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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?

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Chapter 27 clouds

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
CHAPTER 6 - CLOUDS

  • How is a cloud formed?

    • An air parcel containing water vapor (gas) is cooled to the dewpoint such that the water vapor condensesto form water droplets (liquid).

= Liquid water droplets

  • When condensation occurs, the air parcel is ”saturated”(RH=100%)


Unsaturated Parcels

Generic Cloud

Saturated Parcels


Chapter 6 clouds1
CHAPTER 6 - CLOUDS

  • 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 rising parcels

6

6

6

6

Density of Surrounding Air

6

z

CHAPTER 6 - RISING PARCELS

  • 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

3

Density of Main Air Parcel


Chapter 6 rising parcels1

60

60

60

60

Temp. of Surrounding Air

60

z

CHAPTER 6 - RISING PARCELS

  • 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

70

Temp. of Main Air Parcel


CHAPTER 6 - RISING PARCELS

What happens when a parcel rises?

  • It expands

    • Finds itself in an environment with lesser density/pressure

  • It coolsadiabatically

    • 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

60 F

z

70 F


Chapter 6 rising parcels2
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 warmerthan surrounding air parcels at a given altitude, the parcel will continue to rise

    • If parcel becomes colder, it will sink



Chapter 6 case 1
CHAPTER 6 - CASE #1

  • Steep lapse rate (_________ ATMOSPHERE):

    Parcel rises to tropopause Result: Thunderstorms!

UNSTABLE

Parcel cools

...and if drops to dewpoint

Lapse Rate

(steep)

Saturation

z

COLD

Temperature

HOT


Cumulo-nimbus

(Thunderstorm)




Chapter 6 case 2
CHAPTER 6 - CASE #2

  • Steep lapse rate near ground, weak aloft:

    Parcel rises to a point, then stops Result: Cumulus

(weak)

Parcel cools

Lapse Rate

...and if drops to dewpoint

(steep)

Saturation

z

COLD

Temperature

HOT


"Cumulus"

3000 - 8000 feet


Cumulus

3000 - 8000 feet


Alto-cumulus

10000 - 20000 feet


Alto-cumulus

10000 - 20000 feet


Cirro-cumulus

20000-40000 feet


Chapter 6 case 3
CHAPTER 6 - CASE #3

  • Weak lapse rate (_________ ATMOSPHERE):

    Parcel rises only slightly Result: Stratus

STABLE

Lapse Rate

(weak)

z

COLD

Temperature

HOT


"Stratus"

0 - 8000 feet


Stratus

0 - 8000 feet


Altostratus

10000 - 20000 feet


Altostratus

10000 - 20000 feet


Cirrostratus

20000 - 40000 feet


Chapter 6 summary of clouds
CHAPTER 6 - SUMMARY OF CLOUDS

  • Cloud type reveals:

    …how quickly temperature is decreasing w/ height

  • CUMULUSSTRATUS

    Unstable AtmosphereStable Atmosphere

    High Lapse Rate Low Lapse Rate

    Deep, Vertical DevelopmentThin, Spread out


Chapter 6 summary of clouds1
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)


Cirrus

30000 feet +

Composed of Ice Crystals


Contrails

30000 feet +

Composed of Ice Crystals



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