Chapter 6Chapter 6

Humidity, Saturation, and StabilityHumidity, Saturation, and Stability

Weather StudiesWeather Studies Introduction to Atmospheric ScienceIntroduction to Atmospheric Science

American Meteorological SocietyAmerican Meteorological Society

Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

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Case-in-PointCase-in-Point Cloud forests are forests that are perpetually Cloud forests are forests that are perpetually

shrouded in clouds or mistshrouded in clouds or mist– They are found from 2000-3000 m (6500-9800 ft) in They are found from 2000-3000 m (6500-9800 ft) in

elevation in the tropics and subtropics elevation in the tropics and subtropics Onshore and upslope winds that are warm and Onshore and upslope winds that are warm and

humid supply the moisturehumid supply the moisture– Warm air blowing upslope cools through expansionWarm air blowing upslope cools through expansion– Expansional cooling raises the relative humidity to Expansional cooling raises the relative humidity to

saturation and water vapor condenses into low clouds saturation and water vapor condenses into low clouds and fogand fog

– The tree canopy strips moisture from the clouds and this The tree canopy strips moisture from the clouds and this water drips to the forest floorwater drips to the forest floor

Deforestation reduces available moisture and Deforestation reduces available moisture and raises air temperature raises air temperature → clouds form less readily → clouds form less readily and at higher elevationsand at higher elevations

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Case-in-PointCase-in-Point If global warming translates into higher sea If global warming translates into higher sea

surface temperatures (SSTs) in the tropics, surface temperatures (SSTs) in the tropics, cloud forests could be affectedcloud forests could be affected– Air flowing onshore would be warmerAir flowing onshore would be warmer– Greater ascents would be required to produce Greater ascents would be required to produce

cloudsclouds– Clouds would be at higher elevationsClouds would be at higher elevations

Perhaps even lift off the mountainsPerhaps even lift off the mountains

– Cloud forests are extremely sensitive to climate Cloud forests are extremely sensitive to climate variationsvariations They may prove to be early indicators of effects of They may prove to be early indicators of effects of

global-scale climate changeglobal-scale climate change

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Driving QuestionDriving Question How does the cycling of water in the Earth-How does the cycling of water in the Earth-

atmosphere system help maintain a atmosphere system help maintain a habitable planet?habitable planet?– This chapter will tell us:This chapter will tell us:

How the global water cycle functionsHow the global water cycle functions– Especially as it relates to transference between Especially as it relates to transference between

the Earth’s surface and the atmospherethe Earth’s surface and the atmosphere How to quantify the water content of airHow to quantify the water content of air How air becomes saturated through uplift and How air becomes saturated through uplift and

expansional coolingexpansional cooling How atmospheric stability affects the ascent of How atmospheric stability affects the ascent of

airair

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Global Water CycleGlobal Water Cycle

Assumption – the amount of water in the Earth-Assumption – the amount of water in the Earth-atmosphere system is neither increasing or atmosphere system is neither increasing or decreasingdecreasing– Internal processes continually generate and break down Internal processes continually generate and break down

water moleculeswater molecules Volcanoes and meteors (minute amount) add waterVolcanoes and meteors (minute amount) add water Photodissociation of water vapor and chemical reactions break Photodissociation of water vapor and chemical reactions break

down water moleculesdown water molecules

– Fixed quantity of water in Earth-atmosphere system is Fixed quantity of water in Earth-atmosphere system is distributed in 3 phases among various reservoirs, mostly distributed in 3 phases among various reservoirs, mostly the ocean (97.2%) and ice sheets and glaciers (2.15%)the ocean (97.2%) and ice sheets and glaciers (2.15%)

– The sun powers the global water cycle and gravity The sun powers the global water cycle and gravity keeps water from escaping to space, causing water to keeps water from escaping to space, causing water to fall from the sky as precipitation and flow to oceansfall from the sky as precipitation and flow to oceans

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The Global Water CycleThe Global Water Cycle

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Where is the Water Stored?Where is the Water Stored?

Note the small Note the small percentage of the percentage of the total water that is total water that is stored in the stored in the atmosphere.atmosphere.Even though small Even though small in percentage, this is in percentage, this is vital to weather vital to weather processesprocesses

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The Global Water CycleThe Global Water Cycle Transfer processesTransfer processes

1. Phase changes1. Phase changes Evaporation – more molecules enter the atmosphere as vapor Evaporation – more molecules enter the atmosphere as vapor

then return as liquid to the water surfacethen return as liquid to the water surface Condensation – more molecules return to the water surface as Condensation – more molecules return to the water surface as

liquid then enter the atmosphere as vapor liquid then enter the atmosphere as vapor Transpiration – Water that is taken up by plant roots escapes as Transpiration – Water that is taken up by plant roots escapes as

vapor from plant poresvapor from plant pores– Evapotranspiration is the total of evaporation and Evapotranspiration is the total of evaporation and

transpirationtranspiration Sublimation – ice or snow become vapor without first becoming Sublimation – ice or snow become vapor without first becoming

liquidliquid Deposition - water vapor becomes solid without first becoming Deposition - water vapor becomes solid without first becoming

liquidliquid All 3 phases of water exist in the atmosphereAll 3 phases of water exist in the atmosphere

2. Precipitation2. Precipitation Rain, drizzle, snow, ice pellets, and hailRain, drizzle, snow, ice pellets, and hail

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Percent of Precipitation Originating Percent of Precipitation Originating from Land Sourcesfrom Land Sources

Ocean evaporation is the origin of most precipitation.Ocean evaporation is the origin of most precipitation.

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Pathways Pathways Taken by Taken by

Precipitation Precipitation Falling on Land Falling on Land

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The Global Water BudgetThe Global Water Budget

Via precipitation and evaporation, the ocean has a netVia precipitation and evaporation, the ocean has a netloss of water and the land has a net gain.loss of water and the land has a net gain.

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How Humid is it?How Humid is it? Humidity describes the amount of water vapor in Humidity describes the amount of water vapor in

the airthe air– This varies with time of year, from day-to-day, within a This varies with time of year, from day-to-day, within a

single day, and from place-to-placesingle day, and from place-to-place– Humid summer air, and dry winter air cause discomfortHumid summer air, and dry winter air cause discomfort

Ways of measuring humidity:Ways of measuring humidity:– Vapor pressureVapor pressure– Mixing ratioMixing ratio– Specific humiditySpecific humidity– Absolute humidityAbsolute humidity– Relative humidityRelative humidity– DewpointDewpoint– Precipitable waterPrecipitable water

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How Humid is it?How Humid is it? Vapor pressureVapor pressure

– Water vapor disperses among the air molecules and Water vapor disperses among the air molecules and contributes to the total atmospheric pressurecontributes to the total atmospheric pressure This pressure component is called the vapor pressureThis pressure component is called the vapor pressure

Mixing ratioMixing ratio– Mass of water vapor per mass of the remaining dry airMass of water vapor per mass of the remaining dry air

Expressed as grams of water vapor per kilograms of dry airExpressed as grams of water vapor per kilograms of dry air

Specific humiditySpecific humidity– Mass of the water vapor (in grams) per mass of the air Mass of the water vapor (in grams) per mass of the air

containing the vapor (in kilograms)containing the vapor (in kilograms) In this case, the mass of the air includes the mass of the water In this case, the mass of the air includes the mass of the water

vaporvapor

Mixing ratio and specific humidity are so close Mixing ratio and specific humidity are so close they are usually considered equivalentthey are usually considered equivalent

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How Humid is it?How Humid is it? Absolute humidityAbsolute humidity

– The mass of the water vapor per unit volume of humid The mass of the water vapor per unit volume of humid air; normally expressed as grams of water vapor per air; normally expressed as grams of water vapor per cubic meter of aircubic meter of air

Saturated airSaturated air– This is the term given to air at its maximum humidityThis is the term given to air at its maximum humidity– A dynamic equilibrium develops where the liquid water A dynamic equilibrium develops where the liquid water

becomes vapor at the same rate as vapor becomes becomes vapor at the same rate as vapor becomes liquidliquid

– ““Saturation” may be added to various humidity termsSaturation” may be added to various humidity terms Saturation vapor pressure, saturation mixing ratio, saturation Saturation vapor pressure, saturation mixing ratio, saturation

specific humidity, saturation absolute humidityspecific humidity, saturation absolute humidity– Changing the air temperature disturbs equilibrium Changing the air temperature disturbs equilibrium

temporarilytemporarily Example: heating water increases kinetic energy of water Example: heating water increases kinetic energy of water

molecules and they more readily escape the water surface as molecules and they more readily escape the water surface as vapor. If the supply of water is sufficient, a new dynamic vapor. If the supply of water is sufficient, a new dynamic equilibrium is established with more vapor at higher temp.equilibrium is established with more vapor at higher temp.

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Variations with Air Temperature ofVariations with Air Temperature ofVapor Pressure Saturation Mixing RatioVapor Pressure Saturation Mixing Ratio

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How Humid is it?How Humid is it? Relative humidityRelative humidity

– Probably the most familiar measureProbably the most familiar measure– Compares the amount of water vapor present to the Compares the amount of water vapor present to the

amount that would be present if the air were saturatedamount that would be present if the air were saturated– Relative humidity (RH) can be computed from vapor Relative humidity (RH) can be computed from vapor

pressure or mixing ratiopressure or mixing ratio RH = [(vapor pressure)/ (saturation vapor pressure)] x 100 RH = [(vapor pressure)/ (saturation vapor pressure)] x 100 RH = [(mixing ratio)/(saturation mixing ratio)] x 100RH = [(mixing ratio)/(saturation mixing ratio)] x 100

– At constant temperature and pressure, RH varies directly At constant temperature and pressure, RH varies directly with the vapor pressure (or mixing ratio)with the vapor pressure (or mixing ratio)

– If the amount of water vapor in the air remains constant, If the amount of water vapor in the air remains constant, relative humidity varies inversely with temperature relative humidity varies inversely with temperature See next slideSee next slide

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The Relationship of Relative The Relationship of Relative Humidity to TemperatureHumidity to Temperature

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How Humid is it?How Humid is it? DewpointDewpoint

– The temperature to which the air must be cooled The temperature to which the air must be cooled at constant pressure to reach saturationat constant pressure to reach saturation At the dewpoint, air reaches 100% relative humidityAt the dewpoint, air reaches 100% relative humidity Higher with greater concentration of water vapor in airHigher with greater concentration of water vapor in air With high relative humidity, the dewpoint is closer to the With high relative humidity, the dewpoint is closer to the

current temperature than with low relative humiditycurrent temperature than with low relative humidity

– Dew is small drops of water that form on surfaces Dew is small drops of water that form on surfaces by condensation of water vaporby condensation of water vapor

– If the dewpoint is below freezing, frost may form If the dewpoint is below freezing, frost may form on the colder surfaces through depositionon the colder surfaces through deposition Dewpoints below freezing are sometimes referred to as Dewpoints below freezing are sometimes referred to as

frostpointsfrostpoints

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How Humid is it?How Humid is it?

Precipitable waterPrecipitable water– The depth of the water The depth of the water

that would be produced if that would be produced if all the water vapor in a all the water vapor in a vertical column of vertical column of condensed into liquid condensed into liquid waterwater Condensing all the water Condensing all the water

vapor in the atmosphere vapor in the atmosphere would produce a layer of would produce a layer of water covering the entire water covering the entire Earth’s surface to a depth Earth’s surface to a depth of 2.5 cm (1.0 in.)of 2.5 cm (1.0 in.)

– Highest in the tropicsHighest in the tropicsMap of precipitable waterMap of precipitable waterat various locationsat various locations

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Monitoring Water VaporMonitoring Water Vapor Humidity instrumentsHumidity instruments

– HygrometerHygrometer Measures the water vapor concentration of airMeasures the water vapor concentration of air

– Dewpoint hygrometerDewpoint hygrometer Uses a temperature-controlled mirror and an infrared beamUses a temperature-controlled mirror and an infrared beam

– When the mirror temperature reaches a point that When the mirror temperature reaches a point that condensation forms, the reflectivity of the mirror is changed, condensation forms, the reflectivity of the mirror is changed, altering the reflection of the beam. The temperature is altering the reflection of the beam. The temperature is recorded as the dewpoint.recorded as the dewpoint.

These are common at NWS forecast stationsThese are common at NWS forecast stations– Hair hygrometerHair hygrometer

Relates changes in length of a humid hair to humidity – hair Relates changes in length of a humid hair to humidity – hair lengthens as relative humidity increaseslengthens as relative humidity increases

– HygrographHygrograph Provides a record of humidity variations over timeProvides a record of humidity variations over time

– Electronic hygrometerElectronic hygrometer Based on changes in resistance of certain chemicals as they Based on changes in resistance of certain chemicals as they

absorb or release water vapor to the airabsorb or release water vapor to the air

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Monitoring Water VaporMonitoring Water Vapor

The temperature/dewpoint sensor (hygrothermometer) used in the The temperature/dewpoint sensor (hygrothermometer) used in the NWS Automated Surface Observing System (ASOS)NWS Automated Surface Observing System (ASOS)

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Sling psychrometerSling psychrometer– Wick is wetted in distilled waterWick is wetted in distilled water– Instrument is ventilated by whirlingInstrument is ventilated by whirling– Wet-bulb and dry-bulb temperatures are Wet-bulb and dry-bulb temperatures are

recordedrecorded– Dry bulb – actual air temperatureDry bulb – actual air temperature– Water vapor vaporizes from the wick as it Water vapor vaporizes from the wick as it

is whirled and evaporated cooling lowers is whirled and evaporated cooling lowers the temp. to the wet-bulb temperaturethe temp. to the wet-bulb temperature

– Important to remember – use the Important to remember – use the depression of the wet bulb on the chartdepression of the wet bulb on the chart This is the difference between the wet This is the difference between the wet

and dry bulb temperaturesand dry bulb temperatures

Aspirated psychrometers do the same Aspirated psychrometers do the same thing, but use a fan instead whirlingthing, but use a fan instead whirling

Monitoring Water VaporMonitoring Water Vapor

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The difference between the dry-bulb temperature and the wet-bulbThe difference between the dry-bulb temperature and the wet-bulbtemperature, known as the wet bulb depression, is calibrated intemperature, known as the wet bulb depression, is calibrated interms of percentage relative humidity on a psychrometric table.terms of percentage relative humidity on a psychrometric table.

Monitoring Water VaporMonitoring Water Vapor

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The dewpoint can be obtained from measurements of the dry-bulbThe dewpoint can be obtained from measurements of the dry-bulbtemperature and the wet-bulb depression.temperature and the wet-bulb depression.

Monitoring Water VaporMonitoring Water Vapor

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Monitoring Water VaporMonitoring Water Vapor Water vapor satellite Water vapor satellite

imageryimagery– IR imagery using IR imagery using

infrared wavelengths infrared wavelengths that detect water vaporthat detect water vapor

Water vapor imagery indicates presence Water vapor imagery indicates presence of water vapor above 3000 m (10,000 ft) of water vapor above 3000 m (10,000 ft) The whiter the image, the greater the The whiter the image, the greater the moisture content of the airmoisture content of the air

This image shows moisture plumes This image shows moisture plumes extending from the Pacific Ocean into the extending from the Pacific Ocean into the central U.S. and in the southeastern U.S. central U.S. and in the southeastern U.S. from the Gulf of Mexico and Atlantic from the Gulf of Mexico and Atlantic OceanOcean

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How Air Becomes SaturatedHow Air Becomes Saturated As relative humidity nears 100%, condensation or As relative humidity nears 100%, condensation or

deposition becomes more likelydeposition becomes more likely Condensation or deposition will form cloudsCondensation or deposition will form clouds

– Clouds are liquid and/or ice particlesClouds are liquid and/or ice particles Humidity increases when:Humidity increases when:

– Air is cooled; saturation vapor pressure decreases while Air is cooled; saturation vapor pressure decreases while actual vapor pressure remains constantactual vapor pressure remains constant

– Water vapor is added at a constant temperature; vapor Water vapor is added at a constant temperature; vapor pressure increases while saturation vapor pressure pressure increases while saturation vapor pressure remains constantremains constant

As ascending saturated air (RH about 100%) As ascending saturated air (RH about 100%) expands and cools, saturation mixing ratio and expands and cools, saturation mixing ratio and actual mixing ratio decline and some water vapor actual mixing ratio decline and some water vapor is converted to water droplets or ice crystalsis converted to water droplets or ice crystals

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How Air Becomes SaturatedHow Air Becomes Saturated Adiabatic process and lapse rates (review from Adiabatic process and lapse rates (review from

Chapter 5)Chapter 5)– During an adiabatic process, no heat is exchanged During an adiabatic process, no heat is exchanged

between the air parcel and its environmentbetween the air parcel and its environment– Expansional cooling and compressional heating of Expansional cooling and compressional heating of

unsaturated air are referred to as adiabatic processes if unsaturated air are referred to as adiabatic processes if no heat is exchanged with surroundingsno heat is exchanged with surroundings

– Air cools adiabatically as it risesAir cools adiabatically as it rises Lower pressure with altitude allows the air to expandLower pressure with altitude allows the air to expand Unsaturated ascending air cools at 9.8Unsaturated ascending air cools at 9.8°° C/1000 m (5.5 C/1000 m (5.5°° F/1000 ft) F/1000 ft)

and it warms at the same rate upon descent.and it warms at the same rate upon descent.– This is called the dry adiabatic lapse rateThis is called the dry adiabatic lapse rate

– Upon saturation, air continues to cool, but at the Upon saturation, air continues to cool, but at the moist adiabatic lapse rate of moist adiabatic lapse rate of 66°° C/1000 m (3.3 C/1000 m (3.3°° F/1000 F/1000 ft) ft) →→ rate is lower because latent heat released upon rate is lower because latent heat released upon condensation partially offsets cooling as parcel risescondensation partially offsets cooling as parcel rises

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Atmospheric StabilityAtmospheric Stability Air parcels are subject to buoyant forces caused Air parcels are subject to buoyant forces caused

by density differences between the surrounding air by density differences between the surrounding air and the parcel itselfand the parcel itself

Atmospheric stability is the property of ambient air Atmospheric stability is the property of ambient air that either enhances (unstable) or suppresses that either enhances (unstable) or suppresses (stable) vertical motion of air parcels(stable) vertical motion of air parcels– In stable air, an ascending parcel becomes cooler and In stable air, an ascending parcel becomes cooler and

more dense than the surrounding airmore dense than the surrounding air This causes the parcel to sink back to its original altitudeThis causes the parcel to sink back to its original altitude

– In unstable air, an ascending parcel becomes warmer In unstable air, an ascending parcel becomes warmer and less dense than the surrounding airand less dense than the surrounding air This causes the parcel to continue risingThis causes the parcel to continue rising

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Stable AirStable Air

Note that movement of the parcel upward means it is colder than the surrounding air, so it sinks back down to its original altitude

Also, in movement of the parcel downward, it becomes warmer than the surrounding air, and returns to its original altitude

Stable air inhibits vertical Stable air inhibits vertical motionmotion

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Unstable AirUnstable Air

Note 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 descending

Unstable air enhances vertical motion

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Atmospheric StabilityAtmospheric Stability SoundingsSoundings

– These are the temperature profiles of the ambient air These are the temperature profiles of the ambient air through which air parcels are movingthrough which air parcels are moving

– Soundings (and hence stability) can change due to:Soundings (and hence stability) can change due to: Local radiational heating and coolingLocal radiational heating and cooling

– At night, cold ground cools and stabilizes the overlying airAt night, cold ground cools and stabilizes the overlying air

– During day, warm ground warms and destabilizes the overlying airDuring day, warm ground warms and destabilizes the overlying air

Air mass advectionAir mass advection– Air mass is stabilized as it moves over a colder surfaceAir mass is stabilized as it moves over a colder surface

– Air mass is destabilized as it moves over a warmer surfaceAir mass is destabilized as it moves over a warmer surface

Large-scale ascent or descent of airLarge-scale ascent or descent of air– Subsiding air generally becomes more stableSubsiding air generally becomes more stable

– Rising air generally becomes less stableRising air generally becomes less stable

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Atmospheric StabilityAtmospheric Stability Absolute instabilityAbsolute instability

– Occurs when the air temperature is dropping more Occurs when the air temperature is dropping more rapidly with altitude than the dry adiabatic lapse rate rapidly with altitude than the dry adiabatic lapse rate (9.8(9.8°° C/1000 m) C/1000 m)

Conditional instabilityConditional instability– Occurs when the air temperature is dropping with Occurs when the air temperature is dropping with

altitude more rapidly than the moist adiabatic lapse rate altitude more rapidly than the moist adiabatic lapse rate (6(6°° C/1000 m), but less rapidly than the dry adiabatic C/1000 m), but less rapidly than the dry adiabatic lapse ratelapse rate

– Air layer is stable for unsaturated air parcels and Air layer is stable for unsaturated air parcels and unstable for saturated air parcelsunstable for saturated air parcels

– Implies that unsaturated air must be forced upwards in Implies that unsaturated air must be forced upwards in order to reach saturationorder to reach saturation

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Atmospheric StabilityAtmospheric Stability Absolute stabilityAbsolute stability

– Air layer is stable for both unsaturated and saturated air Air layer is stable for both unsaturated and saturated air parcels and occurs when:parcels and occurs when: Temperature of ambient air drops more slowly with altitude than Temperature of ambient air drops more slowly with altitude than

moist adiabatic lapse ratemoist adiabatic lapse rate Temperature does not change with altitude (isothermal)Temperature does not change with altitude (isothermal) Temperature increase with altitude (inversion)Temperature increase with altitude (inversion)

Neutral air layerNeutral air layer– Rising or descending parcel always has same Rising or descending parcel always has same

temperature as ambient airtemperature as ambient air– Neither impedes nor spurs upward or downward motion Neither impedes nor spurs upward or downward motion

of air parcelsof air parcels

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Atmospheric StabilityAtmospheric Stability

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StStüve Diagramsüve Diagrams

Temperature – Horizontal axis, increasing from left to rightTemperature – Horizontal axis, increasing from left to rightPressure – vertical axis, decreasing upwardPressure – vertical axis, decreasing upward

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Lifting Processes - Convective LiftingLifting Processes - Convective Lifting

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Lifting Processes - Frontal LiftingLifting Processes - Frontal Lifting

Frontal uplift occurs where contrasting air masses Frontal uplift occurs where contrasting air masses meet – leads to expansional cooling of rising air, meet – leads to expansional cooling of rising air, and possible cloud and precipitation developmentand possible cloud and precipitation development

Warm front – as a cold and dry air mass retreats, Warm front – as a cold and dry air mass retreats, the warm air advances by riding up and over the the warm air advances by riding up and over the cold aircold air– The leading edge of advancing warm air at the Earth’s The leading edge of advancing warm air at the Earth’s

surface is the warm frontsurface is the warm front Cold front – cold and dry air displaces warm and Cold front – cold and dry air displaces warm and

humid air by sliding under it and forcing the warm humid air by sliding under it and forcing the warm air upwardsair upwards– The leading edge of advancing cold air at the Earth’s The leading edge of advancing cold air at the Earth’s

surface is the cold frontsurface is the cold front

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Lifting Processes – Lifting Processes – Orographic LiftingOrographic Lifting

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Lifting Processes – Lifting Processes – Convergent LiftingConvergent Lifting

When surface winds converge, associated upward When surface winds converge, associated upward motion leads to expansional cooling, increasing motion leads to expansional cooling, increasing relative humidity, and possible cloud and relative humidity, and possible cloud and precipitation formationprecipitation formation

For example, converging winds are largely For example, converging winds are largely responsible for cloudiness and precipitation in a responsible for cloudiness and precipitation in a low-pressure systemlow-pressure system

In another example, converging sea breezes In another example, converging sea breezes contribute to high frequency of thunderstorms in contribute to high frequency of thunderstorms in central Floridacentral Florida