Chapter 4

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 Fig. 4-1, p. 90

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

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.

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

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.

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

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.

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).

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

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

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

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.

Humidity  Measuring humidity Sling psychrometer Hygrometer