Presentation on theme: "Chapter 6 Humidity, Saturation, and Stability"— Presentation transcript:
1 Chapter 6 Humidity, Saturation, and Stability Weather Studies Introduction to Atmospheric Science American Meteorological SocietyChapter 6Humidity, Saturation, and StabilityCredit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants
2 Case-in-PointCloud forests are forests that are perpetually shrouded in clouds or mistThey are found from m ( ft) in elevation in the tropics and subtropicsOnshore and upslope winds that are warm and humid supply the moistureWarm air blowing upslope cools through expansionExpansional cooling raises the relative humidity to saturation and water vapor condenses into low clouds and fogThe tree canopy strips moisture from the clouds and this water drips to the forest floorDeforestation reduces available moisture and raises air temperature → clouds form less readily and at higher elevations
3 Case-in-PointIf global warming translates into higher sea surface temperatures (SSTs) in the tropics, cloud forests could be affectedAir flowing onshore would be warmerGreater ascents would be required to produce cloudsClouds would be at higher elevationsPerhaps even lift off the mountainsCloud forests are extremely sensitive to climate variationsThey may prove to be early indicators of effects of global-scale climate change
4 Driving QuestionHow does the cycling of water in the Earth-atmosphere system help maintain a habitable planet?This chapter will tell us:How the global water cycle functionsEspecially as it relates to transference between the Earth’s surface and the atmosphereHow to quantify the water content of airHow air becomes saturated through uplift and expansional coolingHow atmospheric stability affects the ascent of air
5 Global Water CycleAssumption – the amount of water in the Earth-atmosphere system is neither increasing or decreasingInternal processes continually generate and break down water moleculesVolcanoes and meteors (minute amount) add waterPhotodissociation of water vapor and chemical reactions break down water moleculesFixed quantity of water in Earth-atmosphere system is distributed in 3 phases among various reservoirs, mostly the ocean (97.2%) and ice sheets and glaciers (2.15%)The sun powers the global water cycle and gravity keeps water from escaping to space, causing water to fall from the sky as precipitation and flow to oceans
7 Where is the Water Stored? Note the small percentage of the total water that is stored in the atmosphere.Even though small in percentage, this is vital to weather processes
8 The Global Water Cycle Transfer processes 1. Phase changes Evaporation – more molecules enter the atmosphere as vapor then return as liquid to the water surfaceCondensation – more molecules return to the water surface as liquid then enter the atmosphere as vaporTranspiration – Water that is taken up by plant roots escapes as vapor from plant poresEvapotranspiration is the total of evaporation and transpirationSublimation – ice or snow become vapor without first becoming liquidDeposition - water vapor becomes solid without first becoming liquidAll 3 phases of water exist in the atmosphere2. PrecipitationRain, drizzle, snow, ice pellets, and hail
9 Percent of Precipitation Originating from Land Sources Ocean evaporation is the origin of most precipitation.
10 Pathways Taken by Precipitation Falling on Land
11 The Global Water Budget Via precipitation and evaporation, the ocean has a netloss of water and the land has a net gain.
12 How Humid is it?Humidity describes the amount of water vapor in the airThis varies with time of year, from day-to-day, within a single day, and from place-to-placeHumid summer air, and dry winter air cause discomfortWays of measuring humidity:Vapor pressureMixing ratioSpecific humidityAbsolute humidityRelative humidityDewpointPrecipitable water
13 How Humid is it? Vapor pressure Mixing ratio Specific humidity Water vapor disperses among the air molecules and contributes to the total atmospheric pressureThis pressure component is called the vapor pressureMixing ratioMass of water vapor per mass of the remaining dry airExpressed as grams of water vapor per kilograms of dry airSpecific humidityMass of the water vapor (in grams) per mass of the air containing the vapor (in kilograms)In this case, the mass of the air includes the mass of the water vaporMixing ratio and specific humidity are so close they are usually considered equivalent
14 How Humid is it? Absolute humidity Saturated air The mass of the water vapor per unit volume of humid air; normally expressed as grams of water vapor per cubic meter of airSaturated airThis is the term given to air at its maximum humidityA dynamic equilibrium develops where the liquid water becomes vapor at the same rate as vapor becomes liquid“Saturation” may be added to various humidity termsSaturation vapor pressure, saturation mixing ratio, saturation specific humidity, saturation absolute humidityChanging the air temperature disturbs equilibrium temporarilyExample: heating water increases kinetic energy of water molecules and they more readily escape the water surface as vapor. If the supply of water is sufficient, a new dynamic equilibrium is established with more vapor at higher temp.
