English: Spherical model of the Earth’s atmosphere: Geometry for computing a diagonal path through the Earth’s atmosphere. Atmospheric effects on optical transmission, at an angle z to the normal, can be modelled as a path of length s, as if the atmosphere is concentrated uniformly in approximately the lower 9 km. This path is known as the Airmass. The Air mass coefficient is defined as the ratio ${\displaystyle s/y_{\mathrm {atm} }}$. This model does not apply at non-optical wavelengths where higher layers of the atmosphere affect transmission, for example: the ozone layer at higher ultraviolet frequencies and the ionosphere at lower radio frequencies.