Atmospheric Moisture: Relative Humidity and Dew Point

1
Atmospheric MoistureRelative Humidity and Dew
Point
2
RECAP

• Hydrological cycle transport of water and
  energy.
• Humidity the water content in the air.
• Absolute Humidity the mass of water vapor in a
  fixed volume of air, i.e. the water vapor
  density.
• Specific Humidity the mass of water vapor in a
  fixed total mass of air.
• Water mixing ratio the mass of water vapor in a
  fixed 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. Daltons 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.

3
Humidity

• Humidity the amount of water in the air.
• Absolute humidity the mass of water vapor in a
  unit volume of air.
• Specific humidity the mass of the water vapor
  compared to the total mass of the air parcel.
• Water (mass) mixing ratio The mass of water
  vapor compared to the mass of the rest of the air
  parcel.

4
Vapor pressure

• Partial pressure the pressure of each
• gaseous component in a mixture of gases.
• Daltons law of partial pressure the total
• pressure of a mixture of gases is the sum of
  the partial pressures of each gas component.
• Vapor pressure the partial pressure of H2O
  vapor.
• 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.

5
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)gtPs
• It is an unstable condition
• It occurs in the absence of condensation nuclei.

6
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.

Super-saturated
unsaturated
7
Relative Humidity

• Description how close the air is to being
  saturated.
• Definition
• RH100 the air is saturated. (clouds, fog,
  rain)
• RHlt100 the air is not saturated.
• RHgt100 the air is supersaturated-clean air with
  no condensation nuclei (rare)
• If we add water vapor to the air, RH is
  increasing. If 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.

8
Relative Humidity and Temperature

• RH is usually maximum in the morning (low T) and
  minimum during the afternoon (high T).
• Watering the plants is more effective when RH is
  high less evaporation from the ground (morning,
  evening hours).
• The airs total vapor content is constant
  during the day

9
Variation of SH and RH with latitude.
SH
RH
10
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
• 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.

11
US Dew Point Map
12
Examples of Weather Conditions

• The Gulf area has high dew point but also high
  temperature gt RH is not so high.
• High dew point does not mean high RH!
• Fog and precipitation are associated with high RH.

13
California / Florida

• Why is the weather in California much drier than
  the weather in Florida?

14
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.
• Hot and humid climate (summer Florida). The air
  conditioner cools the air down, the air becomes
  saturated, the excess H2O vapor condenses 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.

The outside air is heated and as it enters the
house
T 20 C Tdew-15 C RH8
T-15 C Tdew-15 C RH100
15
How humid is VERY humid?

• The human body cools down through perspiration
  (evaporation of body liquids from the skin
  surface).
• 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 HIlt80 F. Heat index
  above 90 F is uncomfortable. HIgt100 F is
  dangerous.

16
Heat Index
17
The weight of humid air

• Mean molecular mass of dry air
• Mean molecular mass of wet air.
• 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.

gas m mass of the gas mixture ------------------
-------------------------- N2 28 78 28x78/10032x
21/10028.6 O2 32 21
gas m mass of the gas mixture ------------------
--------------------------- Dry
air 28.6 90 28.6x90/10018x10/10027.5 H2O 18 10