# ATMOSPHERIC HUMIDITY Chapter 4 Circulation of Water in the ...

ATMOSPHERIC HUMIDITY Chapter 4 Circulation of Water in the ...

2/3/11 1234ATMOSPHERIC HUMIDITYChapter 4Circulation of Water in the Atmosphere A general definition of humidity is the amount of water vapor inthe air. Remember, humidity is not constant through time or space,there is constant circulation of water through the hydrologiccycle. 85% of evaporated water in atmosphere comes from oceans. All water vapor in the atmosphere is enough to cover entireglobe 2.5 cm deep.The Many Phases of Water Phase is related to molecular motion, an increase or decreasein motion creates a phase change. Ice is the coolest/slowest phase Water vapor is the warmest/fastest phase Evaporation ⎜⎜⎝⎝ Condensation Sublimation ⎜⎜⎝⎝ Deposition56789Evaporation, Condensation, & Saturation Evaporation is the change of liquid into a gas and requiresheat. Condensation is the change of a gas into a liquid and releasesheat. Cool conditions drive condensation Condensation nuclei Water vapor vs. Steam vs. Clouds Saturation is an equilibrium condition in which for eachmolecule that evaporates, one condenses. “Warm air has a greater capacity for water vapor than doescold air.”Humidity Any of a number of ways of specifying the amount of water1

7891011121314151617Humidity Any of a number of ways of specifying the amount of watervapor in the air. Units: g/cm 3 AKA Water vapor density Not commonly used due to frequent change of volumeHumidity Specific Humidity: mass of water vapor/mass of air Mixing ratio: mass of water vapor/mass of dry air Units: g/kg Neither measurement changes with volume, must add orsubtract water vapor to change. This makes these concepts much more useful.Humidity Vapor pressure: the pressure exerted by water vapormolecules in an air parcel (Dalton’s Law of Partial Pressure) Fraction of total vapor pressure (1% or so) More water molecules = high vapor pressure Saturation vapor pressure: the vapor pressure at which an airparcel will be saturated, changes with temperatureHumidity Special Topic: Vapor Pressure & Boiling Once water boils it requires more energy to increasetemperature. Boiling occurs when the saturation vapor pressure equalsatmospheric pressure. Water boils at a low temperature in the mountains and thusneeds more energy and time to cook items as compared tosea level.Humidity Relative Humidity: (actual water vapor/saturation water vapor)*1002/3/11 2

1617 Relative Humidity: (actual water vapor/saturation water vapor)*1002/3/11 181920212223242526 RH can be changed two ways:○ Change vapor content○ Change saturation A decrease in temperature causes an increase in relativehumidity. What does 50% RH mean? 100%?Humidity Temperature Effects For same amount of water:○ High temp. decreases RH○ Lower temp. increases RH Low RH means more evap. Don’t water lawns when it’s hot. Polar air is very dry even if the RH is near 100%.Dew Point Dew point is the temperature at which saturation occurs. The temperature to which air would have to be cooled (withno change in air pressure or moisture content) for saturationto occur. Cool air parcel to dew point and liquid water condenses. A good measure of actual water vapor content Relative humidity indicates how close to saturation, dewpoint indicates the amount of water vapor. Large difference between temperature and dew point meanshigh relative humidity.Dew Point As the sun goes down, a low dew point means the expectedovernight low temperature will be lower. There will be fewer water molecules in the air to absorb IRenergy from the ground.3

22232425262728293031Humidity Relative Humidity in the Home Due to an increase in temperature of winter air brought into aheated home there is a decrease in relative humidity, causingmore evaporation from body, plants, etc○ Humidifier, chapped lips Swamp coolerHumidity Relative humidity & human comfort “It’s not the heat, it’s the humidity.” High relative humidity equates to less evaporative cooling. Sweat cannot evaporate and cool the body Wet bulb temperature○ The lowest temperature that can be reached by evaporatingwater into the air. Heat IndexHumidity Special Topic: Heavier humid air Due to the molecular weight of water as compared tonitrogen, humid air is lighter than dry air.○ N 2 O 2 H 2 O○ Molecular weight of dry air: ~29 Baseball announcers are incorrect.○ “The air today is heavy because of the high humidity.”Measuring Humidity Sling psychrometer Wet-bulb depression Appendix D Hygrometer Hair hygrometer (0 to 100%: 2.5% increase) Electrical hygrometer○ Carbon’s resistance changes with humidity Infrared hygrometer○ Measure IR passing through air sample2/3/11 32334

2/3/11 323334○ Measure IR passing through air sample Dew-point hygrometer○ Cools a mirror until liquid condensesHomework for Chapter 4 Chapter 4 Questions for Review, p. 108 #4, 7, 10-13, 15-17, 19, 21 Chapter 4 Questions for Thought, p. 108 #7, 11 Chapter 4 Problems and Exercises, p. 109 #3, 6Project for Chapter 4 Project 3: Make a line graph of the temperature, dew-pointand relative humidity for July 4, 2010 Plot the points with a vertical axis that varies from 60 to 100. Give the graph the title July 4, 2010 On the graph: Note when it began to rain. When the temperature rose in the afternoon mark the drop inthe RH and explain why that took place.5

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