in the Halpha line profile. The shift in the wavelength of maximum
opacity to shorter and longer wavelengths generates the red and blue
asymmetries, respectively.
