Condensation

This is a basic lecture for giving an overall idea about condensation.

1. Jannatul Naim Assistant Professor Department of Geography and Environmental Studies University of Rajshahi Lec 05: Condensation

2. Definition of condensation 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. 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 ( 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 . Conditions for Condensation 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 .

4. 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 ).

5. 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.

6. Condensation occurs both- Near the ground surface Up in the air above the ground surface . Forms of Condensation

7. 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) 1.Near the ground surface

8. 1.2 Frost A deposit of minute ice crystals Frost occurs when the dew point of air falls below the freezing point (0 o C) . Conditions : Same as dew Except that condensation taking place below the freezing point.

9. 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 o C , 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. 1.3 Rime

10. 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 1.4 Fog

11. 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.

12. 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 Condensation above the ground surface

13. Rainfall: Next class

14. 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 3. Other forms of Condensation

15. 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

16. 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.

17. Process of Condensation/ Cloud In chapter 5

18. 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.

19. Q & A