GEO 203 Notes – 9/30/10 (Lecture #6: Moisture in the Atmosphere)Major source of water vapor: evaporation from ocean and land (especially tropics)Major sink of water vapor: precipitation (removes water vapor from atmosphere)EARTH’S WATER DISTRIBUTION97% oceans; 3% otherOf 3% other: 77% icecaps/glaciers/inland seas, 22% ground water, 1% otherOf 1% other: 39% atmosphere/soil moisture, 61% lakes, <.4% riversHeat capacity of water: highest of all common solids and liquids—1 cal/g/°CHeat capacity of air: 0.25? cal/g/CDensity of water:Liquid: ~1 g/cm^3Solid: ~0.9 g/cm^3Vapor: ~10^-5 g/cm^3Radiative properties are transparent to visible wavelengths; virtually opaque to many IR wavelengthsAir gets warmer via convection, not by radiationSUBLIMATION: Ice  Vapor; cools surroundingsDEPOSITION: Vapor  Ice; warms surroundings80 calories: ice to waterWater temperature from 0C to 100C: 100 caloriesWater to vapor (evaporation): 540 calories(total: 720cal)Sublimation removes 720 calories from the environment.Deposition releases 720 calories to the environment.WATER VAPORWeighs 18 g/moleMost abundant trace gas in atmosphereVariable amount: 0-4%Most important greenhouse gas; essential to weather and climateSource: evaporation from the ocean and landSink: precipitationLarge amount found close to surface, decreasing quickly with height.Measures of water vapor in atmosphere:• Absolute humidity rv • Density of water vapor in the air• Mass / volume = Density• Absolute humidity = mass of water vapor / volume of air (not covered on exam)• Changes in volume cause changes in absolute humidity• Specific humidity q• RATIO: Mass of water vapor : Total mass of air in unit volume• q = mass of water vapor / total mass of air• Mass / mass (g /g), so no unit of measure• q values normally range from 1 to 20 k/kg• Specific humidity highest (about 17 g water/kg air) at equator; lowest at poles• Typical decrease with altitude• MI: about 8g water/kg air• Mixing ratio r• RATIO: Mass of water : mass of dry air• r = mass of water vapor / total mass of dry air (no unit of measure)• Specific humidity ~=~ mixing ratio• Specific humidity or mixing ratio is conserved as an air parcel rises or sinks in the atmosphere• Vapor pressure e • Air molecules all contribute to air pressure• Vapor pressure = pressure exerted by water vapor molecules in air• Similar to atmospheric pressure, but only due to water vapor molecules• Commonly 2-30mb (of ~1000mb atmospheric pressure) at surface• Since atmospheric total is usually about 1000mb, and water vapor is about 4% of atmosphere at most, should be no more than 40mb• Larger vapor pressure = more water vapor molecules in atmosphere• Saturation vapor pressure es• Maximum possible vapor pressure for given temperature• SVP of water increases with temperature• Relative humidity RH• Dew point temperature Td• GEO 203 Notes – 10/5/10 (Lecture #7: Moisture in the Atmosphere) •• Relative humidity: Actual water vapor / Saturation vapor pressure• Higher relative humidity does not necessarily mean more water vapor in the air• Evaporation; cooling = higher relative humidity• Condensation; warming = lower RH• Temperature reaches maximum in mid-afternoon; relative humidity is somewhat inversely related (lowest when temperature is highest; RH is highest near sunrise)• MISCONCEPTION: Air w/ high RH must have greater water vapor content than air with lower RH• Water vapor content = relative humidity (i.e. 100% = 1.00) x saturation vapor pressure•• Why is the southwest coast of the US hot and dry while the Gulf Coast is hot and moist? • “Patterns of US humidity are strongly governed by wind direction and ocean temperatures. Cooler Pacific waters create lower humidity in the west, while warmer Gulf waters generate highhumidity along the southeast and east coast.”•• RH as an indicator of saturation reveals that desert air is far from saturated, and that cold polar air nears saturation.• High amount of water vapor is found near tropics, decreasing rapidly toward higher latitudes.•• If there is no change in moisture content, then to what temperature must the outside air be cooled in the evening so that it is once again saturated?• Dew point temperature: temperature to which air must be cooled [at constant pressure and constant water vapor content] to become saturated• Higher dew point = more water vapor• At dew point, RH = 100%• Dew point depression: outside air temperature – dew point temperature• Large DPD = drier air/air is farther away from saturation• “Farther away from saturation” does not necessarily mean the air is drier• For every 10°C increase in dew point, water vapor amount in air is nearly doubled• January dew point in MI typically between 10°-20°; July: 60°-70°• Dew point temperature can be used to forecast minimum temperature at night; lower dew point temperature = lower expected minimum temperature• Since water vapor is a greenhouse gas, low dew point = less water vapor = less absorption = lower temperature•• Which environment has higher water vapor in the air: desert air or polar air? • POLAR| Air temp: -2; dew point: -2; RH = 100%• es = 5.3mb• e = 5.3mb• DESERT| Air temp: -35; dew point: 5; RH = 16%• Higher dew point = more water vapor, despite lower RH• es = 56.2mb• e = 9mb•• Chilled Mirror Hygrometer: instrument used to measure dew point• Cools the surface of a mirror until condensation forms•• Wet-bulb temperature: Lowest temperature that can be obtained by evaporating water into the air• Measured with a psychrometer• Sling psychrometer measures water vapor content of air• = temperature of dry bulb – temperature of wet bulb• Larger wet-bulb depression = lower humidity•• Heat index: combines air temperature and humidity to give apparent temperature (how hot it “feels”)• Variables needed in equation: temperature in °F, relative humidity (%)• Generally accurate when temperature exceeds 70°F• At 90°F:• Heat index = 85 when RH = 10%• Heat index = 90 when RH = 30%• Heat index = 106 when RH = 70%• See Appendix D (Humidity and Dew Point Table (Psychrometric Table))•• SUMMARY/REVIEW• Vapor pressure: the pressure exerted by water vapor molecules in given volume of air• Absolute humidity: mass of water vapor : volume of air• Specific humidity: mass of