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 1 P1 Gap 2 P2 Gas 3 P3 Gas 4 P4 Gas 5 P5 Gases 1-5 P P=P1+P2+P3+P4+P5+P6 P air =? Vapor Pressure Air P N 2 P1 O 2 P2 Argon P3 CO 2 P4 H 2 O P5 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 10 o C 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 P r = P air sat 100% 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 = P sat -P air 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 (c) Humidity Probe: Wet bulb 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 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. Figure 7.17
Dry and Wet Adiabatic Rate Dry Adiabatic Cooling: Dry refers to air that is less than saturated. DAR: ~10 o C/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, ~6 o C/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/m 3 Marine air: 1 billion/m 3 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