  Atmosphere-skimming showers are initiated by cosmic rays with incoming
directions such that the full development of the cascade occurs inside the
atmosphere without ever reaching the ground. This new class of showers has been
observed in balloon-borne experiments such as ANITA, but a characterisation of
their properties is lacking. The interplay between the Earth's magnetic field,
the long distances over which atmosphere-skimming showers develop, and the low
density of the atmosphere they traverse gives rise to several effects that are
not seen in downward-going cascades, and require detailed modeling. In this
article, we used the latest version of the ZHAireS-RASPASS shower simulation
program to tackle this problem, and dwell on the particular phenomena that
arises from the peculiar environment on which these showers develop. We focus
in particular on the properties of the longitudinal profile of the shower and
its fluctuations as a function of cosmic-ray energy, direction and primary
mass. We have also studied the phase-space of cosmic-ray arrival directions
where detection in high-altitude experiments is more likely, and have found
that only in a small range of directions the showers are sufficiently developed
before reaching the altitude of the detector. Our results are relevant for the
design of high-altitude and in particular balloon-borne experiments, and for
the interpretation of the data they collect.
