Robert W. Christopherson Charlie Thomsen 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 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 % 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 7.10 Every hurricane carries tremendous amount of water with it.

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 N 2 P1 O 2 P2 Argon P3 CO 2 P4 H 2 O P5 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 10 o C of increase in air temperature. Tropical warm air: wet Polar cold air: dry

Humidity Measurements Relative humidity Specific humidity Dew point temperature Vapor pressure deficit

Relative Humidity Figure 7.8

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. 10g H 2 O/kg Air 10g H2O/kg Air 10g H2O/kg Air 10g H 2 O/kg Air heating

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. 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: ~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.

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 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 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/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 Figure 7.21 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.

Cloud Types and Identification Figure 7.22 Three Classes of clouds: Stratus (low in altitude 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. Christopherson Charlie Thomsen Geosystems 7e An Introduction to Physical Geography End of Chapter 7