
Humidity (absolute humidity) • The amount of water vapour in the air (Holding) Capacity of air • The maximum amount of water vapour in the air
What is the relationship between water vapor holding and temperature? Air hold more water vapour at higher temperature.
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
Humidity Indicator (1)Absolute Humidity (Water Vapour Density) • Mass of water vapour / Volume of air • meaning of mass = weight
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
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
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
The air temperature is 20C. 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
The air temperature is 25C and the relative humidity is 60% • How much moisture will be lost if the temperature drops to 20C? • At 25oC, the air parcel contains ? moisture • 23 X 60% = 13.8 g/m3 • At 20C, 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
How about if the temperature drops from 25C to 15C? • At 15C, 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
Latent heat release Latent heat absorption
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
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
Factors Affecting Evaporation • Temperature • Wind Speed (Movement)
Condensation? Gas Liquid
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
How does air cool? • Method 1: Advection Movement of warm air cold water surface
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
Adiabatic Cooling - Principle Low pressure Adiabatic uplifting Volume of air expands Cooling takes place and condensation facilitated
Reasons for Air Uplifting • Horizontal movement meeting obstacles • Orographic lifting • Wind encounters an obstacle in its path
Reasons for Air Uplifting • Horizontal movement when reduction in wind speed takes place Land Sea Land Sea
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
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