Atmospheric Moisture:Relative Humidity

and Dew Point

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. 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.

•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.

airofvolume

vaporwaterofmassAH =

airofmasstotal

vaporwaterofmassSH =

airdryofmass

vaporwaterofmassMR =

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.

•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.

...)(...)()()( 222 +++++= OHPArPOPNPP

...321 +++= PPPP

nTP ∝

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

•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.

unsaturated

Super-saturate

d

saturated

Relative Humidity

•Description: how close the air is to being saturated.

•Definition:

•RH=100% :the air is saturated. (clouds, fog, rain)

•RH<100% :the air is not saturated.

•RH>100% :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.

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2

2 ×=OHPOHP

RHscapacityvaporwater

contentvaporwaterRH =

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 air’s total vapor content is ~ constant during the day

%100)(

)(

2

2 ×=OHPOHP

RHs

Variation of SH and RH with latitude.

Sp

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fic H

um

idity

(g

/kg

) SH

RH

airofmasstotal

vaporwaterofmassSH =

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2

2 ×=OHPOHP

RHs

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.

unsaturated

Super-saturate

d

saturated

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.

•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.

T=-15 CTdew=-15 CRH=100%

T = 20 CTdew=-15 CRH=8%

The outside air is heated and as it enters the house

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 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:

•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% 28x78/100+32x21/100=28.6O2 32 21%

gas m % mass of the gas mixture---------------------------------------------Dry air 28.6 90% 28.6x90/100+18x10/100=27.5H2O 18 10%