Transcript water vapor
Water Most abundant 71% of surface Hydrologic cycle Phase changes -transfers of latent energyGas (water vapor) Solid (ice) Liquid Phase changes -transfers of latent energyGas (water vapor) Solid (ice) Liquid Phase changes -transfers of latent energyGas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm Solid (ice) Liquid Phase changes -transfers of latent energyGas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm Solid (ice) Liquid melting Phase changes -transfers of latent energyGas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm Solid (ice) Liquid melting + 80 cal/gm LATENT HEAT OF FUSION Phase changes -transfers of latent heat energyGas (water vapor) LATENT HEAT OF VAPORIZATION + 597 cal/gm Solid (ice) Liquid melting + 80 cal/gm LATENT HEAT OF FUSION Phase changes -transfers of latent heat energyGas (water vapor) 597 80 100 +777 cal/gm Solid (ice) LATENT HEAT OF VAPORIZATION + 597 cal/gm Liquid melting + 80 cal/gm LATENT HEAT OF FUSION Phase changes -transfers of latent heat energyGas (water vapor) +777 cal/gm + 597 cal/gm freezing (-80) Solid (ice) Liquid melting + 80 cal/gm Expressions of atmospheric humidity: 1. vapor pressure 2. relative humidity 3. dew point 4. mixing ratio Vapor Pressure Water vapor is one of the gases in the atmosphere that contributes to air pressure. The total pressure of a mixture of gases equals the sum of the pressures exerted by the constituent gases. Dalton’s Law Vapor pressure is the portion of the air pressure due to water vapor. Saturation concept Constant two-way interchange at surface; vapor liquid EVAPORATION: more molecules enter gas phase. CONDENSATION: more enter liquid phase. EQUILIBRIUM: equal amount become liquid and gas. At equilibrium, vapor pressure is SATURATION VAPOR PRESSURE Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the saturation vapor pressure) is higher. Raising temperature increases energy of molecules and more readily escape surface as gas. Initially evaporation prevails, but eventually a new equilibrium is established. Higher temp, higher water vapor conc, higher equilibrium/saturation V.P. Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the saturation vapor pressure) is higher. Notice the difference between vapor pressure and saturation vapor pressure What if…. temperature is 30ºC and vapor pressure is 20 mb? Net evaporation or net condensation? Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the saturation vapor pressure) is higher. if temperature drops to 20º ? Saturation vapor pressure At higher temperatures, the amount of water vapor in the atmosphere (and therefore the vapor pressure) is higher. There is less energy available for evaporation ; condensation will occur, then a new equilibrium is reached. “Warm air can hold more water vapor than cold air” “When temperature is higher, there’s more thermal energy for evaporation , so there is more water vapor in the air” “Warm air expands and can “There’s always plenty of hold more water vapor” room for water vapor molecules”. (Average intermolecular distance is 145 cm.) Now look at inset graph: Now look at inset graph: Below freezing, sat. vapor pressure over ice is different than saturation vapor pressure over (supercooled) water. Now look at inset graph: Below freezing, sat. vapor pressure over ice is different than saturation vapor pressure over (supercooled) water. Which is greater? Does it take more water molecules to saturate air over water or over ice? Over water Relative humidity Amount of moisture in air relative to saturation amount, expressed as %. actual amount water vapor X saturation amount 100 If we have a parcel of air at 10ºC and the vapor pressure is 6 mb, what is the relative humidity? 6 X 100 = 50% 12 If temperature increases (and actual amount of water vapor stays same), what happens to relative humidity? Dew Point Temperature Temperature at which cooling air becomes saturated. Dew Clouds Frost a closer look at dew point temperature : Even “clear” air contains microscopic water droplets, BUT evaporation > condensation so drops don’t survive long enough to clump and grow into cloud droplets. Why do droplets have to grow to become cloud droplets? As temp drops, evaporation rate decreases REMEMBER! Evaporation requires energy! There comes a temperature at which net condensation > net evaporation : “dew point temperature” …then a droplet can grow into a cloud particle. Air is saturated when dew point and air temperature are same. Once air is saturated, dew point and air temp stay same Dew point changes as absolute amount of water vapor changes. NOT temperature-dependent Therefore dew point is better measure than R.H. of how much moisture is in air. Vapor Pressure= 10 mb Morning: Temperature Relative humidity Dew point 10°C ? ? Vapor Pressure= 10 mb Morning: Temperature Relative humidity Dew point 10°C 83% ? Vapor Pressure= 10 mb Morning: Temperature Relative humidity Dew point 10°C 83% 5ºC Vapor Pressure= 10 mb Morning: Temperature 10ºC Relative humidity 83% Afternoon: 30ºC ? Dew point 5ºC ? Vapor Pressure= 10 mb Morning: Temperature 10ºC Relative humidity 83% Afternoon: 30ºC 24% Dew point 5ºC ? Vapor Pressure= 10 mb Morning: Temperature 10ºC Relative humidity 83% Afternoon: 30ºC 24% Dew point 5ºC 5ºC Vapor Pressure= 10 mb Morning: Temperature 10ºC Relative humidity 83% Afternoon: 30ºC 24% Dew point 5ºC 5ºC Has the amount of water vapor changed? Dew points in US rarely exceed 80°F (27°C). Highest dew point ever recorded: Eritrea : 94°F (34°C) (temp. was 115°F (46°C)) Why do we never hear of relative humidities = 100% ? (even if it is raining) Because RH is measured at surface; not at level of clouds. • For same reason there can be clouds in sky and a reported RH < 100% Mixing ratio Mass of water vapor per mass of dry air (g/kg) Saturation mixing ratio: mixing ratio of saturated air. Skew T chart Using these lines on chart: At given temp, can find sat. ratio. At given dew point, can find actual ratio. Can compute relative humidity. Shows dew pt. changes in rising air. Sling psychrometer measures relative humidity Psychrometric tables Wet If bulb depression = dry minus wet you sling the psychrometer and the air temperature (dry bulb) is 40° and the wet bulb temperature is 36°, what is the relative humidity? Relative humidity table What is the dew point? Dew point table