  Extensive air showers initiate the fluorescence emissions from nitrogen
molecules in air. The UV-light is emitted isotropically and can be used for
observing the longitudinal development of extensive air showers in the
atmosphere over tenth of kilometers. This measurement technique is
well-established since it is exploited for many decades by several cosmic ray
experiments. However, a fundamental aspect of the air shower analyses is the
description of the fluorescence emission in dependence on varying atmospheric
conditions. Different fluorescence yields affect directly the energy scaling of
air shower reconstruction. In order to explore the various details of the
nitrogen fluorescence emission in air, a few experimental groups have been
performing dedicated measurements over the last decade. Most of the
measurements are now finished. These experimental groups have been discussing
their techniques and results in a series of Air Fluorescence Workshops
commenced in 2002. At the 8$^{\rm{th}}$ Air Fluorescence Workshop 2011, it was
suggested to develop a common way of describing the nitrogen fluorescence for
application to air shower observations. Here, first analyses for a common
treatment of the major dependences of the emission procedure are presented.
Aspects like the contributions at different wavelengths, the dependence on
pressure as it is decreasing with increasing altitude in the atmosphere, the
temperature dependence, in particular that of the collisional cross sections
between molecules involved, and the collisional de-excitation by water vapor
are discussed.
