Umkehr effect

The Umkehr is the time variation of the ratio of the scattered intensity at two different wavelengths. The word means 'reversal' in German. The Umkehr effect is observed when measurements are made with ultraviolet spectrophotometer of the ratio of the zenith sky light intensities of two wavelengths in the solar ultraviolet when the sun is near the horizon. The shorter of two wavelengths (intensity I) is strongly absorbed and other (intensity I' ) is weakly absorbed. If the value of log(I/I' ) is plotted against the sun's zenith angle, it is observed that this log-intensity ratio decreases as the zenith angle increases until a minimum is reached for a zenith angle of about 8 (when the wavelengths are 3114 and 3324 A). This effect was first noticed by Götz in 1930. The Umkehr measurement is known as customarily N-value and is given by the logarithm base 10 of the ratio of cloudless zenith sky intensitities at two different wavelengths scaled by a multiplicative factor 100 plus a constant which depends on instruments and extraterrestrial radiation. Methods for deriving vertical distribution from the umkehr measurements were developed by Götz, Dobson and Meetham in 1934, using the Dobson ozone spectrophotometer developed by Gordon Dobson. In 1964 Carlton Mateer provided analysis on information content in umkehr measurements.