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