Transcript 01 Condensation

Condensation
Condensation
Atmospheric moisture has its most direct
influence on land only when it is in its
condensed form.
 Condensation is the direct cause of
precipitation.
 It is the reverse of evaporation.
 Water vapour is changed from the vapour
state and becomes droplets of water.

Condensation - conditions
The necessary condition:
cooling of air to below its dew point until
it is saturated
[individual / combined changes in air volume,
pressure, temperature / R.H.]
 The sufficient condition:
presence of condensation nuclei
[hygroscopic particles – wettable substances]

Condensation - conditions

The necessary condition:
cooling of air to below its dew point until
it is saturated
- radiation cooling
- advective cooling
- orographic and frontal uplifting & cooling
- convective or adiabatic cooling
[refer to note p.17 ]
Condensation - conditions

The sufficient condition:
presence of condensation nuclei
- Condensation does not take place easily in
clear or pure air which can be cooled below
its dew point without condensation occurring.
- So, the air is said to be supersaturated.
- It continues to hold water in vapour form
after dew point has been passed.
- Its R.H. is over 100%.
Condensation - conditions

The sufficient condition:
presence of condensation nuclei
[hygroscopic particles – wettable substances]
They attract water-vapour molecules when
the moisture content is near saturation point.
e.g. dust, sea salt from evaporated spray,
sulphur acid from combustion, volcanism,
in numerous supply esp. in urban areas
Condensation - forms
a. Dew and Frost


Dew consists of relatively large water droplets which
collects or deposits on cold or cool ground surfaces at
night under clam conditions when the lower layer of the
atmosphere is saturated due to cooling to dew point at
night.
Frost consists of ice crystals deposited on a cold
surface. The formation of frost is the same of dew. The
different is that the dew point of frost is below 0oC, and
water vapour transform to ice crystals directly through
sublimation process. It is formed at night under calm
conditions when the temperature is below freezing point.
Condensation - forms
b. Mist and Fog
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Mist consists of very fine, condensed water droplets.
These are held in suspension in the air because of
their smaller size.
A mist often forms at very low level
of ground level. The droplets form a thin, fine veil which
hinders the visibility. The visibility is obscure, but still
exceeds 1000 m.
Condensation - forms
b. Mist and Fog


Fog contain small condensed water droplets which
are larger than those in a mist. Thus, it forms a thick
veil which causes poor visibility below 1000 m.
Fog is usually formed by advection of warm, moist air
and by intense radiation at night, namely advection fog
and radiation fog respectively.
Condensation - forms
b.i. Advection Fog
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when warm, moist air passes over a cooler or cold land
or sea surface horizontally;
where cold and warm ocean currents meet each other;
when warm moist air from the ocean merges with cool
dry air from the land.
Condensation - forms
b.i. Advection Fog
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The lower layer of warm air is cooled below dew point
by contact with cooler air or surface, and condensation
results in the formation of advection fog.
Condensation is assisted by the presence of hygroscopic
particles which act as nuclei for condensation.
Advection fog is common in spring in Hong Kong. It often
disperses during the day when the sun appears and
temperature rises. Why?
Condensation - forms
b.ii. Radiation Fog

A radiation fog is formed by condensation when moist air
comes into contact with the cool ground and becomes
cooled due to the radiation of heat from the ground. It
occurs in cold weather when the sky is clear (which
permits maximum radiation) and there is calm, stable
condition.

Hygroscopic particles in the air act as nuclei for the
moisture to condense on. Radiation fog is common in
winter and in industrial regions.
Condensation - forms
c. Clouds

Clouds consist of extremely tiny droplets of water (0.02
to 0.06 mm in diameter), or minute crystals of ice.
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Generally, the formation of cloud is the same of the
formation of fog.
Condensation - forms
c. Clouds
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In order for cloud droplets to form, it is necessary that
microscopic dust particles serve as condensation
nuclei.
There should be a fall of temperature down to dew-point.
- Where the air temperature is well below freezing, clouds
may form of tiny ice crystals. However, water in such
minute quantities can remain liquid far below normal
freezing temperatures, the liquid is said to be
supercooled. Water droplets may exist at temperatures
down to -12oC to -30oC; ice crystals below -30oC.
Condensation - forms
c. Clouds

They can be classified on the basis of two characteristics:
- form: stratiform (blanket like, covering vast areas)
by altitude:
1. High clouds (6000-12000m, with ice crystals)
e.g. Cirrus / Cirrostratus / Cirrocumulus
2. Middle clouds (2000-6000m)
e.g. Altostratus / Altocumulus
3. Low clouds (ground level-2000m)
e.g. Stratus or Nimbostratus / Stratocumulus
Condensation - forms
c. Clouds
- form: stratiform (height > / = horizontal dimensions)
1. Cumulus
2. Cumulonimbus
(extending from a height of 300-600m at the
base up to 9000-12000m)
Condensation - forms
c. Clouds