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Chapter 4. Atmospheric Humidity. 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. Stepped Art. - PowerPoint PPT Presentation

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  • Chapter 4

  • Circulation of Water in the AtmosphereA 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.

  • Stepped ArtFig. 4-1, p. 90

  • The Many Phases of WaterPhase is related to molecular motion, an increase or decrease in motion creates a phase change.Ice is the coolest/slowest phaseWater vapor is the warmest/fastest phase

  • Evaporation, Condensation, & SaturationEvaporation is the change of liquid into a gas -- requires heat.Condensation is the change of a gas into a liquid and releases heat.Condensation nucleiSaturation is an equilibrium condition in which for each molecule that evaporates, one condenses.

  • (a) Water molecules at the surface of the water are evaporating (changing from liquid into vapor) and condensing (changing from vapor into liquid). Since more molecules are evaporating than condensing, net evaporation is occurring. (b) When the number of water molecules escaping from the liquid (evaporating) balances those returning (condensing), the air above the liquid is saturated with water vapor. a)b)

  • HumidityAny of a number of ways of specifying the amount of water vapor in the air.Absolute humidity: mass of water vapor/volume of airWater vapor densityNot commonly used due to frequent change of volumeSpecific Humidity: mass of water vapor/mass of air

  • The water vapor content (humidity) inside this air parcel can be expressed in a number of ways.

  • With the same amount of water vapor in a parcel of air, an increase in volume decreases absolute humidity, whereas a decrease in volume increases absolute humidity.

  • The specific humidity does not change as air rises and descends since it is only dealing with mass.

  • The average specific humidity for each latitude.

    The highest average values are observed in the tropics and the lowest values in polar regions.

  • HumidityVapor pressure: the pressure exerted by water vapor molecules in an air parcel More water molecules = high vapor pressureSaturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature

  • HumidityRelative Humidity: (actual water vapor/saturation water vapor)*100RH can be changed two ways:Change vapor contentChange saturation

    Decrease temperature causes an increase in relative humidity (inverse relationship).

  • (a) At the same air temperature, an increase in the water vapor content of the air increases relative humidity as air approaches saturation.

    (b) With the same water vapor content, an increase in temp-erature causes a decrease in relative humidity asair moves farther away from being saturated.

  • HumidityRelative Humidity and Dew PointDew point is the temperature at which saturation occursCool an air parcel to dew point and liquid water condensesA good measure of actual water vapor contentRelative humidity indicates how close to saturation, dew point indicates the amount of water vapor

  • When When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low.

  • Average surface dew-point temperatures (oF) for (a) January .

  • Average surface dew-point temperatures (oF) for (b) July.

  • Relative humidity averaged for latitudes north and south of the equator.

  • All data represent conditions during a July afternoon at 3 p.m. local time.

  • Relative Humidity in the Home

    T represents temperature

    Td, dew point

    RH, relative humidity

  • HumidityRelative humidity & human comfortIts not the heat, its the humidity.High relative humidity equates to less evaporative cooling.Sweat cannot evaporate and cool the bodyWet bulb temperatureHeat Index

  • Air temperature (F) and relative humidity are combined to determine an apparent temperature or heat index (HI).

  • HumiditySpecial Topic: Heavier humid airDue to the molecular weight of water as compared to nitrogen, humid air is lighter than dry air.Baseball announcers are incorrect.

  • HumidityHumidity is measured by:Sling psychrometer

    Hygrometer

    **FIGURE 4.1 The hydrologic cycle.

    Figure 4.1: The hydrologic cycle.*FIGURE 4.3 The three states of matter. Water as a gas, as a liquid, and as a solid.

    *FIGURE 4.4 (a) Water molecules at the surface of the water are evaporating (changing from liquid into vapor) and condensing (changing from vapor into liquid). Since more molecules are evaporating than condensing, net evaporation is occurring. (b) When the number of water molecules escaping from the liquid (evaporating) balances those returning (condensing), the air above the liquid is saturated with water vapor. (For clarity, only water molecules are illustrated.)

    FIGURE 4.5 Condensation is more likely to occur as the air cools.(a) In the warm air, fast-moving H2O vapor molecules tend to bounceaway after colliding with nuclei. (b) In the cool air, slow-moving vapormolecules are more likely to join together on nuclei. The condensing ofmany billions of water molecules produces tiny liquid water droplets.

    *FIGURE 4.6 The water vapor content (humidity) inside this air parcel can be expressed in a number of ways.FIGURE 4.7 With the same amount of water vapor in a parcel of air, an increase in volume decreases absolute humidity, whereas a decrease in volume increases absolute humidity.

    FIGURE 4.8 The specific humidity does not change as air rises and descends.

    FIGURE 4.9 The average specific humidity for each latitude. The highest average values are observed in the tropics and the lowest values in polar regions.

    **FIGURE 4.11 (a) At the same air temperature, an increase in the water vapor content of the air increases the relative humidity as the air approaches saturation. (b) With the same water vapor content, an increase in air temperature causes a decrease in relative humidity as theair moves farther away from being saturated. *FIGURE 4.12 When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low. These conditions exist in clear weather when the air is calm or of constant wind speed. FIGURE 4.14 Average surface dew-point temperatures (oF) for (a) January and for (b) July.

    FIGURE 4.14 Average surface dew-point temperatures (oF) for (a) January and for (b) July.

    FIGURE 4.16 Relative humidity averaged for latitudes north and south of the equator.

    FIGURE 4.17 Air from the Pacific Ocean is hot and dry over land, whereas air from the Gulf of Mexico is hot and muggy over land. For each city, T represents the air temperature, Td the dew point, and RH the relative humidity. (All data represent conditions during a July afternoon at 3 p.m. local time.)

    FIGURE 4.18 When outside air with an air temperature and a dew point of o15oC (5oF) is brought indoors and heated to a temperatureof 20oC (68oF) (without adding water vapor to the air), the relative humidity drops to 8 percent, placing adverse stress on plants, animals, and humans living inside. (T represents temperature; Td, dew point; and RH, relative humidity.)

    *FIGURE 4.19Air temperature (oF) and relative humidity are combined to determine an apparent temperature or heat index (HI). An air temperature of 95oF with a relative humidity of 55 percent produces an apparent temperature (HI) of 110oF.

    *

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