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Why does most water vapour condense before ultraviolet light can break it apart in the upper atmosphere?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

The premise that water vapour must condense before ultraviolet light can break it apart is accurate. In the Earth's atmosphere, the destruction of water vapour occurs through a process called photodissociation, where ultraviolet light provides the energy necessary to break water molecules into hydrogen and oxygen.1

This process is largely prevented by a "cold trap" located at an altitude of approximately 15 km (about 9.3 miles). Because the temperature of the troposphere decreases rapidly with altitude, the air becomes extremely cold at this height. Since the capacity of air to hold water vapour is strongly dependent on temperature—with warmer air holding significantly more moisture than cooler air—these freezing temperatures cause the vast majority of water vapour to condense into ice crystals or droplets.123

By condensing and returning to lower altitudes, the water is effectively removed from the upper atmosphere, thereby escaping exposure to the ultraviolet radiation that would otherwise cause its dissociation.1