A formula for the ionization rate in extremely intense electromagnetic field is proposed and used for numerical study of QED (quantum-electrodynamical) cascades in noble gases in the field of two counter-propagating laser pulses. It is shown that the number of the electron-positron pairs produced in the cascade increases with the atomic number of the gas where the gas density is taken to be reversely proportional to the atomic number. While the most electrons produced in the laser pulse front are expelled by the ponderomotive force from region occupied by the strong laser field there is a small portion of the electrons staying in the laser field for a long time until the instance when the laser field is strong enough for cascading. This mechanism is relevant for all gases. For high-$Z$ gases there is an additional mechanism associated with the ionization of inner shells at the the instance when the laser field is strong enough for cascading. The role of both mechanisms for cascade initiation is revealed.
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