Why does cooling air cause water vapour to condense?

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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.

Cooling air causes water vapour to condense because the capacity of air to hold water vapour is strongly dependent on temperature. Specifically, air at higher temperatures can hold more water vapour at saturation than air at lower temperatures.1

As air cools, its saturation vapour pressure—the maximum amount of water vapour the air can hold at a given temperature—decreases. When the air temperature drops to the dew point, the air becomes saturated with water vapour. If the air is cooled further, the rate of condensation exceeds the rate of evaporation, causing the excess water vapour to transition into liquid water droplets or ice crystals. This process often requires aerosol particles in the atmosphere to act as condensation nuclei, providing a surface for the water molecules to gather.1234