15 Variations with Air Temperature of Vapor Pressure Saturation Mixing Ratio
16 How Humid is it? Relative humidity Probably the most familiar measure Compares the amount of water vapor present to the amount that would be present if the air were saturatedRelative humidity (RH) can be computed from vapor pressure or mixing ratioRH = [(vapor pressure)/ (saturation vapor pressure)] x 100RH = [(mixing ratio)/(saturation mixing ratio)] x 100At constant temperature and pressure, RH varies directly with the vapor pressure (or mixing ratio)If the amount of water vapor in the air remains constant, relative humidity varies inversely with temperatureSee next slide
17 The Relationship of Relative Humidity to Temperature
18 How Humid is it? Dewpoint The temperature to which the air must be cooled at constant pressure to reach saturationAt the dewpoint, air reaches 100% relative humidityHigher with greater concentration of water vapor in airWith high relative humidity, the dewpoint is closer to the current temperature than with low relative humidityDew is small drops of water that form on surfaces by condensation of water vaporIf the dewpoint is below freezing, frost may form on the colder surfaces through depositionDewpoints below freezing are sometimes referred to as frostpoints
19 How Humid is it? Precipitable water The depth of the water that would be produced if all the water vapor in a vertical column of condensed into liquid waterCondensing all the water vapor in the atmosphere would produce a layer of water covering the entire Earth’s surface to a depth of 2.5 cm (1.0 in.)Highest in the tropicsMap of precipitable waterat various locations
20 Monitoring Water Vapor Humidity instrumentsHygrometerMeasures the water vapor concentration of airDewpoint hygrometerUses a temperature-controlled mirror and an infrared beamWhen the mirror temperature reaches a point that condensation forms, the reflectivity of the mirror is changed, altering the reflection of the beam. The temperature is recorded as the dewpoint.These are common at NWS forecast stationsHair hygrometerRelates changes in length of a humid hair to humidity – hair lengthens as relative humidity increasesHygrographProvides a record of humidity variations over timeElectronic hygrometerBased on changes in resistance of certain chemicals as they absorb or release water vapor to the air
21 Monitoring Water Vapor The temperature/dewpoint sensor (hygrothermometer) used in the NWS Automated Surface Observing System (ASOS)
22 Monitoring Water Vapor Sling psychrometerWick is wetted in distilled waterInstrument is ventilated by whirlingWet-bulb and dry-bulb temperatures are recordedDry bulb – actual air temperatureWater vapor vaporizes from the wick as it is whirled and evaporated cooling lowers the temp. to the wet-bulb temperatureImportant to remember – use the depression of the wet bulb on the chartThis is the difference between the wet and dry bulb temperaturesAspirated psychrometers do the same thing, but use a fan instead whirling
23 Monitoring Water Vapor The difference between the dry-bulb temperature and the wet-bulbtemperature, known as the wet bulb depression, is calibrated interms of percentage relative humidity on a psychrometric table.
24 Monitoring Water Vapor The dewpoint can be obtained from measurements of the dry-bulbtemperature and the wet-bulb depression.
