Atmospheric Moisture. State Changes of Water Humidity Adiabatic Cooling What Makes Air Rise?...

Atmospheric Moisture

Atmospheric Moisture

• State Changes of Water• Humidity• Adiabatic Cooling• What Makes Air Rise?• Atmospheric Stability

Terms• Adiabatic

– Without the transfer of heat

• Exothermic– Heat-releasing

• Endothermic– Heat-absorbing

Terms• Humidity

– How much water vapor the air is holding

• Parcel– A volume of air assumed to have the same

properties throughout (temp, humidity, etc.)– Assumed to behave independently of

surrounding (ambient) air

• Environmental lapse rate (ELR)

Terms

• Environmental lapse rate (ELR)

– Rate at which temperature drops with increasing altitude in the troposphere

– Variable, but average = 6.5°C/km

State Changes of Water

PSCI 131: Atmospheric Moisture

Water’s State Changes


• Transfer of heat between water molecules & their surroundings

• Either endothermic or exothermic

• Heat source: reradiation of solar energy from ground

PSCI 131: Atmospheric Moisture: Water’s State Changes

Blue arrows: exothermic Red arrows: endothermic

Water’s State Changes


• Why does sweating cool you off?

• Sweat evaporates (endothermic), absorbing heat from your skin

Humidity


HumidityPSCI 131: Atmospheric Moisture

• Evaporation from Earth’s surface (especially oceans) adds water vapor to the air

• A given mass of air at a given temperature can only hold so much water in vapor form

• If this limit is exceeded, excess vapor condenses to liquid

Ways of Expressing Air’s Humidity

PSCI 131: Atmospheric Moisture: Humidity

• Relative humidity

• Dewpoint temperature

Relative HumidityPSCI 131: Atmospheric Moisture: Humidity

• Saturation content

– Maximum amount of vapor an air parcel can hold

– Controlled by air’s temperature


• Vapor content– How much vapor the parcel is actually

holding

• RH = vapor content / saturation content


• Relative humidity will change if either of the following happens:

– Temperature changes (more common cause)

– Vapor content changes


• Example – Parcel’s vapor content is 10g (given)– Parcel temp: 25 degrees C– Therefore, saturation content is 20g (from table)

• RH = 10g/ 20g = 50%

Relative Humidity


• Example (cont.)– Parcel’s vapor content changes to 14g– Parcel temp remains 25 degrees C– Therefore, saturation content is still 20g

• RH = 14g/ 20g = 70%

• RH has risen because vapor content has risen


• Example (cont.)– Vapor content is still 14g– Parcel cools to 20 degrees C– New saturation content is 14g

• RH = 14g/ 14g = 100%

• RH has risen because temp has fallen

• Air is saturated

Relative Humidity: Summary


• Higher vapor content = higher RH

• Lower temp = lower sat. content = higher RH

Relative Humidity: Summary


• RH highest when temp is lowest

• & vice versa

Dewpoint Temperature


• Temp at which air parcel is saturated (100% RH)

• More vapor in parcel (more humid) = higher dewpoint

Adiabatic Cooling


Adiabatic CoolingPSCI 131: Atmospheric Moisture: Adiabatic Cooling

Air parcel rises, pressure drops, air cools (no heat energy transferred).

Adiabatic Cooling Rates

• Dry adiabatic lapse rate (DAR): 10° C /km– If RH less than 100%

• Wet adiabatic lapse rate (WAR): 5-9° C /km– If RH equals100%– Depends on vapor content– Always less than DAR

PSCI 131: Atmospheric Moisture : Adiabatic Cooling

Condensation Level

• Altitude at which rising, cooling air’s temp reaches dew point and condensation begins

PSCI 131: Atmospheric Moisture: Adiabatic Cooling

Condensation LevelPSCI 131: Atmospheric Moisture: Adiabatic Cooling

• Several things are true when air has risen to its condensation level:– Air has cooled to its dewpoint– RH = 100%– Condensation can occur (usually as clouds)


Condensation Level

• Adiabatic lapse rates (WAR and DAR): rates at which air parcel cools as it rises

• Env lapse rate (ELR): rate at which temperature around parcel decreases with altitude


Adiabatic Lapse Rates vs Env Lapse Rate

What Makes Air Rise?


Four Processes

• Orographic Lifting

PSCI 131: Atmospheric Moisture: What Makes Air Rise?

Four Processes

Frontal wedging


Four ProcessesConvergence


Four Processes

Convective Lifting


Atmospheric Stability


What Is It?

• Balance between DAR, WAR, and ELR

• How likely a given air parcel is to rise, and how far up it will go

• The “weather maker”

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Three Combinations

• Absolute stability

• Absolute instability

• Conditional instability


• Adiabatic lapse rates (WAR and DAR): rates at which air parcel cools as it rises

• Env lapse rate (ELR): rate at which temperature around parcel decreases with altitude

Adiabatic Lapse Rates vs Env Lapse Rate


Absolute Instability

• ELR > DAR > WAR

• Surrounding air cools faster than parcel, so parcel is always warmer

• Parcel “wants” to keep rising

• Think of a hot-air balloon


Absolute Instability


Absolute Instability• Weather effects

– Lots of condensation– Heavy clouds– Lots of precip– Storms


Absolute Stability

•DAR > WAR > ELR

•Rising parcel cools faster than surrounding air, so parcel is always colder

•“Wants” to descend


Absolute Stability

•Will only rise if forced to– Orographic lifting, frontal wedging, etc.

•A “cold-air balloon”


Absolute Stability


Absolute Stability

•Weather effects– Fair weather– Light to moderate clouds and precip

• If forced above condensation level

– No storms


Conditional Instability

• DAR > ELR > WAR

• Rising parcel cools faster than surrounding air while RH is less than 100% and air is cooling at DAR– Parcel always cooler while below

condensation level– Must be forced to rise



• Surrounding air cools faster than parcel once RH = 100% –Parcel always warmer above condensation level

• Think of a cold-air balloon that is forced to rise to condensation level, at which it changes into a hot-air balloon


Conditional InstabilityPSCI 131: Atmospheric Moisture: Atmospheric Stability


• Weather effects–Fair weather–Heavy clouds and precip/storms if forced above condensation level


End of Chapter

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Atmospheric Moisture. State Changes of Water Humidity Adiabatic Cooling What Makes Air Rise?...

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