Atmospheric Moisture:
Relative Humidity
and Dew Point
RECAP
•
•
• Absolute Humidity: the mass of water vapor in a fixed
•
• Water mixing ratio: the mass of water vapor in a fixed
•
•
Hydrological cycle: transport of water and energy.
Humidity: the water content in the air.
volume of air, i.e. the water vapor density.
Specific Humidity: the mass of water vapor in a fixed
total mass of air.
mass of the remaining dry air.
Actual vapor pressure: the amount of water vapor in
terms of the amount of pressure exerted by the water
vapor molecules alone. Dalton’s law.
Saturation vapor pressure: the pressure that the water
vapor molecules would exert if the air were saturated,
i.e. the maximal vapor pressure at a given temperature T.
•
•
Humidity
Humidity: the amount of water in the air.
Absolute humidity: the mass of water vapor in a unit
volume of air.
AH 
mass of water vapor
volume of air
• compared to the total mass of the air parcel.
Specific humidity: the mass of the water vapor
SH 
mass of water vapor
total mass of air
• Water (mass) mixing ratio: The mass of water vapor
compared to the mass of the rest of the air parcel.
MR 
mass of water vapor
mass of dry air
Vapor pressure
• Partial pressure: the pressure of each
gaseous component in a mixture of gases.
• Dalton’s law of partial pressure: the total
pressure of a mixture of gases is the sum of the partial
pressures of each gas component.
P  P1  P2  P3  ...
P  P( N2 )  P(O2 )  P( Ar)  ...  P( H 2O)  ...
• Vapor pressure: the partial pressure of H O vapor.
2
♦ What is the H2O vapor pressure if 1% of the air is H2O and
the total air pressure is 1bar?
• The
pressure of a gas is proportional to the number of molecules
and to the temperature of the gas.
P  nT
•
•
Saturation Vapor Pressure
What is it?
♦ This is the partial pressure of H2O when the air is
saturated.
♦ This is the maximum H2O partial pressure before
the H2O molecules condense out.
Supersaturation: P(H2O)>Ps
♦ It is an unstable condition
♦ It occurs in the absence of condensation nuclei.
Saturation Vapor
Pressure
saturated
• The saturation vapor
•
pressure Ps, depends on
the temperature. It
increases with
temperature.
Ps over water is larger
than it is over an ice
surface at the same
temperature.
Supersaturated
unsaturated
•
• Definition:
Relative Humidity
Description: how close the air is to being saturated.
water vapor content
RH 
water vapor capacity
P ( H 2O )
RH 
100%
Ps ( H 2O)
• RH=100% :the air is saturated. (clouds, fog, rain)
•
•
• If we add water vapor to the air, RH is increasing. If
•
RH<100% :the air is not saturated.
RH>100% :the air is supersaturated-clean air with no
condensation nuclei (rare)
we remove water vapor from the air RH is decreasing.
An increase in temperature results in a decrease of
RH, and a decrease of T results in an increase in RH.
Relative Humidity and Temperature
•
• Watering the plants is more effective when RH is high: less
•
RH is usually maximum in the morning (low T) and minimum
during the afternoon (high T).
evaporation from the ground (morning, evening hours).
The air’s total vapor content is ~ constant during the day
P ( H 2O )
RH 
100%
Ps ( H 2O)
Specific Humidity (g/kg)
Variation of SH and RH with latitude.
SH
SH 
mass of water vapor
total mass of air
P ( H 2O )
RH 
100%
Ps ( H 2O)
RH
Dew Point
• The
temperature to which the air has to be cooled (with no
change in the air pressure or water content) for saturation to
occur.
• If the air is saturated
saturated
♦ How much is RH?
♦ What is the air T?
• The dew point is a measure of the
water vapor content in the
atmosphere. A high dew point
temperature corresponds to high
H2O content.
• Adding water vapor increases
the dew point and removing
water vapor decreases the
dew point.
Supersaturated
unsaturated
US Dew Point Map
•
•
•
Examples of Weather Conditions
The Gulf area has high dew
point but also high
temperature => RH is not
so high.
High dew point does not
mean high RH!
Fog and precipitation are
associated with high RH.
•
California / Florida
Why is the weather in California much drier than the
weather in Florida?
•
Humidity in Your Home
Cold climate (winter in upstate NY). The water content is the same
inside and outside the house.The RH inside is much lower than RH
outside.
T=-15 C
Tdew=-15 C
RH=100%
The outside air is
heated and as it
enters the house
T = 20 C
Tdew=-15 C
RH=8%
• air down, the air becomes saturated, the excess H O vapor condenses
Hot and humid climate (summer Florida). The air conditioner cools the
2
in the cooling unit. As a result the air inside the house has lower water
content (lower dew point). The relative humidity in the house decreases
as the cooled saturated air from the cooling unit is mixed with the
warm unsaturated air in the room.
• Hot and dry climate (summer in Arizona). Evaporative cooling systems:
The hot dry air from outside flows across pads saturated with water.
Water is evaporated, the air cools down because it provides the energy
needed for the evaporation. As a result the air in the house becomes
cooler, with higher dew point and RH.
How humid is “VERY” humid?
• (evaporation of body liquids from the skin surface).
•
The human body cools down through perspiration
•
•
If the relative humidity is high, the evaporation is
inhibited and we cannot cool efficiently. The body
temperature rises.
Heat Index: shows what the air temperature feels
like. It takes into account the humidity and the actual
temperature of the air.
Humans feel comfortable if HI<80 F. Heat index
above 90 F is uncomfortable. HI>100 F is dangerous.
Heat Index
The weight of humid air
• Mean molecular mass of dry air:
gas
m
%
mass of the gas mixture
-------------------------------------------N2
28
78%
28x78/100+32x21/100=28.6
O2
32
21%
• Mean molecular mass of wet air.
gas
m
%
mass of the gas mixture
--------------------------------------------Dry air 28.6
90%
28.6x90/100+18x10/100=27.5
H2O
18
10%
• Moist
air is lighter and less dense than dry air at the same
temperature.
• Moist
air rises more readily. Evaporation enhances convection in
the atmosphere.