Download Chapter 7 Water and Atmospheric Moisture Water and Atmospheric

Chapter 7
Water and
Atmospheric
Moisture
Robert W. Christopherson
Charlie Thomsen
Water kept both the terrestrial and marine ecosystems closely linked with the atmosphere.
(1) Air carries water vapor and the associated latent heat to redistribute
(2) Water enters the air through evaporation (soil and water surface) and transpiration (leaves).
(3) Water returns to Earth through precipitation (rain, snow, dew, hail)
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 3
P3
Gap 2
P2
Gas 1
P1
Gases 1-5
P
Gas 4
P4
Gas 5
P5
P=P1+P2+P3+P4+P5+P6
Pair=?
Vapor Pressure
Air
P
Argon
P3
O2
P2
N2
P1
CO2
P4
Vapor Pressure (P5): the press of water created by water vapor in the air.
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
H2O
P5
Humidity Measurements
Relative humidity
Specific humidity
Dew point temperature
Vapor pressure deficit
Relative Humidity
r=
Pair
× 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.
Figure 7.13
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.
The lower the dew point temp, the drier the air.
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 (200 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