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Atmospheric Moisture
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Atmospheric Moisture
• State Changes of Water• Humidity• Adiabatic Cooling• What Makes Air Rise?• Atmospheric Stability
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Terms• Adiabatic
– Without the transfer of heat
• Exothermic– Heat-releasing
• Endothermic– Heat-absorbing
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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)
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Terms
• Environmental lapse rate (ELR)
– Rate at which temperature drops with increasing altitude in the troposphere
– Variable, but average = 6.5°C/km
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State Changes of Water
PSCI 131: Atmospheric Moisture
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Water’s State Changes
PSCI 131: Atmospheric Moisture
• Transfer of heat between water molecules & their surroundings
• Either endothermic or exothermic
• Heat source: reradiation of solar energy from ground
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PSCI 131: Atmospheric Moisture: Water’s State Changes
Blue arrows: exothermic Red arrows: endothermic
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Water’s State Changes
PSCI 131: Atmospheric Moisture
• Why does sweating cool you off?
• Sweat evaporates (endothermic), absorbing heat from your skin
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Humidity
PSCI 131: Atmospheric Moisture
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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
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Ways of Expressing Air’s Humidity
PSCI 131: Atmospheric Moisture: Humidity
• Relative humidity
• Dewpoint temperature
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Relative HumidityPSCI 131: Atmospheric Moisture: Humidity
• Saturation content
– Maximum amount of vapor an air parcel can hold
– Controlled by air’s temperature
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Relative HumidityPSCI 131: Atmospheric Moisture: Humidity
• Vapor content– How much vapor the parcel is actually
holding
• RH = vapor content / saturation content
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Relative HumidityPSCI 131: Atmospheric Moisture: Humidity
• Relative humidity will change if either of the following happens:
– Temperature changes (more common cause)
– Vapor content changes
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Relative HumidityPSCI 131: Atmospheric Moisture: Humidity
• Example – Parcel’s vapor content is 10g (given)– Parcel temp: 25 degrees C– Therefore, saturation content is 20g (from table)
• RH = 10g/ 20g = 50%
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Relative Humidity
PSCI 131: Atmospheric Moisture: 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
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Relative HumidityPSCI 131: Atmospheric Moisture: Humidity
• 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
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Relative Humidity: Summary
PSCI 131: Atmospheric Moisture: Humidity
• Higher vapor content = higher RH
• Lower temp = lower sat. content = higher RH
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Relative Humidity: Summary
PSCI 131: Atmospheric Moisture: Humidity
• RH highest when temp is lowest
• & vice versa
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Dewpoint Temperature
PSCI 131: Atmospheric Moisture: Humidity
• Temp at which air parcel is saturated (100% RH)
• More vapor in parcel (more humid) = higher dewpoint
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Adiabatic Cooling
PSCI 131: Atmospheric Moisture
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Adiabatic CoolingPSCI 131: Atmospheric Moisture: Adiabatic Cooling
Air parcel rises, pressure drops, air cools (no heat energy transferred).
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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
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Condensation Level
• Altitude at which rising, cooling air’s temp reaches dew point and condensation begins
PSCI 131: Atmospheric Moisture: Adiabatic Cooling
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Condensation LevelPSCI 131: Atmospheric Moisture: Adiabatic Cooling
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• 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)
PSCI 131: Atmospheric Moisture: Adiabatic Cooling
Condensation Level
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• 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
PSCI 131: Atmospheric Moisture: Adiabatic Cooling
Adiabatic Lapse Rates vs Env Lapse Rate
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What Makes Air Rise?
PSCI 131: Atmospheric Moisture
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Four Processes
• Orographic Lifting
PSCI 131: Atmospheric Moisture: What Makes Air Rise?
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Four Processes
Frontal wedging
PSCI 131: Atmospheric Moisture: What Makes Air Rise?
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Four ProcessesConvergence
PSCI 131: Atmospheric Moisture: What Makes Air Rise?
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Four Processes
Convective Lifting
PSCI 131: Atmospheric Moisture: What Makes Air Rise?
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Atmospheric Stability
PSCI 131: Atmospheric Moisture
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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
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Three Combinations
• Absolute stability
• Absolute instability
• Conditional instability
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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• 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
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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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
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Absolute Instability
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Absolute Instability• Weather effects
– Lots of condensation– Heavy clouds– Lots of precip– Storms
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Absolute Stability
•DAR > WAR > ELR
•Rising parcel cools faster than surrounding air, so parcel is always colder
•“Wants” to descend
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Absolute Stability
•Will only rise if forced to– Orographic lifting, frontal wedging, etc.
•A “cold-air balloon”
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Absolute Stability
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Absolute Stability
•Weather effects– Fair weather– Light to moderate clouds and precip
• If forced above condensation level
– No storms
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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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
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Conditional Instability
• 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
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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Conditional InstabilityPSCI 131: Atmospheric Moisture: Atmospheric Stability
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Conditional Instability
• Weather effects–Fair weather–Heavy clouds and precip/storms if forced above condensation level
PSCI 131: Atmospheric Moisture: Atmospheric Stability
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End of Chapter