  Several experiments measure the fluorescence light produced by extensive air
showers in the atmosphere. This light is converted into a longitudinal shower
profile from which information on the primary energy and composition is
derived. The fluorescence yield, as the conversion factor between light profile
measured by EAS experiments and physical interpretation of showers, has been
measured in several laboratory experiments. The results, however, differ
considerably. In this article, a model calculation of the fluorescence emission
from relevant band systems of nitrogen in dependence on wavelength and
atmospheric conditions is presented. Different calculations are compared to
each other in combination with varying input parameters. The predictions are
compared with measurements and the altitude-dependence of the fluorescence
yield is discussed in detail.
