Clouds Flashcards
What are clouds?
Simply the presence of water droplets or ice crystals suspended in the atmosphere.
How are clouds formed?
When water vapour in the air condenses. For condensation to occur, air must be cooled below the dew point by either:
Adiabatic cooling
Heat loss by conduction or evaporation
Temperature decrease as a result of mixing
When does condensation occur?
When the air is cooled until it becomes saturated.
What is the coalescence mechanism?
Small raindrops in cloud coalesce, creating a large drop that can no longer remain aloft. As it falls it collides with smaller drops and coalesces further, falling faster and coalescing even more.
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What is super-cooling?
Water droplets with a temperature below 0 are said to be super-cooled.
What is free convection?
The almost vertical movement of air, rising of its own accord. E.g. Thermals.
What is slant-wise ascent?
Frontal or Orographic lifting.
Precipitation caused by frontal lifting is transient whereas precipitation caused by orographic lifting continues as long as the air flows.
What is Forced Convection?
If are is forced to rise by slant-wise ascent and then encounters a conditionally unstable layer, convection may take place, resulting in cumuliform cloud.
Likely to occur in mountainous regions as well as along an advancing front.
What is Turbulent Mixing?
When layers of air move at different speeds and in different directions, mixing will occur at their common boundary.
Can there be a diurnal variation of clouds?
Yes. Clouds formed by free convection have a very clear diurnal cycle, as they are triggered by surface heating.
When is the maximum occurrence of convective clouds likely?
The maximum occurrence of convective clouds is normally found in the late afternoon.
What do convective clouds forming in the morning indicate?
A conditionally unstable atmosphere.
What are the three levels of cloud?
Cloud Low (CL)
Cloud Medium (CM)
Cloud High (CH)
What are High Clouds (CH)?
Bases 20,000ft+
Cirrus (Ci)
Cirrocumulus (Cc)
Cirrostratus (Cs)
