Chapter 7Water and Atmospheric

Moisture

Geosystems 6eAn Introduction to Physical Geography

Robert W. ChristophersonCharles E. Thomsen

Water and Atmospheric Moisture

TOPICS:Water on Earth

Unique Properties of Water  

Humidity  

Atmospheric Stability  

Clouds and Fog  

Water on Earth  Earth’s water originated from

Icy comets

Hydrogen and oxygen

OutgassingExample: geysers 

Eustasy, Glacio-eustasy, Isostasy

Ocean and Freshwater Distribution

Figure 7.3

Unique Properties of Water  The water molecule: H2O

Polarity: + to – and – to += hydrogen bonding

Surface tension

Capillarity

Unique Properties of Water  Heat properties

Change of state requires energy be absorbed or released!

Phase changes:Freezing/melting (solid to liquid to solid)

Condensation/vaporization (gas to liquid to gas)

Sublimation/deposition (gas to solid to gas)

Evaporation & CondensationThe process by which molecules break free of a liquid volume is evaporationWhen water vapor molecules randomly collide with the water surface and bond with adjacent molecules is condensation

Three States of Water

Figure 7.5

HumidityHumidity refers to water vapor in the air

Relative Humidity is the ratio (expressed as %) of the amount of water vapor that is actually in the air compared to the maximum water vapor possible in the air at a given temperature

Air becomes saturated when the rate of evaporation and the rate of condensation reach equilibrium = 100% relative humidity

HumidityEquation for Relative Humidity 

actual water vapor

(specific humidity)

RH = ------------------------------ X 100 max amount that can be held at that temp

(saturation specific humidity)

Relative Humidity

Figure 7.8

Cooling

Warming

Actualwater vapor

Relative HumidityRelative humidity is the indication of how close the air is to saturation and when condensation will begin

Dew-point temperature not really a temperature, but a measure of moisture content

When air temperature tries to decrease below the dew point, surplus water vapor is removed from the air by condensation

Dew Point Temperature

Distribution of Water Vapor

January

Distribution of Water Vapor

July

Humidity Patterns

Figure 7.11

Vapor PressureOne of 2 measures of relative humidity

As water molecules evaporate and become part of the air, they become water-vapor molecules. In this state, they exert a portion of the air pressure called vapor pressure

Saturation vapor pressure

Increasing temperature means an increase in saturation vapor pressure– kinetic energy

Specific HumidityOne of 2 measures of relative humidity

Specific humidity is a measure of humidity that remains constant as temperature and pressure change

Specific humidity is the mass of water vapor (g) per mass of air (kg) at any specified temperature

Maximum specific humidity

Condensation NucleiPure water droplets are uncommon

Homogeneous nucleation

Hygroscopic aerosols

Heterogeneous nucleation

Condensation nuclei

Potential Test QuestionRelative humidity has an inverse

relationship withA) dew-point temperature

B) temperature

C) air pressure

D) eustasy

Potential Test QuestionWhich of the following has the highest water

vapor content in the air?

A) southern Alabama

B) Oklahoma

C) eastern Montana

D) southern Missouri

Adiabatic Processes  Adiabatic processes

Begins with a parcel of air

Bouyancy caused initially by differences in (near) surface temperature

Less dense, warmer air rises, more dense, colder air sinks, after which…

Ascending or descending air will undergo changes in temperature with no exchange of heat. This is an adiabatic process.

Adiabatic Processes  Adiabatic processes

Ascending or descending air will undergo changes in temperature with no exchange of heat. This is an adiabatic process.Changes in temperature will be due solely to changes in pressure! (Example?)Note difference with parcel of air undergoing changes in temperature when we discussed humidity! No motion was implied!Heat WAS exchanged = diabatic process.

Adiabatic Processes  Adiabatic processes

Note also differences in the volumes of the parcels of air between diabatic (heat exchange) and adiabatic (no-heat exchange):

Cooling

Diabatic Process  

Adiabatic Process!

Buoyancy

Figure 7.15

Adiabatic Processes

Figure 7.17