  A detailed simulation of vertical showers in atmosphere produced by primary
gammas and protons, in the energy range 1-100 TeV, has been performed by means
of the FLUKA Monte Carlo code, with the aim of studying the time structure of
the shower front at different detector heights. It turns out that the time
delay distribution can be fitted using few parameters coincident with the
distribution central moments. Such parameters exhibit a smooth behaviour as a
function of energy. These results can be used both for detector design and for
the interpretation of the existing measurements. Differences in the time
structure between gamma and proton induced showers are found and explained in
terms of the non-relativistic component of extensive air showers.
