  The zenith angle distributions of atmospheric neutrinos are determined by the
possible presence of neutrino oscillations and the combination of three most
important contributions: (1) geomagnetic effects on the primary cosmic rays,
that suppress the primary flux in the Earth's magnetic equatorial region, (2)
the zenith angle dependence of the neutrino yields, due to the fact that
inclined showers produce more neutrinos, and (3) geometrical effects due to the
spherical shell geometry of the neutrino production volume. The last effect has
been recognized only recently and results in an important enhancement of the
flux of sub--GeV neutrinos for horizontal directions. In this work we discuss
the geometrical effect and its relevance in the interpretation of the
atmospheric neutrino data.
