Measuring Atmospheric Moisture and Relative Humidity

1. Lab 5: Atmospheric Moisture

2. Relative Humidity • Sling Psychrometers: measures Relative Humidity • Dry bulb temp • Web bulb temp DB – WB = Wet bulb depression DB vs. WB: Big difference = dry air Small difference = moist air

3. Relative Humidity • Swing for ~ 60 seconds • If you have a fraction, change to a whole number • Record DB & WB temps • Individuals or groups • 4 locations: • Inside • Outside

4. Measuring Relative Humidity

5. Atmospheric Moisture: Latent Heat • Water comes in three phases: • Solid • Liquid • Vapor • Unique to this atmospheric component *** • Latent heat transfer: process of water changing phase • Energy is consumed or released • Sensible heat: heat we can feel & measure • Latent heat: energy in the form of heat • Sensible heat: potential energy in the form of thermal energy or heat • Latent heat: amount of energy in the form of heat released or absorbed by a chemical substance during a change of state. • Kelvin is the most reliable measurement of temperature because zero kelvin represents the temperature at which all molecular motion stops.

6. Heat: average kinetic energy of a given amount of liquid • Condensation = warming process • Evaporation = cooling process

7. Atmospheric Moisture: Latent Heat

8. Amount of water vapor in the air depends on amount of energy available to change liquid → gas. • As temperature increases, the liquid water molecules start moving faster…it’s more likely to evaporate • Energy NOT used to boil the water is used to change the state of the water from liquid to gas

9. Two ways to measure atmospheric moisture: • Vapor pressure – • Water vapor molecules exert pressure proportional to their concentration in the atmosphere • Maximum is called saturation vapor pressure • SVP increases with temperature

10. 2. Mixing Ratio – • Mass of water vapor in the air • Ratio of water vapor mass to the mass of dry air • Units of grams of water vapor per kilogram of dry air • saturation mixing ratio: air is saturated • Depends on temperature (↑ with temperature) • Represents max weight of water vapor/kilogram of dry air

11. Relative Humidity: measures how close the air sample is to saturation • It is a ratio of actual water vapor in the air to the saturation level at a given temperature. • The greater the difference between air temperature & the dew point, the lower the RH (dry air) • RH is temperature dependant

13. Winter? Warming up already dry air will DECREASE Relative Humidity! • Does winter indicate air that would be dry or moist? In the winter, air is generally rather dry. Cold air has less capacity to hold water vapor than warm air. • Heat in your home – becomes dry

15. Summer? • In the summer, air is generally moist. Warm air can hold greater amounts of water vapor than cold air. • Cooling already moist air will INCREASE Relative Humidity!

16. Dew Point • Dew point: temperature to which air must be cooled to reach saturation (RH = 100%) • Generally seen as dew in the morning on the grass.

17. Dew Point Find MR of 17 Create a STRAIGHT line to the dew point line Create another STRAIGHT line down to the temperature Read the temperature. A 30° air sample would have to cool to ~ 22° to reach saturation (100% humidity)