  Suppose granular material is shaken vertically with z(t)=A_0 cos(\omega_0 t).
Can we expect to find convection if A_0\omega_0^2 < g? By means of theoretical
analysis and computer simulation we find that there is no critical
\Gamma=|A_0|\omega_0^2/g for the onset of convection. Instead we propose a
modified criterion which coincides with \Gamma=1 for small frequency \omega_0.
