chapter 27 - clouds
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Chapter 27 - Clouds. Use of Power Point design and animations with permission from Dr. Joby Hilliker ( West Chester University, West Chester, PA. CHAPTER 6 - CLOUDS. How is a cloud formed? - PowerPoint PPT PresentationTRANSCRIPT
Chapter 27 - Clouds
Use of Power Point design and animations with permission from Dr. Joby Hilliker
( West Chester University, West Chester, PA
How is a cloud formed? An air parcel containing water vapor (gas) is cooled to the dewpoint
such that the water vapor condenses to form water droplets (liquid).
CHAPTER 6 - CLOUDS
= Liquid water droplets
When condensation occurs, the air parcel is ”saturated” (RH=100%)
When condensation occurs, the air parcel is ”saturated” (RH=100%)
The temp. at which saturation (and condensation) will occur is the dewpoint The farther the dewpoint is below the air
temp, the lower the RH, and the less likely clouds will form.
CHAPTER 6 - CLOUDS
How can an air parcel cool to the dewpoint? Get it to rise!
How can this happen? Density of parcel needs to be less than surrounding air
CHAPTER 6 - RISING PARCELS
Density of Main Air Parcel
Density of Surrounding
Air
Translate density into temperature: Warmer air is less dense; Colder air is more dense
A parcel will rise if it’s warmer than surrounding air. “Warm air rises” concept
CHAPTER 6 - RISING PARCELS
Temp. of Main Air Parcel
Temp. of Surrounding
Air
What happens when a parcel rises? It expands
Finds itself in an environment with lesser density/pressure
It cools adiabatically Takes work (thermal energy) to expand parcel adiabatic = no heat exchange between parcel and environment parcels cools at the dry adiabatic lapse rate (DALR), which is
5.5°F / 1000 feet
CHAPTER 6 - RISING PARCELS
To determine if a parcel will rise, compare: a. the parcel’s temperature which cools at a constant rate
…at a given altitude to…b. temp. of surrounding parcels changes on a daily basis; is measured (i.e., the atmosphere)
RULE #1: As long as the rising parcel is warmer than surrounding air
parcels at a given altitude, the parcel will continue to rise If parcel becomes colder, it will sink
CHAPTER 6 - RISING PARCELS
Steep lapse rate (_________ ATMOSPHERE): Parcel rises to tropopause Result: Thunderstorms!
CHAPTER 6 - CASE #1
Temperature HOTCOLD
UNSTABLE
Steep lapse rate near ground, weak aloft: Parcel rises to a point, then stops Result: Cumulus
CHAPTER 6 - CASE #2
Temperature HOTCOLD
Weak lapse rate (_________ ATMOSPHERE): Parcel rises only slightly Result: Stratus
CHAPTER 6 - CASE #3
STABLE
Temperature HOTCOLD
Cloud type reveals: …how quickly temperature is decreasing w/ height
CUMULUS STRATUSUnstable Atmosphere Stable AtmosphereHigh Lapse Rate Low Lapse RateDeep, Vertical Development Thin, Spread out
CHAPTER 6 - SUMMARY OF CLOUDS
If parcel can rise: Cloud Top = where parcel is no
longer warmer than surrounding air Cloud Base = where saturation
occurs
What could I infer about atmosphere if it is CLEAR? Sinking air Lack of moisture (low dewpoint)
CHAPTER 6 - SUMMARY OF CLOUDS
Height Cloud Base Stable Unstable
<8000 feet STRATUS CUMULUS
10000-20000 feet Add prefix “Alto-”
>20000 feet Add prefix “Cirro-”
With rain? Add “Nimbo-”
CLOUD NOMENCLATURE