 Solution

Low temperature favours association into the dimer and, especially, suppresses its thermally activated dissociation back to chlorine monoxide. More of the dimer can therefore follow photodissociation instead of the chemically unproductive thermal return. The supplied branching fraction is a local condition, not a temperature-independent universal yield.

Cold polar air also permits polar stratospheric clouds and efficient heterogeneous conversion of chlorine reservoirs into photolabile forms. The stratospheric polar vortex isolates and cools that air. Darkness permits activation and storage, but rapid catalytic ozone loss requires the return of sunlight. These chemical and meteorological conditions explain the springtime maximum; low temperature alone does not supply the photodissociation photons.