The scattered light intensity from the zenith observed on the ground varies due to changes in solar elevation angle. When the intensity ratio of two arbitrary wavelengths within the ultraviolet region of zenith light are observed in succession using a Dobson spectrophotometer or a Brewer spectrophotometer, the intensity ratio between the two wavelengths increases at the beginning when the solar elevation angle is decreasing, and then starts decreasing at some point. The aspect of change in intensity ratio is referred to as inversion, and observation of the inversion aspect is referred to as Umkehr observation. The atmosphere can be divided into several layers based on elevation. The amount of ozone in each layer can be obtained by using data measured through Umkehr observation.