  The vertical profile of atmospheric depth is an important element in
extensive air shower studies. The depth of shower maximum is one of the most
important characteristics of the shower. In the fluorescence technique of
shower detection, the geometrical reconstruction provides the altitude of
shower maximum, so that an accurate profile of atmospheric depth is needed to
convert this altitude to the depth of shower maximum. In this paper the
temporal variation of experimentally measured profiles of atmospheric depth at
different sites is studied and implications for shower reconstruction are
shown. The atmospheric profiles vary on time scales from hours to years. It is
shown that the daily variation of the profile is as important as its seasonal
variation and should be accounted for in air shower studies. For precise shower
reconstruction, the daily profiles determined locally at the site of the air
shower detector are recommended.
