Moisture, Clouds, and Precipitation

Moisture, Clouds, and PrecipitationChapter 18Water In The AtmosphereWater VaporSource of all Condensation and PrecipitationMost important gas in the atmosphereOnly makes up about 4% of atmosphereWaters Change Of StatePresent in all statesSolid, Liquid and GasChanges states as it goes through water cyclePasses through atmosphere as gas (water vapor)

Atmosphere holds enough water vapor to cover globe in 2mm deep layer

Solid To Liquid - MeltingEnergy must be transferred in form of heatIce water stays at 0 degrees until all ice has meltedEnergy added is used to break apart crystals in iceLatent HeatHeat used to melt ice didnt produce temperature change it was hiddenThis hidden energy gets stored in liquid waterWont get released until liquid returns to solidThis is the energy source for storms

Liquid to Gas - EvaporationEnergy absorbed during evaporation gives water molecules enough motion to escape surface of liquid and become gasKnown as Latent Heat of VaporizationWhen you step out of shower and are freezing it is because the water on your skin is evaporatingThe energy needed to cause evaporation is coming from your skinYour skin then feels cooler Gas To Liquid - CondensationMakes clouds and fogWater molecules have to release energyLatent Heat of CondensationViolent storms happen when a lot of this energy is released at onceSolid to Gas SublimationSkips liquid phaseDry Ice is frozen carbon dioxideWhen exposed to room temperature it sublimates to gaseous stateDepositionWater vapor goes directly to solidFrostHumidityAmount of water vapor in the airSaturationThink of jar with lid on that is half full of water and the rest is dry airThe water will evaporate causing movement in molecules of dry airWhen more molecules escape liquid water the pressure increasesThis forces some molecules to condense When the number of molecules evaporating are equal to the number of molecules condensing the air is said to be saturatedWarm air holds more water vapor than cold air (pg 506)Relative HumidityThe ratio of the airs actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressureMeasures how near the air is to saturationDoes not measure quantity of water vapor in airCan change in two waysAdd or remove water vaporChange temperatureRelative Humidity

Dew PointTemperature that air has to cool to in order to reach its saturation pointDuring summer, since warm air can hold a lot of water vapor, the warm days mean a lot of moisture in airWhen sun goes down the air coolsCool air cant hold as much water vapor so it condenses and we wake up with dew covered surfacesHigh Dew Point = moist airLow Dew Point = dry airMeasuring HumidityHygrometerPsychrometerA type of hygrometerHas two thermometersOne wet-bulbOne dry bulb

Measuring HumidityHow A Psychrometer WorksAir must continually pass over bulbWater evaporates from wet bulbThe heat absorbed by evaporation causes wet-bulb temperature to drop (like your skin getting cold after shower)Amount of cooling is directly related to amount of water vapor in airDry air will cause more evaporation and greater drop in temperatureUse standard tables to determine relative humidityRelative Humidity Table pg 525

Why is Winter Air Dry?Water vapor content of air stays constantWarm air holds more water vapor than cold airIn winter outside air is cold so it doesnt hold a lot of moistureWhen you pump that air into your house and warm it up the relative humidity drops to less than 10%Causes static electricity, dry skin, sinus headaches, etc.

18.1 ReviewWhat is the most important gas for understanding atmospheric processes?What happens to heat during a change of state?How does the temperature of air influence its ability to hold water?List two ways relative humidity can be changed?What does relative humidity describe about air?If the dew point is low, is the air moist or dry?Cloud FormationClouds form when air is cooled to its dew pointAir must be saturatedBut these clouds form during warmest part of day so how is that possible?

Air Compression and ExpansionThink of pumping up bicycle tireWhen you put air in the hose gets warmCompressing air increases motion of gas moleculesThis causes temperature to riseTire losing airWhen air is released it expands and cools

Adiabatic Temperature ChangesChanges that happen even though heat isnt added or subtractedExpansion and CoolingAs you go higher in atmosphere pressure decreasesWith less pressure a parcel of air is able to expand as it moves higherDry Adiabatic RateRate of cooling for unsaturated air100C/1000 metersEventually parcel of air reaches its dew pointLatent heat is released and clouds formWet Adiabatic RateRate of cooling for saturated air50C/1000 meters

Adiabatic Cooling

Processes That Lift AirAir would prefer not to move aroundFour mechanisms cause air to riseOrographic LiftingFrontal WedgingConvergenceLocalized Convective Lifting

