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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


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


Basic cloud forms

PSCI 131: Atmospheric Moisture: Condensation & Cloud Formation

Basic Cloud Forms

  • Cirrus

  • Cumulus

  • Stratus


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


Cloud heights basic names3

PSCI 131: Atmospheric Moisture: Condensation & Cloud Formation

Cloud Heights & Basic Names

  • Vertical clouds

    • Cumulonimbus (thunderheads)


Cloud heights basic names4

PSCI 131: Atmospheric Moisture: Condensation & Cloud Formation

Cloud Heights & Basic Names



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