Water and Atmospheric Moisture
Moisture, Clouds, and PrecipitatoinHumidityGlobal Precipitation Precipitation ProcessesLifting Mechanisms
Big Question: What Causes Air to Precipitate?
Global PrecipitationDark Blue > 80 inchesBeige < 10 inches
Hydrologic Cycle
Relative Humidity
The ratio between the actual amount of water vapor held in the atmosphere compared to the amount required for saturation. It is influenced by temperature.
HumidityHumidity: water vapor content of airCapacity primarily a function of temperatureRelative humidity (RH) (actual water vapor content) (max. water vapor capacity of the air)
RH affected by evaporation/condensationtemperaturex 100
Saturation vsAir TemperatureThe actual amount ofWater air can hold changesWith air temperature
Air at 104 F can hold 3 timesAs much water as 68 F air !(47 grams vs only 15 grams)
Air at 68 F can hold 4 timesAs much water as air at 0 F(15 grams vs only 4 grams)32 F68 F104 F4 grams15 grams47 grams
Relative Humidity ExamplesExamples At 22 C (72 F), one kg of air can hold 50 grams of water vapor.RH if holding 25 grams? 50 grams? 60 grams?What happens if water is added? Removed?What happens to RH if temp. drops? Increases?
Saturation and Dew PointSaturated v. unsaturated airDew-point temperaturetemperature to which air must be cooled to reach saturation (100% RH)water on outside of glassice on your car window
Adiabatic Cooling: Clouds and CondensationCloud base = dew point altitude
Relative Humidity and Temp.RH fluctuates over a day or season.
Measuring Relative HumiditySling psychrometerHair hygrometer
After Saturation Occurs the AirMust Release Extra Water as FluidWater forms on the outside of a cold glass as the coldAir surrounding the glass chills the air to the Dew Point Temperature
The resulting water is not from the glass, the water is from condensation of moisture in the air around the glass
In Nature Extra Moisture isTransformed to Water DropletsCold air next to the rain-soaked cliff is chilled To The Dew Point Temperature & creates a Misty Cloud along a RockyMountain slope
Air near theSlope is 100%Saturated
Fog: A Cloud on the GroundFog by San Francisco BayThis Advectional fog forming in San Francisco is due to Moist Ocean air being moved horizontally over cooler land surfaces
This fog is a commonSummer condition Along the California coast As high temperatures Farther inland draw moist ocean Air over coastal areas
Yakima River, Washington
Valley Fog in the Valleys of the Appalachian MountainsValley Fog: Distributed by Topography
An oftenVery dense Type ofValley FogCalled TuleFog in theCentralValley ofCaliforniaPacificOcean
Temperature InversionsWhen warmer air overlies cooler air, pollutants and fog are trapped beneath the inversion.Common Winter Radiation Inversion in Valleys
Temperature InversionsCommon Summer Inversion in Los Angeles
Types of Lifting of AirAir Lifting processes create clouds & clouds Are the only means of precipitation on EarthFour types of lifting are recognized:
Convergent Lifting Convectional LiftingOrographic Lifting Frontal Lifting
Convergent Lifting of AirConvergent Lifting occurs When Air masses meet & are forced To rise vertically
This process is best seen at The Equator where the Trades Winds Meet & rise to form towering Clouds & heavy Precipitation
Convergent Lifting & Global Precipitation PatternsConvergent Lifting provides the greatest amount of PrecipitationOn EarthNear theEquator
Shown In darkGreen onThis map
Local Convectional LiftingAnywhere theLand is warmer Air will rise
In this exampleA plowed field Creates warmer Air temperatures Than nearbyGreen cropland & Local air rises
Convectional Lifting Over FloridaWarmer temperatures over the peninsula of Florida, which is land, cause air to rise compared to the cooler oceans nearby
Rising air in thisShuttle Picture isShown by a Cloud patternwhich generallyfollows the shape of the southern Florida peninsula
Orographic Lifting of AirWhen air movingHorizontally Encounters aMountain it mustRise over the crest
As it rises, it coolsTo create clouds,And most oftenprecipitationMoistAirMoistureLostDryAirRun off NO Run off
The Orographic EffectWet (Windward) Side Dry (Leeward) SideAlong the west coast of United States air moving eastwardFrom the ocean contains moisture as this air moves overThe coastal mountains, clouds & precipitation occur Leaving Only dry air on the inland side of the mountains
Frontal Lifting of AirAlthough not a mountain range, masses of moving airCreate the same effect Unlike mountains air massesCan provide lifting in many different locations
Fronts can lift air Which is stable,Creating clouds& large amountsOf precipitationAs rain, snow,Sleet or hail
Precipitation FormationRequires condensation nuclei (solid particles)saturation (air at dew point)Result is temperature dependent Result varies geographically
Collision-Coalescence Process = Raindrop Bergeron Ice-Crystal Process = Snowflake
Precipitation Types / PropertiesWhat about hail?
Snowflakes and TemperatureSnow crystal images from an electron microscope
Seasonal and Global Variation in Lifting Mechanisms and PrecipitationConvective increases when insolation is most intense and when marine air moves over land massesOrographic requires elevation changeFrontal midlatitudes onlyConvergent near ITCZ only
SummaryHumidityRelative HumidityRelationship to TemperatureDew PointLCLPrecipitation (Rain, Snow, Sleet, Hail)Ice Crystal Process, Collision-CoalescenceLifting MechanismsConvective, Orographic, Frontal, ConvergenceAtmospheric Stability