chapter 7 water and atmospheric moisture
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Chapter 7 Water and Atmospheric Moisture. Water and Atmospheric Moisture. Water on Earth Unique Properties of Water Humidity Atmospheric Stability Clouds and Fog . Water on Earth . - PowerPoint PPT PresentationTRANSCRIPT
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