Robert W. ChristophersonCharlie ThomsenChapter 7Water and Atmospheric MoistureWater 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 MoistureWater on EarthUnique Properties of WaterHumidityAtmospheric Stability Clouds and FogWater on EarthWorldwide 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 todayLand and Water HemispheresFigure 7.271% of the Earth surface areas are covered with water, mostly by ocean.Ocean and Freshwater DistributionFigure 7.3BaikalUnique Properties of WaterHeat propertiesPhase change: naturally exists in liquid, gas and solid phases on Earth. Phase changes always associated with heat changes: Latent HeatVaporizationCondensation sublimationHeat properties of water in nature:Three States of WaterFigure 7.5Ice is lighter than water, thus ice floats keeping the bottom of the ocean unfrozen.Water expands when frozen.Phase ChangesFigure 7.7Water Vapor in the AtmosphereFigure 7.10Spatial distribution of water in the air as measured by GOES-8 satellite. Light areas more water. Aleutian LowThe 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 AtmosphereFigure 7.10Every hurricane carries tremendous amount of water with it.The Law of Partial PressureGas 1P1Gap 2P2Gas 3P3Gas 4 P4Gas 5P5Gases 1-5PP=P1+P2+P3+P4+P5+P6Pair=?Vapor PressureN2P1O2P2ArgonP3CO2P4H2OP5Air PVapor Pressure (P5): the press of water created by water vapor in the air.Saturation Vapor PressureFigure 7.12The 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: dryHumidity MeasurementsRelative humiditySpecific humidityDew point temperatureVapor pressure deficitRelative HumidityFigure 7.8%100×=satairPPrSpecific HumidityFigure 7.13Definition: The mass of water vapor (in grams) per mass of air (in kilograms).Not influenced by temperature or pressure.Vapor Pressure Deficit and Dew Point TemperatureVapor Pressure Deficit = Psat-PairThe 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 PatternsFigure 7.11Diurnal Cycles Seasonal Cycles Humidity InstrumentsFigure 7.14Dry bulbWet 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 conditionsAn 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 unstableif it continues to rise when disturbed upward until it reaches an altitude where the surrounding air has a similar density and temperature.Buoyancy and GravityFigure 7.15Adiabatic ProcessesFigure 7.17The 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 RateFigure 7.17Dry 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 HeatingFigure 7.17Adiabatic ProcessesDry adiabatic rate10 C°/1000 m5.5 F°/1000 ftMoist adiabatic rate6 C°/1000 m3.3 F°/1000 ftAtmospheric Temperatures and StabilityFigure 7.18env lapse rate > DARenv lapse rate <MAR/ DARMAR < env lapse rate < DARThree Examples of StabilityFigure 7.19Clouds and FogCloud Formation ProcessesCloud Types and IdentificationFogCloud Formation ProcessesMoisture 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/m3Marine air: 1 billion/m3Artificial Precipitation: Using airplane or cannon to add condensation nuclei into the clouds to facilitate moisture droplet formationMoisture DropletsFigure 7.20Raindrop and Snowflake FormationFigure 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 IdentificationFigure 7.22Three Classes of clouds: Stratus (low in altitude < 2000m ), Cumulus (2000~6000m), and Cirrus (>6000 m).CirrusFigure 7.22AltocumulusFigure 7.22CumulusFigure 7.22AltostratusFigure 7.22NimbostratusFigure 7.22StratusFigure 7.22FogDefinition: Cloud layer on the ground.Advection fogEvaporation fogUpslope fogValley fogRadiation fogAdvection FogFigure 7.24Advection: 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 FogFigure 7.25During 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 FogFigure 7.25Figure 7.26Cold air from upslope drawn into valley to cold the warm air, causing water vapor to condense and form moisture dropletsEvaporation and Radiation FogFigure 7.28When long wave radiation cools the surface and chills the air nearby below dew point temperature, moisture droplets occur (i.e. clouds  fog).Robert