Humidity and Condensation

1. Humidity and Condensation Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida

2. Hydrologic Cycle • Evaporation-Transportation-Condensation • Leads to Precipitation

3. How Much Water is in the Air? • Vapor pressure is the push force that H2O molecules exert • Actual vapor pressure is low (0 – 70 mb) • If it increases that indicates a larger number of water molecules

4. Saturation Vapor Pressure • Upper limit to the amount of water vapor that may be found in the air • Wind removes vapor molecules therefore increases evaporation • Warmer water evaporates easier Number of water molecules leaving (evaporating from) the liquid balances those returning (condensing into)

5. It’s All Relative (Humidity) • Ratio of the air’s vapor content to its capacity RH = (actual vapor pres. / saturation vapor pres.) x 100 • RH can change by changing the water vapor content OR by changing the air temperature • Increase Temp = Decrease RH • Increase Vapor = Increase RH

6. Do the Dew (Point) • The temperature at which air would have to be cooled for saturation to occur • When temperature & dew point are close, the air’s relative humidity is high (and vice versa) • Good indicator of the air’s actual water vapor content • High dew point = High vapor content • Low dew point = Low vapor content

7. Go to Visualization A Function of Temperature • Saturation vapor pressure is the pressure that H2O molecules would exert if the air was saturated • At higher temps it takes more water vapor to saturate the air, which exert a greater pressure

9. Dew Point vs. Relative Humidity • Even though RH can be 100% the air may be dry! • Humid air is lighter than dry air because H2O molecules with much less than N2 or O2

10. It’s the Humidity! • Heat index combines air temperature with RH to determine the “apparent temperature” • Hygrometers are used to measure humidity

11. Dew and Frost • Most likely to form on calm nights with a nocturnal inversion • Surfaces near the ground cool below the dew point and water condenses on them • If dew point is below 32°F and air cools below this temperature, then frost forms by deposition

12. Haze and Fog • Above 75% RH some water vapor starts to condense onto tiny particles known as Condensation Nuclei • Haze: As the size of the particles grows, they scatter visible light in all directions • Fog: With RH near 100% the particles grow large & condensation increases with visibility further restricted to below 1 km • A CLOUD ON THE GROUND!

13. Types of Fog: Cooling Process • Radiation Fog: forms with clear skies and light winds when cold air at the surface cools to the dew point (ground & valley) • Advection Fog: forms when warm, moist air moves over a colder surface • Upslope Fog: forms when moist air rises up mountain sides, expanding, cooling to the dew point temperature

14. Types of Fog: Evaporation • Evaporation Fog: forms when mixing of two unsaturated air masses leads to evaporation which enriches the air with water vapor • “Seeing” your breath • Steam Fog when cold air moves or forms over warm water, like a pool, pond or lake • Rain on asphalt on a summer day