Concept of Humidity

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

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

Spatial Distribution of RH globally

High RH along equator and 60 o N&S Because of low pressure Ascending air leads to adiabatic cooling Low RH along 30 o N&S Because of high pressure Descending air leads to adiabatic heating

The air temperature is 20  C. The absolute humidity is 9.4 g/m3 the relative humidity (9.4 g/m 3 / 17.3 g/m 3 X100% = 54.3% What is the dew point temperature? Holding capacity = 17.3 g/m 3 10 o C

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 25 o C, the air parcel contains ? moisture 23 X 60% = 13.8 g/m 3 At 20  C, the holding capacity of air is 17.3 g/m g/m 3 > 13.8 g/m 3 It is unsaturated, therefore, no moisture loss

How about if the temperature drops from 25  C to 15  C? At 15  C, the holding capacity of air is 12.8 g/m g/m 3 > 13.8 g/m 3 Condensation will take place 12.8 g/m g/m 3 = - 1 g/m 3 moisture loss = 1 g/m 3

Changing States of Moisture

Latent heat release Latent heat absorption

More Important Processes in Our Course

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 20hPa 15hPa 10hPa 5hPa High vapour pressure Low vapour pressure When the environment is drier, the gradient is steeper

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

Method 2 Radiation

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 Sea Land

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