Evaporation and Condensation: Principles and Processes

Evaporation transforms water from liquid to gas, driven by factors like temperature and air movement. Condensation, on the other hand, converts water vapor into liquid or solid forms due to cooling and the presence of condensation nuclei. These processes are fundamental to the water cycle, influencing weather patterns and atmospheric conditions.

Evaporation Mechanism

• Definition: The process by which water is transformed from liquid to gaseous state.
• Latent Heat of Evaporation: The amount of heat that is required to convert a unit mass of liquid substance into a gaseous phase at constant temperature conditions.
• Factors influencing evaporation
  • Higher temperatures lead to increased water absorption and retention capacity in a given volume of air.
  • Air with a lower moisture content has the ability to absorb and retain moisture. The movement of air replaces saturated air with unsaturated air, which promotes evaporation. Consequently, greater air movement results in higher rates of evaporation.
  • Evaporation Variability: Factors like temperature, air pressure, wind, and water salinity impact evaporation rates. 
    • Globally, oceans have higher evaporation due to extensive coverage. Evaporation decreases from equator to poles. 
    • The western North Atlantic shows the highest rates.

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Condensation: Processes and Conditions

• Definition: Process of transformation of water vapour into water; caused by the loss of heat.
• Sublimation: Process of condensation of water vapour directly into solid form. 
• Conditions for Condensation
  • Temperature of the air reduced to dew point, with its volume remaining constant.
  • Both the volume and the temperature are reduced.  
  • Moisture is added to the air through evaporation.
• Condensation Nuclei: Small particles in the atmosphere that provide surfaces for water vapour to condense. 
• Example: Dust, smoke, pollen, and salt; 
• Factors Influencing Condensation: 
  • Air movement; 
  • Temperature: Lower temperature favours condensation; 
  • Humidity: Higher humidity promotes condensation; 
  • Altitude and pressure change.
• Forms of Condensation
• • When Dew Point > Freezing Point: Outcomes are Dew, fog, and clouds,
  • When Dew Point < Freezing Point: Outcomes are White frost, snow, hailstones and cirrus clouds.
• Dew: When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface); 
  • The dew point is above the freezing point.
• Ideal Conditions for Its Formations: clear sky as clouds will reflect the long wave radiations coming out of the earth’s surface, thus keeping the earth warm.
  • Calm air, high relative humidity, and a cold and long night. (Dew point< Freezing point= Formation of frost) [UPSC  2019]
• Ideal Conditions for the Formation of White Frost: same as those for the formation of dew, except that the air temperature must be at or below the freezing point.
• Fog: Essentially a cloud that forms either at or extremely close to the Earth’s surface.
  • Occurs when the temperature of an air mass with a substantial amount of water vapour, suddenly drops, leading to condensation on tiny dust particles within the air mass itself;
  • Prevalent in areas where warm currents of air come in contact with cold currents; 
  • Drier than the mist; 
• Ideal Conditions: High humidity, dew point, presence of condensation nuclei; 
• Types of Fog: 
  • Cooling Fogs – Radiation Fog: formed due to night cooling; 
  • Advection Fog: when warm air moves over the cold surface; 
  • Upslope Fog: when air rises over a slope, orographic lifting.
  • Evaporation Fogs – Steam Fog: forms over warm water, common over lakes; 
• Mist: More moisture content than fog; Occurs in moist air (>75% humidity); short-lived; Common over mountains.
• Haze: Reduced visibility; Linked to low humidity (<75%); Caused by uneven light refraction and industrial particles.
• Smog: Combination of smoke and fog; caused due to Urban/industrial air pollution; Greyish, brownish, hazy appearance; 
  • 3 Types: Classical (London), Photochemical (Summer/Los Angeles) and VOG (Volcanic Smog)
• Types of Smog
  • Classical Smog: Caused by coal combustion, high concentration of sulphur dioxide and particulate matter; 
    • Weather Conditions: Cold and humid climate.
  • Photochemical Smog: A mixture of pollutants that are formed when nitrogen oxides and volatile organic compounds (VOCs) react to sunlight, creating a brown haze; 
    • Creates Secondary Pollutants: Ozone, PAN, and aldehydes.
  • VOG (Volcanic Smog): From volcanic eruptions; a hazy mixture of SO2 gas and aerosols.

Conclusion

Evaporation and Condensation are essential processes in the Earth’s water cycle, regulating the distribution of water in the atmosphere and on the Earth’s surface. Understanding the factors influencing these processes helps us comprehend weather phenomena like fog, dew, and clouds, contributing to better management of water resources and environmental sustainability.

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