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Atmospheric Moisture. Atmospheric Moisture. State Changes of Water Humidity Adiabatic Cooling What Makes Air Rise? Atmospheric Stability Condensation & Cloud Formation. Terms. Adiabatic Without the transfer of heat Exothermic Heat-releasing Endothermic Heat-absorbing. Terms.

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atmospheric moisture1
Atmospheric Moisture
  • State Changes of Water
  • Humidity
  • Adiabatic Cooling
  • What Makes Air Rise?
  • Atmospheric Stability
  • Condensation & Cloud Formation
terms
Terms
  • Adiabatic
    • Without the transfer of heat
  • Exothermic
    • Heat-releasing
  • Endothermic
    • Heat-absorbing
terms1
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)
terms2
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
State Changes of Water

PSCI 131: Atmospheric Moisture

water s state changes
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
slide8

PSCI 131: Atmospheric Moisture: Water’s State Changes

Blue arrows: exothermic Red arrows: endothermic

water s state changes1
Water’s State Changes

PSCI 131: Atmospheric Moisture

  • Why does sweating cool you off?
  • Sweat evaporates (endothermic), absorbing heat from your skin
humidity
Humidity

PSCI 131: Atmospheric Moisture

humidity1
Humidity

PSCI 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
Ways of Expressing Air’s Humidity

PSCI 131: Atmospheric Moisture: Humidity

  • Relative humidity
  • Dewpoint temperature
relative humidity
Relative Humidity

PSCI 131: Atmospheric Moisture: Humidity

  • Saturation content
    • Maximum amount of vapor an air parcel can hold
    • Controlled by air’s temperature
relative humidity1
Relative Humidity

PSCI 131: Atmospheric Moisture: Humidity

  • Vapor content
    • How much vapor the parcel is actually holding
  • RH = vapor content / saturation content
relative humidity2
Relative Humidity

PSCI 131: Atmospheric Moisture: Humidity

  • Relative humidity will change if either of the following happens:
    • Temperature changes (more common cause)
    • Vapor content changes
relative humidity3
Relative Humidity

PSCI 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%
relative humidity4
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
relative humidity5
Relative Humidity

PSCI 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
relative humidity summary
Relative Humidity: Summary

PSCI 131: Atmospheric Moisture: Humidity

  • Higher vapor content = higher RH
  • Lower temp = lower sat. content = higher RH
relative humidity summary1
Relative Humidity: Summary

PSCI 131: Atmospheric Moisture: Humidity

  • RH highest when temp is lowest
  • & vice versa
dewpoint temperature
Dewpoint Temperature

PSCI 131: Atmospheric Moisture: Humidity

  • Temp at which air parcel is saturated (100% RH)
  • More vapor in parcel (more humid) = higher dewpoint
adiabatic cooling
Adiabatic Cooling

PSCI 131: Atmospheric Moisture

adiabatic cooling1

PSCI 131: Atmospheric Moisture: Adiabatic Cooling

Adiabatic Cooling

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

adiabatic cooling rates

PSCI 131: Atmospheric Moisture : Adiabatic Cooling

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
condensation level

PSCI 131: Atmospheric Moisture: Adiabatic Cooling

Condensation Level
  • Altitude at which rising, cooling air’s temp reaches dew point and condensation begins
condensation level2

PSCI 131: Atmospheric Moisture: Adiabatic Cooling

Condensation Level
  • 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)
adiabatic lapse rates vs env lapse rate

PSCI 131: Atmospheric Moisture: Adiabatic Cooling

Adiabatic Lapse Rates vs Env Lapse Rate
  • 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
what makes air rise
What Makes Air Rise?

PSCI 131: Atmospheric Moisture

atmospheric stability
Atmospheric Stability

PSCI 131: Atmospheric Moisture

what is it

PSCI 131: Atmospheric Moisture: 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”
three combinations

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Three Combinations
  • Absolute stability
  • Absolute instability
  • Conditional instability
adiabatic lapse rates vs env lapse rate1

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Adiabatic Lapse Rates vs Env Lapse Rate
  • 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
absolute instability

PSCI 131: Atmospheric Moisture: Atmospheric Stability

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 instability2

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Instability
  • Weather effects
    • Lots of condensation
    • Heavy clouds
    • Lots of precip
    • Storms
absolute stability

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Stability
  • DAR > WAR > ELR
  • Rising parcel cools faster than surrounding air, so parcel is always colder
  • “Wants” to descend
absolute stability1

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Stability
  • Will only rise if forced to
    • Orographic lifting, frontal wedging, etc.
  • A “cold-air balloon”
absolute stability3

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Stability
  • Weather effects
    • Fair weather
    • Light to moderate clouds and precip
      • If forced above condensation level
    • No storms
conditional instability

PSCI 131: Atmospheric Moisture: Atmospheric Stability

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
conditional instability1

PSCI 131: Atmospheric Moisture: Atmospheric Stability

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
conditional instability3

PSCI 131: Atmospheric Moisture: Atmospheric Stability

Conditional Instability
  • Weather effects
    • Fair weather
    • Heavy clouds and precip/storms if forced above condensation level
condensation cloud formation
Condensation & Cloud Formation

PSCI 131: Atmospheric Moisture

what are clouds

PSCI 131: Atmospheric Moisture: Condensation & Cloud Formation

What Are Clouds?
  • Aggregates of water droplets or ice crystals
  • Classified by form and height
cloud heights basic names

PSCI 131: Atmospheric Moisture: Condensation & Cloud Formation

Cloud Heights & Basic Names
  • High: about 20,000 feet +
    • Cirrus
    • Cirrostratus
    • Cirrocumulus
cloud heights basic names1

PSCI 131: Atmospheric Moisture: Condensation & Cloud Formation

Cloud Heights & Basic Names
  • Middle: about 6,500 – 20,000 feet
    • Altostratus
    • Altocumulus
    • “Alto”: middle
cloud heights basic names2

PSCI 131: Atmospheric Moisture: Condensation & Cloud Formation

Cloud Heights & Basic Names
  • Low: below about 6,500 feet
    • Cumulus
    • Stratus
    • Stratocumulus
    • Nimbostratus
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