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Stratus formed by turbulence will occur when:
  • A
    absolute instability exists at low level.
  • B
    the wind speed is less than 10 kts and the air is heated by the Earth's surface.
  • C
    the wind speed is greater than 10 kts and the condensation level is situated just above the turbulent layer.
  • D
    in the friction layer mixing occurs by turbulence and the condensation level is situated below the top of the turbulent layer.
Refer to figures.
Learning Objective 050.04.01.01.02: Describe cloud formation based on the following lifting processes: unorganised lifting in thin layers and turbulent mixing; forced lifting at fronts or over mountains; free convection.

TURBULENCE INVERSION/CLOUD

This type of cloud formation is relatively rare, and requires a layer of significant turbulence (often behind high ground on a windy day, but sometimes when surface conditions change, causing mechanical turbulence, as in the diagram above. The layer also needs to be absolutely stable to stop rising air turning into a convection current.

Initially, an area of turbulence forms. This causes the higher up air to be dragged down and the lower air to be dragged upwards, which happens many times, and "shakes up" the air. As the air is both rising and descending at the DALR (dry adiabatic lapse rate), this "shake up" of the layer causes the previously stable air (quite a small ELR - environmental lapse rate), to have an environmental lapse rate the same as the DALR, 3ºC/1000ft. This means that the air now cools at a much faster rate with increasing altitude.

This does not add or remove any energy from the system, as that is not possible. What has happened is that the air half way up the layer has stayed the same temperature, but the air above is cooler than before, and the air below is warmer than before. The energy has been re-distributed, with a higher environmental lapse rate.

This means that the higher up air may drop below the dewpoint (if the air contains enough moisture), leading to a layered stratus cloud that begins at the condensation level and ends at the top of the turbulent layer. This scenario means that the condensation level (the height the air reaches the dewpoint) is below the top of the turbulent layer


An inversion will often be found at the top of the turbulence layer, where the cold air at the top of the turbulent layer now sits below warmer air in the unaffected layer. This gets the name of a "turbulence inversion". There are YouTube videos that visually demonstrate this effect. You can find them by searching for "turbulence inversions".

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