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Class #3: Humidity, condensation, and clouds. Chapters 4 and 5. Atmospheric Humidity. Chapter 4. Circulation of Water in the Atmosphere. A general definition of humidity is the amount of water vapor in the air.
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Class #3: Humidity, condensation, and clouds Chapters 4 and 5 Class #3 July 9, 2010
Atmospheric Humidity Chapter 4 Class #3 July 9, 2010
Circulation of Water in the Atmosphere • A general definition of humidity is the amount of water vapor in the air. • Remember, humidity is not constant through time or space, there is constant circulation of water through the hydrologic cycle. Class #3 July 9, 2010
Stepped Art Class #3 July 9, 2010 Fig. 4-1, p. 90
Class #3 July 9, 2010 Fig. 4-2, p. 91
The Many Phases of Water • Phase is related to molecular motion, an increase or decrease in motion creates a phase change. • Ice is the coolest/slowest phase • Water vapor is the warmest/fastest phase Class #3 July 9, 2010
Evaporation, Condensation, & Saturation • Evaporation is the change of liquid into a gas a requires heat. • Condensation is the change of a gas into a liquid and releases heat. • Condensation nuclei • Saturation is an equilibrium condition in which for each molecule that evaporates, one condenses. Class #3 July 9, 2010
Humidity • Any of a number of ways of specifying the amount of water vapor in the air. • Absolute humidity: mass of water vapor/volume of air • Water vapor density • Not commonly used due to frequent change of volume Class #3 July 9, 2010
Humidity • Specific Humidity: mass of water vapor/mass of air • Mixing ratio: mass of water vapor/mass of dry air • Neither measurement changes with volume, must add or subtract water vapor. Class #3 July 9, 2010
Humidity • Vapor pressure: the pressure exerted by water vapor molecules 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 air parcel will be saturated, changes with temperature Class #3 July 9, 2010
Humidity • Special Topic: Vapor Pressure & Boiling • Once water boils it requires more energy to increase temperature. • Water boils at a low temperature in the mountains and thus needs more energy and time to cook items as compared to sea level. Class #3 July 9, 2010
Humidity • Relative Humidity: (actual water vapor/saturation water vapor)*100 • RH can be changed two ways: • Change vapor content • Change saturation • Decrease temperature causes an increase in relative humidity (inverse relationship). Class #3 July 9, 2010
Humidity • Relative Humidity and Dew Point • Dew point is the temperature at which saturation occurs • 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, dew point indicates the amount of water vapor Class #3 July 9, 2010
Class #3 July 9, 2010 Fig. 4-13, p. 98
Class #3 July 9, 2010 Fig. 4-13, p. 98
Class #3 July 9, 2010 Fig. 4-13, p. 98
Class #3 July 9, 2010 Fig. 4-13, p. 98
Class #3 July 9, 2010 Fig. 4-15a, p. 100
Class #3 July 9, 2010 Fig. 4-15b, p. 100
Humidity • Relative Humidity in the Home • Due to an increase in temperature in a heated home there is a decrease in relative humidity, causing more evaporation from body, plants, etc • Humidifier, chapped lips • Swamp cooler Class #3 July 9, 2010
Humidity • 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 • Heat Index Class #3 July 9, 2010
Humidity • Special Topic: Heavier humid air • Due to the molecular weight of water as compared to nitrogen, humid air is lighter than dry air. • Baseball announcers are incorrect. Class #3 July 9, 2010
Humidity • Measuring humidity • Sling psychrometer • Hygrometer Class #3 July 9, 2010
Condensation: DEW, Fog, & clouds Chapter 5 Class #3 July 9, 2010
The Formation of Dew & Frost • Dew forms on objects near the ground surface when they cool below the dew point temperature. • More likely on clear nights due to increased radiative cooling • White frost forms when temperature cools below the dew point and the dew point is below 0°C Class #3 July 9, 2010
Condensation Nuclei • Particles suspended in the air that around which water condenses or freezes. • Hydrophobic/hygroscopic Class #3 July 9, 2010
Class #3 July 9, 2010 Table 5-1, p. 113
Haze • Dry condensation nuclei (above dew point) reflect and scatter sunlight creating blueish haze. • Wet condensation nuclei (75% relative humidity) reflect and scatter sunlight creating grayish or white haze. Class #3 July 9, 2010
