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Atmospheric Humidity

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.

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Atmospheric Humidity

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  1. Chapter 4 Atmospheric Humidity

  2. 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.

  3. Stepped Art Fig. 4-1, p. 90

  4. 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

  5. Evaporation, Condensation, & Saturation • Evaporation is the change of liquid into a gas -- 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.

  6. (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) a) (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.

  7. 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 • Specific Humidity: mass of water vapor/mass of air

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

  9. 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.

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

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

  12. Humidity • Vapor pressure: the pressure exerted by water vapor molecules in an air parcel • More water molecules = high vapor pressure • Saturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature

  13. 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).

  14. (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 as air moves farther away from being saturated.

  15. Humidity • Relative Humidity and Dew Point • Dew point is the temperature at which saturation occurs • Cool an 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

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

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

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

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

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

  21. Relative Humidity in the Home T represents temperature Td, dew point RH, relative humidity

  22. 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

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

  24. 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.

  25. Humidity • Humidity is measured by: • Sling psychrometer • Hygrometer

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