= 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 \b[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.
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