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 (Daltons 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 Its not the heat, its 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