25 Monitoring Water Vapor Water vapor satellite imageryIR imagery using infrared wavelengths that detect water vaporWater vapor imagery indicates presence of water vapor above 3000 m (10,000 ft) The whiter the image, the greater the moisture content of the airThis image shows moisture plumes extending from the Pacific Ocean into the central U.S. and in the southeastern U.S. from the Gulf of Mexico and Atlantic Ocean
26 How Air Becomes Saturated As relative humidity nears 100%, condensation or deposition becomes more likelyCondensation or deposition will form cloudsClouds are liquid and/or ice particlesHumidity increases when:Air is cooled; saturation vapor pressure decreases while actual vapor pressure remains constantWater vapor is added at a constant temperature; vapor pressure increases while saturation vapor pressure remains constantAs ascending saturated air (RH about 100%) expands and cools, saturation mixing ratio and actual mixing ratio decline and some water vapor is converted to water droplets or ice crystals
27 How Air Becomes Saturated Adiabatic process and lapse rates (review from Chapter 5)During an adiabatic process, no heat is exchanged between the air parcel and its environmentExpansional cooling and compressional heating of unsaturated air are referred to as adiabatic processes if no heat is exchanged with surroundingsAir cools adiabatically as it risesLower pressure with altitude allows the air to expandUnsaturated ascending air cools at 9.8° C/1000 m (5.5° F/1000 ft) and it warms at the same rate upon descent.This is called the dry adiabatic lapse rateUpon saturation, air continues to cool, but at the moist adiabatic lapse rate of 6° C/1000 m (3.3° F/1000 ft) → rate is lower because latent heat released upon condensation partially offsets cooling as parcel rises
28 Atmospheric Stability Air parcels are subject to buoyant forces caused by density differences between the surrounding air and the parcel itselfAtmospheric stability is the property of ambient air that either enhances (unstable) or suppresses (stable) vertical motion of air parcelsIn stable air, an ascending parcel becomes cooler and more dense than the surrounding airThis causes the parcel to sink back to its original altitudeIn unstable air, an ascending parcel becomes warmer and less dense than the surrounding airThis causes the parcel to continue rising
29 Stable AirNote that movement of the parcel upward means it is colder than the surrounding air, so it sinks back down to its original altitudeAlso, in movement of the parcel downward, it becomes warmer than the surrounding air, and returns to its original altitudeStable air inhibits vertical motion
30 Unstable AirNote that movement of the parcel upward means it is warmer than the surrounding air, so it continues rising.Also, in movement of the parcel downward, it becomes colder than the surrounding air, and continues descendingUnstable air enhances vertical motion
31 Atmospheric Stability SoundingsThese are the temperature profiles of the ambient air through which air parcels are movingSoundings (and hence stability) can change due to:Local radiational heating and coolingAt night, cold ground cools and stabilizes the overlying airDuring day, warm ground warms and destabilizes the overlying airAir mass advectionAir mass is stabilized as it moves over a colder surfaceAir mass is destabilized as it moves over a warmer surfaceLarge-scale ascent or descent of airSubsiding air generally becomes more stableRising air generally becomes less stable
32 Atmospheric Stability Absolute instabilityOccurs when the air temperature is dropping more rapidly with altitude than the dry adiabatic lapse rate (9.8° C/1000 m)Conditional instabilityOccurs when the air temperature is dropping with altitude more rapidly than the moist adiabatic lapse rate (6° C/1000 m), but less rapidly than the dry adiabatic lapse rateAir layer is stable for unsaturated air parcels and unstable for saturated air parcelsImplies that unsaturated air must be forced upwards in order to reach saturation
33 Atmospheric Stability Absolute stabilityAir layer is stable for both unsaturated and saturated air parcels and occurs when:Temperature of ambient air drops more slowly with altitude than moist adiabatic lapse rateTemperature does not change with altitude (isothermal)Temperature increase with altitude (inversion)Neutral air layerRising or descending parcel always has same temperature as ambient airNeither impedes nor spurs upward or downward motion of air parcels
37 Lifting Processes - Frontal Lifting Frontal uplift occurs where contrasting air masses meet – leads to expansional cooling of rising air, and possible cloud and precipitation developmentWarm front – as a cold and dry air mass retreats, the warm air advances by riding up and over the cold airThe leading edge of advancing warm air at the Earth’s surface is the warm frontCold front – cold and dry air displaces warm and humid air by sliding under it and forcing the warm air upwardsThe leading edge of advancing cold air at the Earth’s surface is the cold front
39 Lifting Processes – Convergent Lifting When surface winds converge, associated upward motion leads to expansional cooling, increasing relative humidity, and possible cloud and precipitation formationFor example, converging winds are largely responsible for cloudiness and precipitation in a low-pressure systemIn another example, converging sea breezes contribute to high frequency of thunderstorms in central Florida