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
mg/cubic m
hPa
Spatial Distribution of RH globally
Spatial Distribution of RH globally
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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 20C. The
absolute humidity is 9.4 g/m3
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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 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
How about if the temperature drops from
25C to 15C?
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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
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
• Vapourthe
pressure
gradient
When
environment
is
– difference of water vapour content between
drier,
two the
placesgradient 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
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
Sea
Land
Reasons for Air Uplifting
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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