EO 301.1 Describe the four factors that control the
evolution of clouds.
NUCLEATION PROCESS
Changes of state
- Vapour to liquid
- Liquid to solid
- Vapour to solid
› Leading to the formation of
- Water droplets
- Ice crystals
Dimensions and concentration of hygroscopic nuclei
Three types of nuclei can be distinguished according to their dimensions:
- Aitken nuclei, with radius of less than 0.1 micrometre
o Aitken nuclei are very small and, even though there are a great
number of them, they play a negligible role in condensation due to
their small size.
- Large nuclei, with radius from 0.1 to 1 micrometre – know these
o Large nuclei are the most active. Their concentration varies between
10/cm³ for the largest among them to 1000/cm³ for the smallest.
- Giant nuclei, with radius of 1 micrometre or more
o Giant nuclei play a negligible role in condensation, not because of
their dimension, but because there aren’t very many of them
The process of breaking up of an ice crystal into a large number of
fragments is referred to as
fragmentation
FOUR FACTORS THAT CONTROL THE EVOLUTION OF CLOUDS
Clouds form when water vapour condenses into water droplets or undergoes
deposition into ice crystals. The evolution of clouds depends on:
- Amount of water vapour available
- Abundance of condensation and freezing nuclei
- Stability of the air
- Lift
EO 301.2 Describe the four types of vertical motion
that lead to the formation of clouds.
Most clouds form when moist air is lifted up and cooled down to the
saturation point, also called the lifted condensation level (LCL). The types of lift are listed below:
- Convection
- Mechanical turbulence
- Orographic lift
- Large scale lift
Clouds form when water vapour gas condenses into liquid water
droplets or undergoes deposition into ice crystals. The evolution and
type of cloud depend on what four factors? Discuss and explain each factor
Amount of water vapor available
Stability of the air
Amount of lift -Most clouds form when
What is the growth rate of a droplet through the condensation process
alone - not important
Growth rate: fast then slow
- 1 µm – 10 µm: one second
- 10 µm – 100 µm: six minutes
- 100 µm – 1 mm: three hours
- 1 mm – 3 mm: several days
- Name the four types of vertical motion that assist in the formation of
clouds. Discuss and explain each type
-dont worry about this one
Convective lift -There are two main processes that initiate convection in the atmosphere: Daytime heating, Cold air advection. Often, in unstable air, a parcel of air whose temperature is warmer than
surrounding air will become buoyant and lift up. This commonly occurs in the boundary layer due to daytime heating in the warmer seasons. As the sun heats a land surface, the lower layers of the atmosphere warm up and become unstable.
- Mechanical turbulence
-Mechanical turbulence refers to an eddying motion of the air caused by
friction between the air and the ground as the air flows over the Earth’s
surface. The intensity of the mechanical turbulence and the height to which it
will extend depend on: - Roughness of the underlying surface
- Strength of the wind
- Stability of the air
When wind blows over a rough surface, the air currents twist and swirl.
If the wind is of moderate strength, mechanical turbulence will cause air to
mix in the lower levels. - Orographic lift- the result of air being forced to higher terrain. The extent of the cloud that forms depends on the moisture in the air and
the type of cloud depends on the stability of the air.
The air descending on the downwind side of a slope will be compressed and
heated and this will cause cloud in this area to dissipate - Large scale lift
-This is a very important lifting process. A large region of horizontal
convergence of at least several hundred kilometres in the boundary layer is coupled with a large region of horizontal divergence in the upper
troposphere. Consequently, an upward vertical motion is created causing the air between these two levels to rise. As a result, the rising air cools adiabatically and condensation occurs.
The extent and type of clouds associated with convergence can vary greatly
and depend on the moisture content of the air and its stability as well as on
the horizontal and vertical extent of the convergent area.
