Concept of HumidityConcept 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

hPamg/cubic m

Spatial Distribution of RH globally

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

Changing States Changing States of Moistureof Moisture

Changing States Changing States of Moistureof Moisture

Latent heat release

Latent heat absorption

More Important Processes in Our Course

EvaporationLiquid 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

15hPa10hPa

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 Necessary Conditions of CondensationCondensation

• 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 SeaLand 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