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In a valley an inversion can be formed by..
  • A
    turbulent air due to rotors.
  • B
    katabatic winds with cold air.
  • C
    advection of moist air from the sea.
  • D
    heat from the sun in the afternoon.

Refer to figure.

A katabatic wind is caused by a flow of cold air down a hill or mountain side at night, when the highlands radiate heat and are cooled.

  • The air in contact with the higher-level ground is also cooled, and it becomes denser than the air at the same elevation but away from the slope; it, therefore, begins to flow downhill.
  • The katabatic effect is most marked if the mountain side is snow covered, if the sky is clear to assist radiation and if the PG is slack.
  • Speeds average 10 kt and the flow of cold air into the valley helps frost and fog to form.
In valleys, inversions are also called cold-air pools and form as a result of both cooling of the ground due to long-wave radiation and nocturnal down-slope winds (Katabatic winds). The colder air which flows down the valley walls may “collect” at the bottom of the valley, creating a cold pool of air near the surface and low temperatures on the ground. They usually extend from the bottom of the valley up to the boundary-layer top. In winter, inversions can last for a few days to several weeks, being triggered and maintained by anticyclonic conditions at synoptic scale

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