Chapter 4

Circulation of Water in the Atmosphere A general definition of humidity is the

amount of water vapor in the air. Remember, humidity is not constant

through time or space, there is constant circulation of water through

the hydrologic cycle.

Stepped Art

Fig. 4-1, p. 90

The Many Phases of Water Phase is related to molecular motion, an

increase or decrease in motion creates a phase change.Ice is the coolest/slowest phaseWater vapor is the warmest/fastest

phase

Evaporation, Condensation, & Saturation Evaporation is the change of liquid into a

gas -- requires heat. Condensation is the change of a gas

into a liquid and releases heat.Condensation nuclei

Saturation is an equilibrium condition in which for each molecule that evaporates, one condenses.

(a) Water molecules at the surface of the water are evaporating (changing from liquid into vapor) and condensing (changing from vapor into liquid). Since more molecules are evaporating than condensing, net evaporation is occurring.

(b) When the number of water molecules escaping from the liquid (evaporating) balances those returning (condensing), the air above

the liquid is saturated with water vapor.

a)

b)

Humidity

Any of a number of ways of specifying the amount of water vapor in the air.

Absolute humidity: mass of water vapor/volume of airWater vapor densityNot commonly used due to frequent change

of volume

Specific Humidity: mass of water vapor/mass of air

The water vapor content (humidity) inside this air parcel can be expressed in a number of ways.

With the same amount of water vapor in a parcel of air, an increase in volume decreases absolute humidity, whereas a decrease in volume increases absolute humidity.

The specific humidity does not change as air rises and descends – since it is only dealing with mass.

The average specific humidity for each latitude.

The highest average values are observed in the tropics and the lowest values in polar regions.

Humidity

Vapor pressure: the pressure exerted by water vapor molecules in an air parcel More water molecules = high vapor

pressure

Saturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature

Humidity

Relative Humidity: (actual water vapor/saturation water vapor)*100RH can be changed two ways:

○ Change vapor content○ Change saturation

Decrease temperature causes an increase in relative humidity (inverse relationship).

(a) At the same air temperature, an increase in the water vapor content of the air increases relative humidity as air approaches saturation.

(b) With the same water vapor content, an increase in temp-erature causes a decrease in relative humidity asair moves farther away from being saturated.

Humidity

Relative Humidity and Dew PointDew point is the temperature at which

saturation occursCool an air parcel to dew point and liquid

water condensesA good measure of actual water vapor

contentRelative humidity indicates how close to

saturation, dew point indicates the amount of water vapor

When

When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low.

Average surface dew-point temperatures (oF) for (a) January .

Average surface dew-point temperatures (oF) for (b) July.

Relative humidity averaged for latitudes north and south of the equator.

All data represent conditions during a July afternoon at 3 p.m. local time.

Relative Humidity in the Home

T represents temperature

Td, dew point

RH, relative humidity

Humidity

Relative humidity & human comfort“It’s not the heat, it’s the humidity.”High relative humidity equates to less

evaporative cooling.Sweat cannot evaporate and cool the bodyWet bulb temperatureHeat Index

Air temperature (°F) and relative humidity are combined to determine an apparent temperature or heat index (HI).

Humidity

Special Topic: Heavier humid airDue to the molecular weight of water as

compared to nitrogen, humid air is lighter than dry air.

Baseball announcers are incorrect.

Humidity

Humidity is measured by:Sling psychrometer

Hygrometer