2. • The process of condensation that changes
water from vapor to liquid renders the
water visible.
Condensation is the process of water vapour
changing to the liquid state.
(Lal, 2008)
The movement of water from a gaseous state
into a liquid state; the opposite of evaporation.
(Davie,2008)
Examples: Dew, fog, frost, Cloud etc
3.
4. • If dew point is above freezing point, condensation will occurs in liquid form (e.g., dew,
fog, rainfall).
• Whenever the dew point temperature falls below the freezing point (0 degree Celsius),
water vapour may convert directly into ice by the process of sublimation (e.g., frost, ice,
snow etc.).
• The temperature at which condensation occurs (always below freezing point) is called
frost point or frost point temperature.
Condensation depends on two factor:
• Relative Humidity
• Degree of Cooling
• The air having 100 percent relative humidity is called saturated air. And when the air
becomes saturated as relative humidity becomes 100%, hence the condensation begins.
• Saturation can happen either reducing air temperature or adding water vapour into the
air.
• The temperature at which an air becomes saturated is called dew point.
5. • Besides, water vapour needs some kind of surface on which it may
condense.
• For dew or frost solid objects at the ground do this job.
• But away from the earth surface, condensation of water vapour provided
by dust particles (salt particles, smoke) or aerosols (Condensation nuclei ).
6.
7. Condensation process:
There are 4 processes for condensation:
1 Cooling by expansion (Adiabatic process)
2. Conduction of heat from the air to a cold surface
3. Direct radiation- cooling of the air
4. Mixing of warm and cold masses of saturated air.
9. 1.1 Dew
The water droplets formed by condensation of water
vapour on a relatively cold surface of an object.
Conditions for dew formation are:
• Clear skies (cloud free) and calm air
• Long winter night so that the radiation cooling can
take place.
• Appreciable quantity of moisture in the air that
means high humidity in the lowest layers of air
• the temperature of the substance must reach the
dew point.
• contact with any cold substance (Green grass,
leaves, metal wires, threads of spider webs etc are
substances that collect dew)
10. 1.2 Frost
A deposit of minute ice crystals
Frost occurs when the dew point of air falls below
the freezing point (0oC) .
Conditions:
• Same as dew
• Except that condensation taking place below the
freezing point.
11. • Cloud and fog droplets are very much smaller than rain
droplets and when, on a very cold, foggy night super-
cooled droplets impact upon objects which have been
cooled below 0oC, they immediately begin to freeze in
the same way as for Glaze.
• However, because of the small size, the heat released on
freezing can be quickly conducted away either to the
object or to the air, so that the whole droplet will freeze
very rapidly and will not spread out as in the case of
Freezing Rain.
• As more and more cold fog droplets strike the windward
side of the object they will build up a mass of porous ice
or rime, with many air spaces between the solidified
droplets. It is because rime contains these air spaces that
it appears white in contrast to glazed frost which is clear,
solid ice with little or no trapped air.
12. • Extremely small water droplets suspending in
the atmosphere and reducing the horizontal
visibility is fog.
• Conditions necessary for formation of Fog:
i) The ground should not get much heated during
daytime.
ii) The air must not be very dry
iii) Wind velocity should be calm.
iv) Inversion, radiation or cold mass advection must takes
place.
•Classification of Fog:
A) Thick Fog : Restricts visibility up to 45 meters
B) Moderate Fog : Restricts visibility up to 450 meters
C) Thin Fog : Restricts visibility up to 900 meters
13. 1.5 Mist
Mist is less dense fog. The suspended water droplets
restrict. Visibility between 1000 to 2000 meters or 4 on the
coded scale (IMD) The obscurity is known as mist. Relative
humidity is at least 75%. Mist disappears with rising sun.
1.6 Smog:
The combined effect of smoke and fog droplets may reduce
visibility and this phenomenon is called smog.
1.7 Haze:
Some solid particles like dust, smoke from fire and industry
restrict visibility is haze.
14. Cloud: Next Class
– Cooling process determines cloud type and
location
– Cooling from below produces fog and strati form
(layer) clouds
• Often forms near ground level
16. 3.1 Snowfall
• The fall of larger snowflakes from the clouds on
the ground surface is called snowfall.
• The snowfall occurs when the freezing level is so
close to the ground surface (less than 300m from
the surface)
• The aggregation of ice crystals reach the ground
without being melted in a solid form of
precipitation as snow
17. 3.2 Sleet
• Sleet refers to a mixture of snow and rain.
• Translucent ice having a diameter of 5mm or less.
3.3 Hail
• Consists of large pellets or spheres of ice.
• In fact, hail is a form of solid precipitation where in
small balls or pieces of ice, known as hailstones,
having a diameter of 5 to 50mm fall downward
known as hailstorms.
• Occurs from cumulonimbus cloud
18. 3.4 Drizzle
• The fall of numerous uniform minute
droplets of water having diameter of less
than 0.5 mm is called drizzle.
• Drizzle fall continuously from low stratus
cloud but the total amount of water
received on the ground surface is
significantly low.
20. References
• Ahmed, R. (1997). Abhaoa O Jalbauy Vijnan. Department of
Geography and Environmental Studies, RU: Rajshahi.
• Barry, R. G., & Chorley, R. J. (1978). Atmosphere, weather and
climate. Methuen: London and Newyork.
• Lal, D.S. (2008). Climatology. Chatanya Publishing House:
Allahabad.
• Singh, Dr. S. (2007). Climatology. Prayag Pustak Bhawan: Allahabad.
• Trewartha, G. T. (1968). An introduction to climate. Mcgraw-Hill
Book Company, Inc.; New York; London.
