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Concept of Humidity

Concept of Humidity

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Concept of Humidity

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  1. Concept of Humidity

  2. Humidity (absolute humidity) • The amount of water vapour in the air (Holding) Capacity of air • The maximum amount of water vapour in the air

  3. What is the relationship between water vapor holding and temperature? Air hold more water vapour at higher temperature.

  4. Conditions of Saturated & Unsaturated • Saturated: an air mass holding maximum amount of water vapour at that temperature (dew point temperature) • Unsaturated: an air mass holding less than the saturated amount • Supersaturated: an air holding more than the maximum amount

  5. Humidity Indicator (1)Absolute Humidity (Water Vapour Density) • Mass of water vapour / Volume of air • meaning of mass = weight

  6. Why do we seldom use the indicator of absolute humidity? • Because the humidity concentration depends on temperature • BUT • absolute humidity does not consider such change

  7. Water vapour content / water vapour capacity Vapour pressure / saturated vapour pressure Humidity Indicator (2)Relative Humidity • Different formula, but same thing mg/cubic m hPa

  8. Spatial Distribution of RH globally

  9. Spatial Distribution of RH globally • High RH along equator and 60oN&S • Because of low pressure • Ascending air leads to adiabatic cooling • Low RH along 30oN&S • Because of high pressure • Descending air leads to adiabatic heating

  10. The air temperature is 20C. The absolute humidity is 9.4 g/m3 • the relative humidity • (9.4 g/m3 / 17.3 g/m3 X100% • = 54.3% • What is the dew point temperature? • Holding capacity = 17.3 g/m3 • 10oC

  11. The air temperature is 25C and the relative humidity is 60% • How much moisture will be lost if the temperature drops to 20C? • At 25oC, the air parcel contains ? moisture • 23 X 60% = 13.8 g/m3 • At 20C, the holding capacity of air is • 17.3 g/m3 • 17.3 g/m3 > 13.8 g/m3 • It is unsaturated, therefore, no moisture loss

  12. How about if the temperature drops from 25C to 15C? • At 15C, the holding capacity of air is • 12.8 g/m3 • 12.8 g/m3 > 13.8 g/m3 • Condensation will take place • 12.8 g/m3 - 13.8 g/m3 = - 1 g/m3 • moisture loss = 1 g/m3

  13. Changing States of Moisture

  14. Latent heat release Latent heat absorption

  15. More Important Processes in Our Course

  16. Evaporation Liquid  Gas • Evaporation implies an addition of energy (about 600 calories/gram), which is used in overcoming the intermolecular attraction to individual water molecules so that they are able to leave the water surface and become water vapour • Latent heat absorption • Fall in temperature

  17. Factors Affecting Evaporation • Vapour pressure gradient • difference of water vapour content between two places When the environment is drier, the gradient is steeper Low vapour pressure 5hPa 10hPa 15hPa 20hPa High vapour pressure

  18. Factors Affecting Evaporation • Temperature • Wind Speed (Movement)

  19. Condensation? Gas  Liquid

  20. Necessary Conditions of Condensation • RH > 80% • Presence of atmospheric nuclei / hydroscopic nuclei / condensation nuclei for the moisture to coat on • air cooling (air cool -> holding capacity of air decreases -> water releases) • that temperature when water releases is called dew point temperature

  21. How does air cool? • Method 1: Advection Movement of warm air cold water surface

  22. Method 2 Radiation

  23. Non-adiabetic (Diabetic) Cooling-does not involves uplifting • Advection • Radiation • On a clear winter’s night, radiation cooling will leads to the formation of • Dew, fog and frost

  24. Adiabatic Cooling - Principle Low pressure Adiabatic uplifting Volume of air expands Cooling takes place and condensation facilitated

  25. Reasons for Air Uplifting • Horizontal movement meeting obstacles • Orographic lifting • Wind encounters an obstacle in its path

  26. Reasons for Air Uplifting • Horizontal movement when reduction in wind speed takes place Land Sea Land Sea

  27. Reasons for Air Uplifting • Convection uplifting • When air is heated from below • Warm air expands and the density gets lower • Replace by colder, denser air from above • Up-rising convection starts

  28. Reasons for Air Uplifting • Low pressure / convergence • Convergence of polar air mass and the warmer westerlies • Frontal uplifting • Along the Inter-tropical Convergence Zone, • Meeting of NE and SE Trades