Orographic LiftingElevated terrain acts as barrierCauses air to rise and reach dew pointClouds form and it rains on windward sideOther side of barrier is dry because all of precipitation fell outAir descends and warms up making it even drierRain Shadow Desert forms

Frontal WedgingWarm air meets cold air and a front formsWarm air is less dense so it rises over cold air

ConvergenceWhen two air masses run into each other the air has to go somewhereIt cant go down so it has to go upLeads to adiabatic cooling and possible cloud formationThis is why it rains many afternoons in Florida

Localized Convective LiftingUnequal heating of Earths surface can cause pockets of air to be warmed more than othersRising parcels of air are called thermalsBirds use this to soar high in skyPeople use it when they want to go hang glidingIf parcel rises above condensation level then clouds form and may produce rain

StabilityStable air resists vertical movementStable when air is cooler than surrounding environmentThis means it is more dense and doesnt rise up to form clouds or cause precipitationStable air tends to remain in original positionUnstable air tends to rise it is warmer and less dense than its surroundingsStability MeasurementsWe can tell if air is stable by knowing temperature of the atmosphere at various heightsMeasured using aircraft and radiosondesEnvironmental Lapse RateRate of change of air temperature with heightDegrees of StabilityStable airTemperature decreases gradually with increasing altitudeTemperature inversionMost stable airTemperature increases with heightHappens on clear nightsEarth cools off quickly and air at surface is cooled while air above surface is still warmStability and Daily WeatherStable airYou wouldnt think clouds would form if air was stableBut, remember, other processes cause air to liftWhen this happens clouds are usually widespread with little vertical thicknessPrecipitation, if any, is light to moderateUnstable airTowering clouds with powerful storms

CondensationAir must be saturated and there must be a surface for the water vapor to condense onKnown as Condensation NucleiUsually microscopic particles of dust, smoke or saltWater condenses and forms clouds, but they are so small they are still light enough to remain in the airSo a cloud is made of millions and millions of tiny water droplets18.2 ReviewDescribe what happens to air temperature when work is done on the air to compress it.What does stability mean in terms of air movement?What are the four mechanisms that cause air to rise?Describe conditions that cause condensation of liquid water in air.What is a temperature inversion?Hypothesize about other regions on Earth, other than the Florida peninsula, where convergence might cause cloud development and precipitation.CloudsVisible mixtures of tiny droplets of water or tiny crystals of iceShow what is going on in the atmosphereKnowing the characteristics of cloud types helps you predict the weather.Types based on form and heightCloud Form #1 - CirrusHigh, white and thinOccur as patches or veil-like sheetsWispy fibers give feathery appearance

Cloud Form #2 - CumulusRounded individual cloud massUsually have flat base and rise as domes or towersFluffy tops described as cauliflower head

Cloud Form #3 - StratusSheets or layers that cover the skyNo distinct unitsUsually bring precipitation

Cloud Height #1 High Base above 6000 metersCirrus, cirrostratus and cirrocumulusThin and whiteMade of ice crystalsUsually no precipitationIf followed by cirrocumulus or cirrostratus may warn of approaching storm

Cloud Height #2 - Middle2000 6000 metersPrefix alto as part of their nameAltocumulus are larger and denser than cirrocumulusAltostratus are white or grayish sheet covering skyOften accompanied by light snow or drizzle

Cloud Height #3 - LowForm below 2000 metersStratus, stratocumulus and nimbostratusUniform, fog-like layerCovers most of skyNimbostratus main precipitation makersForm in stable air when air is forced up

Clouds of Vertical DevelopmentThese dont fit in any of the other categoriesThey may start in one layer and extend into anotherForm in unstable airCumulus are usually fair weather but grow out of control under right circumstances become cumulonimbusProduce rain or thunderstorms

FogA cloud with its base at or very near the groundPhysically same as cloudDifference is method and place of formationClouds form when air risesFog forms three waysRadiant coolingMovement of air over cold surfaceWater vapor added to reach saturation levelFogs Caused by CoolingWarm moist air over Pacific moves over cold California Current then carried onshore by prevailing windsCool, clear, calm nights air near surface of Earth cools when Earth loses heat gained during day and reaches its dew pointAs the air cools it becomes dense and sits in low areas

Fogs Caused By EvaporationCool air moves over warm waterWater evaporates from surface and air above reaches saturationKnown as Steam FogMost common in fall and early winter

How Precipitation FormsCloud droplets