  Surface neutron counter data are often used as a proxy for atmospheric
ionisation from cosmic rays in studies of extraterrestrial effects on climate.
Neutron counter instrumentation was developed in the 1950s and relationships
between neutron counts, ionisation and meteorological conditions were
investigated thoroughly using the techniques available at the time; the
analysis can now be extended using modern data. Whilst surface neutron counts
are shown to be a good proxy for ionisation rate, the usual meteorological
correction applied to surface neutron measurements, using surface atmospheric
pressure, does not completely compensate for tropospheric effects on neutron
data. Residual correlations remain between neutron counts, atmospheric pressure
and geopotential height, obtained from meteorological reanalysis data. These
correlations may be caused by variations in the height and temperature of the
atmospheric layer at ~100hPa. This is where the primary cosmic rays interact
with atmospheric air, producing a cascade of secondary ionising particles.
