Chapter 7 Water and Atmospheric Moisture

Robert W. ChristophersonCharlie Thomsen

Chapter 7Water and

Atmospheric Moisture

Water and Atmospheric MoistureWater on Earth  

Unique Properties of Water  

Humidity  

Atmospheric Stability  

Clouds and Fog  

Water on Earth  Worldwide equilibrium: On a global scale there is no net gain or loss of water even though we have floods and drought somewhere every year, i.e. Earth is a ? system in terms of matter).

Distribution of Earth’s water today  

Land and Water Hemispheres

Figure 7.2

71% of the Earth surface areas are covered with water, mostly by ocean.

Ocean and Freshwater Distribution

Figure 7.3

Baikal

Unique Properties of Water  Heat properties 

Phase change: naturally exists in liquid, gas and solid phases on Earth.

Phase changes always associated with heat changes: Latent Heat 

Vaporization

Condensation

sublimation

Heat properties of water in nature:  

Three States of Water

Figure 7.5

Ice is lighter than water, thus ice floats keeping the bottom of the ocean unfrozen.

Water expands when frozen.

Phase Changes

Figure 7.7

Water Vapor in the Atmosphere

Figure 7.10

Spatial distribution of water in the air as measured by GOES-8 satellite.

Light areas more water.

Aleutian Low

The air circulation transfers water from humid tropical region to dry continents on a grant scale. Resident time of water in the air is only ~8 days.

Water Vapor in the Atmosphere

Figure 7.10Every hurricane carries tremendous amount of water with it.

The Law of Partial Pressure

Gas 1P1

Gap 2P2

Gas 3P3

Gas 4 P4

Gas 5P5

Gases 1-5P

P=P1+P2+P3+P4+P5+P6

Pair=?

Vapor Pressure

N2

P1

O2

P2

ArgonP3

CO2

P4

H2OP5

Air P

Vapor Pressure (P5): the press of water created by water vapor in the air.

Saturated Vapor Pressure

Dry Air

Saturated Vapor Pressure is reached when water molecules leaving the water surface and the water molecules coming back to the water surface are balanced.

Water

Air Water Vapor

Water

Saturation Vapor Pressure

Figure 7.12

The partial pressure created by water vapor when the air contains the maximum amount of water vapor it can hold.

At subfreezing temperature, saturation vapor pressure is greater above water surface than over an ice surface.

Saturation vapor pressure nearly doubles for every 10oC of increase in air temperature.

Tropical warm air: wetPolar cold air: dry

Humidity MeasurementsRelative humidity  

Specific humidity

Dew point temperature

Vapor pressure deficit 

Relative Humidity

Figure 7.8

%100sat

air

P

Pr

Specific Humidity

Figure 7.13

Definition: The mass of water vapor (in grams) per mass of air (in kilograms).

Not influenced by temperature or pressure.

10gH2O/kg Air

10gH2O/kg Air

10gH2O/kg Air

10gH2O/kg Air

heating

Vapor Pressure Deficit and Dew Point TemperatureVapor Pressure Deficit = Psat- Pair

The bigger VPD, the drier the air.

Dew Point Temperature: Reduce the temperature of an unsaturated parcel of air at constant barometric pressure until the actual vapor pressure equal the saturation vapor pressure. The temperature is call the dew point temperature.

• A

VPD

Temporal Humidity Patterns

Figure 7.11

Diurnal Cycles

Seasonal Cycles

Humidity Instruments

Figure 7.14

Dry bulb

Wet bulb(c) Humidity Probe:

Atmospheric Stability  Adiabatic processes: A process involves no heat exchange between the parcel of an atmosphere and its surroundings.

  Stable and unstable atmospheric conditions 

An air parcel is stable if it resists displacement upward, i.e. when disturbed, it tends to return to its starting place. An air parcel is unstable if it continues to rise when disturbed upward until it reaches an altitude where the surrounding air has a similar density and temperature.

Buoyancy and Gravity

Figure 7.15

Adiabatic Processes

Figure 7.17

The air parcel use its kinetic energy to export work out, thus lower temperature as it expands.

The air parcel receive work from outside and increase its kinetic energy, thus a higher temperature as it is compressed.

Dry and Wet Adiabatic Rate

Figure 7.17

Dry Adiabatic Cooling: Dry refers to air that is less than saturated. DAR: ~10oC/1000m.Moist Adiabatic Cooling: Wet refers to vapor condensation, condensation releases latent heat, which warms the air parcel. Thus MAR is always smaller than DAR, ~6oC/1000m.

Adiabatic Heating

Figure 7.17

Adiabatic Processes  Dry adiabatic rate

10 C°/1000 m

5.5 F°/1000 ft

Moist adiabatic rate6 C°/1000 m

3.3 F°/1000 ft

Atmospheric Temperatures and Stability

Figure 7.18

env lapse rate > DAR

env lapse rate <MAR/ DAR

MAR < env lapse rate < DAR

 Three Examples of Stability

Figure 7.19

Clouds and FogCloud Formation Processes  

Cloud Types and Identification  

Fog  

Cloud Formation Processes  Moisture droplet:

Tiny water drop (~20μm in diameter) that make up clouds. An average rain drop (2000 μm in diameter) needs a million or more such droplets.

Cloud-condensation nuclei: When relative humidity is reach 100%, water vapor does not necessarily

condense unless tiny particles (2 μm in diameter) exist so that the water can hang on.

Continental air: 10 billion/m3

Marine air: 1 billion/m3

Artificial Precipitation: Using airplane or cannon to add condensation nuclei into the clouds to

facilitate moisture droplet formation

Moisture Droplets

Figure 7.20

Raindrop and Snowflake Formation

Figure 7.21Recall at subfreezing temperature, air around ice surface is more saturated that that around water, making it possible snow flakes draws water from supercooled water droplets.

Cloud Types and Identification  

Figure 7.22

Three Classes of clouds: Stratus (low in altitude < 2000m ), Cumulus (2000~6000m), and Cirrus (>6000 m).

Cirrus

Figure 7.22

Altocumulus

Figure 7.22

Cumulus

Figure 7.22

Altostratus

Figure 7.22

Nimbostratus

Figure 7.22

Stratus

Figure 7.22

Fog  Definition: Cloud layer on the ground.

Advection fog

Evaporation fog

Upslope fog

Valley fog

Radiation fog

Advection Fog

Figure 7.24

Advection: migration of air from one place to another place, or wind. When warm air migrates to cold region, water vapor in the warm air condense to form moisture droplet.

Evaporation Fog

Figure 7.25

During the early morning of a sunny winter day, water surface temperature is higher than the surrounding air. The evaporated water then condense in the nearby cold air, forming fog.

Valley Fog

Figure 7.25 Figure 7.26

Cold air from upslope drawn into valley to cold the warm air, causing water vapor to condense and form moisture droplets

Evaporation and Radiation Fog

Figure 7.28

When long wave radiation cools the surface and chills the air nearby below dew point temperature, moisture droplets occur (i.e. clouds fog).

Robert W. ChristophersonCharlie Thomsen

Geosystems 7eAn Introduction to Physical Geography

End of Chapter 7