Umkehr effect
From Glossary of Meteorology
An anomaly of the relative zenith radiances of scattered sunlight at certain wavelengths in the ultraviolet as the sun approaches the horizon, due to the presence of the ozone layer.
The ratio of radiances measured in the zenith direction at two wavelengths, one strongly absorbed by ozone, the other not so strongly absorbed, shows a steady decrease with solar zenith angle until a zenith angle of about 86°, whereupon the ratio increases. The details of this reversal can be used to determine the vertical distribution of ozone concentration.
