Transcript Water in the Air

Chapter 7
Water and
Atmospheric
Moisture
Robert W. Christopherson
Charlie Thomsen
Water and Atmospheric Moisture
Water 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
71% of the Earth surface
areas are covered with water,
mostly by ocean.
Figure 7.2
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
Ice is lighter than water, thus
ice floats keeping the bottom
of the ocean unfrozen.
Water expands when frozen.
Figure 7.5
Phase Changes
Figure 7.7
Water Vapor in the Atmosphere
Aleutian Low
Spatial distribution of water in the
air as measured by GOES-8
satellite.
Light areas more water.
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.
Figure 7.10
Water Vapor in the Atmosphere
Every hurricane carries tremendous amount of water with it.
Figure 7.10
The Law of Partial Pressure
Gas 1
P1
Gap 2
P2
Gas 3
P3
Gases 1-5
P
Gas 4
P4
P=P1+P2+P3+P4+P5+P6
Pair=?
Gas 5
P5
Vapor Pressure
Air
P
N2
P1
O2
P2
Argon
P3
CO2
P4
Vapor Pressure (P5): the press of water created by water vapor in the air.
H2O
P5
Saturated Vapor Pressure
Dry Air
Water
Air
Water Vapor
Water
Saturated Vapor Pressure is reached when water molecules leaving the water surface
and the water molecules coming back to the water surface are balanced.
Saturation
Vapor
Pressure
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: wet
Polar cold air: dry
Figure 7.12
Humidity Measurements
Relative humidity
Specific humidity
Dew point temperature
Vapor pressure deficit
Relative Humidity
Pair
r
 100%
Psat
Figure 7.8
Specific Humidity
Definition: The mass of water
vapor (in grams) per mass of air
(in kilograms).
Not influenced by temperature or
pressure.
10g
H2O/kg Air
10g
H2O/kg Air
10g
H2O/kg Air
10g
H2O/kg Air
Figure 7.13
heating
Vapor Pressure
Deficit and Dew
Point Temperature
Vapor 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.
VPD
•A
Temporal Humidity Patterns
Diurnal Cycles
Seasonal Cycles
Figure 7.11
Humidity Instruments
Dry bulb
Wet bulb
(c) Humidity Probe:
Figure 7.14
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
The air parcel receive
work from outside
and increase its
kinetic energy, thus a
higher temperature as
it is compressed.
The air parcel use its
kinetic energy to
export work out, thus
lower temperature as
it expands.
Figure 7.17
Dry and Wet Adiabatic Rate
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.
Figure 7.17
Adiabatic Heating
Figure 7.17
Adiabatic Processes
Dry adiabatic rate
10 C°/1000 m
5.5 F°/1000 ft
Moist adiabatic rate
6 C°/1000 m
3.3 F°/1000 ft
Atmospheric Temperatures and Stability
MAR < env lapse rate < DAR
env lapse rate <MAR/ DAR
env lapse rate > DAR
Figure 7.18
Three
Examples
of Stability
Figure 7.19
Clouds and Fog
Cloud 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
Recall 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.
Figure 7.21
Cloud Types and Identification
Three Classes of clouds: Stratus (low in altitude < 2000m ), Cumulus (2000~6000m), and
Cirrus (>6000 m).
Figure 7.22
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
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.
Figure 7.24
Evaporation Fog
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.
Figure 7.25
Valley Fog
Cold air from upslope drawn into valley to cold the warm air, causing water
vapor to condense and form moisture droplets
Figure 7.25
Figure 7.26
Evaporation and Radiation Fog
When long wave radiation cools
the surface and chills the air
nearby below dew point
temperature, moisture droplets
occur (i.e. clouds  fog).
Figure 7.28
End of Chapter 7
Geosystems 7e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen