Moisture and Atmospheric Stability … and Instability How does is relate to cloud development and precipitation?

Moisture and Atmospheric Moisture and Atmospheric Stability … and InstabilityStability … and Instability

How does is relate to cloudHow does is relate to cloud

development and development and precipitation?precipitation?

Water: A Unique Water: A Unique Substance!Substance!

• What is Water:What is Water:– Water is the only liquid on the surface of Water is the only liquid on the surface of

the Earth in large quantitiesthe Earth in large quantities– It exists in all forms on EarthIt exists in all forms on Earth– Ice (solid state) is less dense than liquidIce (solid state) is less dense than liquid

(which is why icebergs ‘float’)(which is why icebergs ‘float’)

Ice Forms on top of Ice Forms on top of Water because it is less Water because it is less

DenseDense

Moisture in the AtmosphereMoisture in the Atmosphere

• Water is part of a distinct system called Water is part of a distinct system called the the hydrological cyclehydrological cycle

• Water is removed from the surface & Water is removed from the surface & into the atmosphere by two processes:into the atmosphere by two processes:– EvaporationEvaporation: Water removed off a free : Water removed off a free

water surface, like a lake, river, ocean, or water surface, like a lake, river, ocean, or even soileven soil

– TranspirationTranspiration: Water released into the air : Water released into the air by the stomata in leavesby the stomata in leaves

Hydrologic CycleHydrologic Cyclehttp://www.youtube.com/watch?v=0_c0ZzZfC8c&feature=related

http://www.youtube.com/watch?v=0_c0ZzZfC8c&feature=related

http://www.youtube.com/watch?v=0_c0ZzZfC8c&feature=related

Terms to remember!Terms to remember!

• EvaporationEvaporation - liquid to gas - liquid to gas

• CondensationCondensation - gas to liquid - gas to liquid

• PrecipitationPrecipitation - liquid/solid falling to - liquid/solid falling to groundground

• SublimationSublimation - solid to gas - solid to gas

• DepositionDeposition – gas to solid – gas to solid

• Transpiration Transpiration - gas released by plants- gas released by plants

• Runoff/InfiltrationRunoff/Infiltration - precipitation soaking - precipitation soaking

into soilinto soil

‘‘Phases’ of WaterPhases’ of Water

Solid (ice):Solid (ice): Water molecules are arranged Water molecules are arranged and bonded together in a lattice network and bonded together in a lattice network and are fixed in placeand are fixed in place

Liquid (water):Liquid (water): Water molecules are Water molecules are loosely bonded together but are able to loosely bonded together but are able to move around freelymove around freely

Gas (water vapor):Gas (water vapor): Water molecules are Water molecules are not bonded to each other and are freely not bonded to each other and are freely flying around in the atmosphereflying around in the atmosphere

Change of Phase of WaterChange of Phase of Water Very Important Concept!Very Important Concept!

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Phase Changes of WaterPhase Changes of Water• MeltingMelting: Going from a solid to a liquid: Going from a solid to a liquid

HEAT IS ABSORBED BY THE SOLID (ICE) BUT HEAT IS ABSORBED BY THE SOLID (ICE) BUT REMOVED FROM THE AIR AROUND IT!REMOVED FROM THE AIR AROUND IT!

When an Ice Cube When an Ice Cube melts it cools the melts it cools the environment environment slightly as it takes slightly as it takes Heat from the Heat from the atmosphere in atmosphere in order to melt the order to melt the ice. The ice. The surrounding air surrounding air becomes cooler becomes cooler during this processduring this process

Change of Phase of WaterChange of Phase of Water Very Important Concept!Very Important Concept!

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Phase Changes of WaterPhase Changes of Water

• Melting: Going from a solid to a liquidMelting: Going from a solid to a liquid

As the Ice ABSORBS Heat and takes it As the Ice ABSORBS Heat and takes it from from

the Atmosphere (thus cooling the air!)the Atmosphere (thus cooling the air!)

• EvaporationEvaporation: Going from a liquid to a gas also: Going from a liquid to a gas also

takes heat from the atmosphere which takes heat from the atmosphere which

results in a cooling effectresults in a cooling effect

For example: As standing water/puddles on For example: As standing water/puddles on streets and sidewalks dries up during a sunny streets and sidewalks dries up during a sunny morning, it slows the normal rise inmorning, it slows the normal rise in

Change of Phase of WaterChange of Phase of Water Very Important Concept Very Important Concept

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• Melting: Going from a solid to a liquidMelting: Going from a solid to a liquid

• Evaporation: Going from a liquid to a gasEvaporation: Going from a liquid to a gas

• SublimationSublimation: : Going directly from a solid to a Going directly from a solid to a gas gas withoutwithout entering the liquid phase entering the liquid phase ALSO ALSO TAKES heat energy from the atmosphere – TAKES heat energy from the atmosphere – thus cooling the airthus cooling the air

Example – Snow melting on a sunny day Example – Snow melting on a sunny day

often goes directly from a solid to water often goes directly from a solid to water

vapor statevapor state

Phase Changes of WaterPhase Changes of Water• Melting: Going from a solid to a liquidMelting: Going from a solid to a liquid

• Evaporation: Going from a liquid to a gasEvaporation: Going from a liquid to a gas

• Sublimation: Going directly from a solid to a gas Sublimation: Going directly from a solid to a gas withoutwithout entering the liquid phase entering the liquid phase

ALL OF THE ABOVE: ALL OF THE ABOVE: AS HEAT IS ABSORBED AS HEAT IS ABSORBED BY BY SOLID ICE, OR WHEN LIQUID WATER IS REMOVED SOLID ICE, OR WHEN LIQUID WATER IS REMOVED BY EVAPORATION, THE BY EVAPORATION, THE ATMOSPHERIC ATMOSPHERIC ENVIRONMENT COOLS SLIGHTLYENVIRONMENT COOLS SLIGHTLY

SNOW MELTING AS IT FALLS THROUGH THE ATMOSPHERE SNOW MELTING AS IT FALLS THROUGH THE ATMOSPHERE WHEN THE AIR TEMPERATURE IS ABOVE FREEZING WILL WHEN THE AIR TEMPERATURE IS ABOVE FREEZING WILL CAUSE THE AIR TEMPERATURE TO FALL. THAT’S WHY EVEN CAUSE THE AIR TEMPERATURE TO FALL. THAT’S WHY EVEN WITH AN INITIAL TEMPERATURE OF WITH AN INITIAL TEMPERATURE OF 40°, SNOW CAN FALL AND 40°, SNOW CAN FALL AND WITHIN AN HOUR, THE TEMPERATURE CAN DROP TO NEAR WITHIN AN HOUR, THE TEMPERATURE CAN DROP TO NEAR FREEZING!FREEZING!

Change of Phase of WaterChange of Phase of Water Very Important ConceptVery Important Concept

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• DepositionDeposition: Going directly from a : Going directly from a gas to a gas to a solidsolid

HEAT IS RELEASED INTO THE HEAT IS RELEASED INTO THE ATMOSPHERE ATMOSPHERE

FROST will form on a sub-freezing FROST will form on a sub-freezing surface when there is a high level of surface when there is a high level of moisture in the lower level of the moisture in the lower level of the atmosphere (RELATIVE humidity close to atmosphere (RELATIVE humidity close to 100%)100%)

Phase Changes of WaterPhase Changes of Water• DepositionDeposition: Going directly from a : Going directly from a gas to a solidgas to a solid

- - Water vapor freezing into ice crystals in a cloud Water vapor freezing into ice crystals in a cloud - Frost forming in a freezer is an example of deposition- Frost forming in a freezer is an example of deposition

- Frost forming on grass/roads on a cold morning - Frost forming on grass/roads on a cold morning


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• Deposition: Going directly from a gas to a Deposition: Going directly from a gas to a solidsolid

• CondensationCondensation: Going from a : Going from a gas to a liquid gas to a liquid releasing Heat Energyreleasing Heat Energy

EXAMPLE: WATER VAPOR IN THE ATMOSPHERE EXAMPLE: WATER VAPOR IN THE ATMOSPHERE CONDENSES INTO SMALL WATER DROPLETS WHICH CONDENSES INTO SMALL WATER DROPLETS WHICH FORM CLOUDS. HEAT ENERGY FROM THE GAS FORM CLOUDS. HEAT ENERGY FROM THE GAS REMAINS WITHIN THE CLOUD, THUS WARMING THE REMAINS WITHIN THE CLOUD, THUS WARMING THE ATMOSPHEREATMOSPHERE


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• Deposition: Going directly from a gas to a Deposition: Going directly from a gas to a solidsolid

• Condensation: Going from a gas to a liquidCondensation: Going from a gas to a liquid releasing Heat Energyreleasing Heat Energy

• Freezing:Freezing: Going from a liquid to a solid will Going from a liquid to a solid will releaserelease

heat energy into the atmosphereheat energy into the atmosphere

THE HEAT ENERGY RELEASED or THE HEAT ENERGY RELEASED or GAINED DURING A GAINED DURING A CHANGE OF CHANGE OF

PHASEPHASE IS A ‘HIDDEN’ HEAT THAT IS A ‘HIDDEN’ HEAT THAT IS CALLED:IS CALLED:

LATENT HEATLATENT HEATTHIS IS THIS IS EXTREMELY EXTREMELY IMPORTANT FOR IMPORTANT FOR THE DEVELOPMENT OF CLOUDS AND THE DEVELOPMENT OF CLOUDS AND

EVENTUALLY PRECIPITATION AND EVENTUALLY PRECIPITATION AND PLAYS A MAJOR ROLE IN THE PLAYS A MAJOR ROLE IN THE

INTENSIFICATION OF STORM SYSTEMS!INTENSIFICATION OF STORM SYSTEMS!

Latent HeatLatent Heat

• Latent heatLatent heat: : Adding heat to melt iceAdding heat to melt ice does does notnot result in a result in a

temperature change temperature change BUT:BUT:– Melting Melting 1 gram of ice requires 80 calories 1 gram of ice requires 80 calories

ofof

heat energy (taken from the heat energy (taken from the atmosphere)atmosphere)

•Latent heat of meltingLatent heat of melting

- Freezing Freezing 1 gram of water 1 gram of water releases releases 80 80 caloriescalories

of of heat energy into the atmosphereheat energy into the atmosphere


• Latent heat Latent heat is also involved with is also involved with evaporationevaporation, , the process of converting from a liquid to a gasthe process of converting from a liquid to a gas – The The latent heat of latent heat of vaporizationvaporization is the energy is the energy

absorbedabsorbed by the water during evaporation. by the water during evaporation.•~ ~ 600600 calories/gram required for water! calories/gram required for water!

– Evaporation cools the airEvaporation cools the air as all that as all that heat heat energy is taken from the airenergy is taken from the air to vaporize the to vaporize the liquid water!liquid water!


• CondensationCondensation is the reverse process,is the reverse process, converting a converting a gas to a liquidgas to a liquid– Water vapor changes from to a liquid state and Water vapor changes from to a liquid state and

releases latent heat to the atmosphere –releases latent heat to the atmosphere –

~ ~ 600600 calories/gram of heat energy calories/gram of heat energy

– The The energy released warms the surrounding air energy released warms the surrounding air and helps drive storm intensification!and helps drive storm intensification!

Phase Change Summary

WE WILL TALK MORE ABOUT WE WILL TALK MORE ABOUT CONDENSATION AND HOW IT CONDENSATION AND HOW IT

RELATES TO CLOUD RELATES TO CLOUD DEVELOPMENT AFTER WE DEVELOPMENT AFTER WE

DISCUSS HUMIDITYDISCUSS HUMIDITYand the REAL difference and the REAL difference

between moist and dry air!between moist and dry air!

Humidity: Water Vapor in the AirHumidity: Water Vapor in the Air

• HumidityHumidity is the amount of water vapor in the air. is the amount of water vapor in the air.– Absolute humidityAbsolute humidity is the actual mass or amount of is the actual mass or amount of

water vaporwater vapor in a given volume of air in a given volume of air

– Relative humidity Relative humidity indicates how close air is to indicates how close air is to saturation saturation rather than the actual quantity of water rather than the actual quantity of water vapor in the air vapor in the air ** Warmer air holds more moisture ** Warmer air holds more moisture than colder air **than colder air **

– Dew pointDew point is the temperature to which air needs to be is the temperature to which air needs to be cooled to reach saturatiocooled to reach saturation. n. When the air temperature When the air temperature drops to the dewpoint, the air is saturated and the drops to the dewpoint, the air is saturated and the relative humidity reaches 100%relative humidity reaches 100%

Changes in Changes in Relative Humidity Relative Humidity with Temperaturewith Temperature

Assuming there is no change in the actual water content of the atmosphere, as temperatures rise through the day, the relative humidity falls with no change in the absolute amount of water vapor in the air

As the temperature rises…

there is a drop in the relative humidity

Amount of Water Vapor Required for Amount of Water Vapor Required for Saturation Saturation

Rises with TemperatureRises with Temperature

With an Air With an Air Temp of 15°C Temp of 15°C saturationsaturationis reached with is reached with 10g/Kg of water 10g/Kg of water

With an Air Temp With an Air Temp of 25°C of 25°C saturationsaturationis reached with is reached with 20g/Kg of water 20g/Kg of water

Dew-Point TemperaturesOn a typical spring day

Very Humid!Very Humid!

VERY DRYVERY DRY

Relative Humidity Relative Humidity the bottom line…the bottom line…

• How relative humidity changes:How relative humidity changes:– If water vapor is added, relative humidity If water vapor is added, relative humidity

goes up.goes up.– If water vapor is removed, relative If water vapor is removed, relative

humidity goes down.humidity goes down.– 100% relative humidity equals saturation100% relative humidity equals saturation– A decrease in temperature equals an A decrease in temperature equals an

increase in relative humidity.increase in relative humidity.

CONSIDER THE ATMOSPHERECONSIDER THE ATMOSPHERE

WITH ONLY THE PERMANENT GASESWITH ONLY THE PERMANENT GASES

AND NO WATER VAPORAND NO WATER VAPOR

Is Humid Air Heavier than Dry Is Humid Air Heavier than Dry Air?Air?

• Water vapor is Water vapor is more buoyantmore buoyant than dry air – BUT WHY? than dry air – BUT WHY?

• The chemical make-up of a parcel of dry air is comprised The chemical make-up of a parcel of dry air is comprised of mainly of Nof mainly of N 2 2 and 0 and 0 2 2, (78% and 21%), (78% and 21%)

• Nitrogen has a molecular weight of 28 x (78%) = Nitrogen has a molecular weight of 28 x (78%) = 2222

• Oxygen has a molecular weight of 32 x (21%) = Oxygen has a molecular weight of 32 x (21%) = 77

• Therefore Therefore molecular weight of dry air = 22+7 = 29molecular weight of dry air = 22+7 = 29

• HH220 molecular weight: 0 molecular weight:

HH2 2 molecular weight = 1 x 2 = 2 molecular weight = 1 x 2 = 2

O molecular weight = 16 O molecular weight = 16

• Therefore Therefore molecular weight of water = 2+16 = 18molecular weight of water = 2+16 = 18


• Water VaporWater Vapor has a molecular weight of has a molecular weight of 1818• Dry Air Dry Air has a molecular weight of has a molecular weight of 2929

18 IS < than 2918 IS < than 29So So Water vapor is Water vapor is more buoyant more buoyant than than

dry air!dry air! Therefore, when there is water vapor in Therefore, when there is water vapor in

the atmosphere, it becomes buoyant the atmosphere, it becomes buoyant and thus begins to rise!and thus begins to rise!


Lets dig Deeper into the Lets dig Deeper into the TWO things that make up TWO things that make up

HUMIDITYHUMIDITY

Temperature and DewpointTemperature and Dewpoint

Temperature vs DewpointTemperature vs Dewpoint

• Temperature is the Temperature is the average kinetic average kinetic energy of the energy of the molecules of the molecules of the atmosphere. It atmosphere. It measures the measures the degree of degree of heatheat in in the airthe air

• Dewpoint is the Dewpoint is the actual amount of actual amount of moisture in the moisture in the atmosphere. It atmosphere. It measures the measures the degree of degree of moisturemoisture in the airin the air

Water Vapor in the Air

For the purposes of this discussion, we will often refer to a ‘parcel’ of air – best thought of as a ‘balloon’ filled with moist air

As the parcel of air moves up or down, the moist air does NOT mix with the surrounding environment

AS YOU AS YOU MOVE UP MOVE UP FROM THE SURFACE OF FROM THE SURFACE OF THE EARTH THE TEMPERATURE DECREASES THE EARTH THE TEMPERATURE DECREASES IN THE TROPOSPHERE…..IN THE TROPOSPHERE…..SO HOW DOES SO HOW DOES THAT AFFECT THAT AFFECT THE TEMPERATURE OF THE THE TEMPERATURE OF THE RISING AIR PARCELRISING AIR PARCEL??

What actually happensWhat actually happens within an enclosed within an enclosed ““air air parcelparcel”” Because the

atmosphere is NORMALLY cooler as you rise in altitude, the temperature of the parcel should also decrease as it rises – but why?

70%

85%

100%

HOW DOES LOWERINGHOW DOES LOWERING PRESSURE HELP CLOUDS PRESSURE HELP CLOUDS DEVELOP? DEVELOP?

RECALL THAT AIR PRESSURE LOWERS AS THE ALTITUDE RECALL THAT AIR PRESSURE LOWERS AS THE ALTITUDE INCREASES. THIS ALLOWS AIR ‘PARCELS’ THAT RISE FROM INCREASES. THIS ALLOWS AIR ‘PARCELS’ THAT RISE FROM

THE SURFACE TO THE SURFACE TO INITIALLY INITIALLY ““EXPANDEXPAND””

Time to expand our minds!Time to expand our minds!

• In physics, work is In physics, work is defined as a force defined as a force being applied over a being applied over a distancedistance


• In physics, work is In physics, work is defined as a force being defined as a force being applied over a distanceapplied over a distance

• In order to do work, In order to do work, energy must be energy must be expendedexpended

• When our parcel expands When our parcel expands while being lifted, it is while being lifted, it is doing work by pushing doing work by pushing outout against the against the environment a certain environment a certain distancedistance


• Therefore, the parcel must use Therefore, the parcel must use up energy in order to expand.up energy in order to expand.


• Therefore, the parcel Therefore, the parcel must use up energy in must use up energy in order to expand.order to expand.

• The energy that is The energy that is available to be used up available to be used up is the is the kinetic energy kinetic energy of the moleculesof the molecules

• Otherwise known as:Otherwise known as: TEMPERATURE !TEMPERATURE !


• Therefore, the parcel must Therefore, the parcel must use up energy in order to use up energy in order to expand.expand.

• The energy that is The energy that is available to be used up is available to be used up is the the kinetic energy of kinetic energy of the moleculesthe molecules..

• Otherwise known as:Otherwise known as: TEMPERATURE !TEMPERATURE !

Therefore, the Therefore, the Temperature in a Temperature in a parcel also decreases parcel also decreases as it rises in the as it rises in the atmosphere.atmosphere.

Finally, Lets Talk About Finally, Lets Talk About Stability and InstabilityStability and Instability

Specifically, this refers to the Specifically, this refers to the temperature profile of the temperature profile of the

atmosphereatmosphere

Air that is Warmer than Air that is Warmer than itsits

Surroundings RisesSurroundings Rises

It’s said to be more It’s said to be more buoyant …buoyant …

Generally Speaking…Generally Speaking…

Instability: Warm air at the surface and cold air above Instability: Warm air at the surface and cold air above allows air parcels that are warm at the surface to rise…allows air parcels that are warm at the surface to rise…

Stability: Cool air at the surface but ‘warmer’ air above. Stability: Cool air at the surface but ‘warmer’ air above. This inhibits surface parcels from rising and normally This inhibits surface parcels from rising and normally forces the parcel to return to lower atmosphereforces the parcel to return to lower atmosphere

Adiabatic Temperature Adiabatic Temperature ChangesChanges

Air Cools at Dry Adiabatic Rate until Reaching Dew PointThen Cools at Wet Adiabatic Rate

Rising AirCooler thanEnvironmentTends Toward

Stability

Stable air does not allow warm air parcels to rise.

Unstable air allows warm air parcels to rise.

Rising Air that is warmer than its environment will continue to riseand the air is said to be unstable

Air that is not saturated will rise and cool at the ‘dry adiabatic’ Air that is not saturated will rise and cool at the ‘dry adiabatic’ rate of about 5°F/1,000 ft. Once it cools to the dewpoint rate of about 5°F/1,000 ft. Once it cools to the dewpoint (saturation) the air parcel will cool at the ‘wet adiabatic’ rate of (saturation) the air parcel will cool at the ‘wet adiabatic’ rate of 3°/1,000’. Remember, when saturation occurs, so does 3°/1,000’. Remember, when saturation occurs, so does condensation - and that releases heat which warms’ the condensation - and that releases heat which warms’ the parcel.parcel.

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Moisture and Atmospheric Stability … and Instability How does is relate to cloud development and precipitation?