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If an orographic cloud descends down a slope, its temperature increases at..
  • A
    a lower rate than in dry air, as evaporation, absorbs heat.
  • B
    a lower rate than in dry air, as condensation gives out heat.
  • C
    a higher rate than in dry air, as it gives up latent evaporation heat.
  • D
    the same rate as if the air mass were dry.

Orographic cloud => saturated air mass

As an air parcel descends, it is compressed, and its temperature increases adiabatically. The rate at which temperature increases is dependent on the type of air parcel (whether it is saturated or unsaturated/dry).

THE DRY ADIABATIC LAPSE RATE DALR – Lapse rate for rising dry (unsaturated) air.

Constant/fixed value of 1ºC/100 m (about 3ºC/1000 ft)
THE SATURATED LAPSE RATE SALR – Lapse rate for rising saturated air.

Constant/fixed value of 0.6ºC/100 m (about 1.8ºC/1000 ft)

=> Saturated air warms up at a slower rate than unsaturated/dry air!

  • If the air continues to rise above its condensation level, it starts to cool down at the Saturated Adiabatic Lapse Rate (1.8°C per 1 000 ft), which is lower than the DALR => because latent heat is released during the condensation process (warming up the surroundings) - thus the rate of cooling is slowed down.
  • On the other hand, when saturated air descends it is warmed-up at a slower rate (SALR 1.8°C per 1 000 ft) than the dry air (DALR 3ºC per 1000 ft), because evaporation takes place during which latent heat is absorbed - thus slowing down the rate of warming of the parcel.
Rising unsaturated (dry) air cools down more rapidly than saturated (moist) air.
Sinking unsaturated (dry) air warms-up more rapidly than saturated (moist) air.

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