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# Chapter 27 - Clouds - PowerPoint PPT Presentation

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

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?

• 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%)

Generic Cloud

Saturated Parcels

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.

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

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

• 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 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

• 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

(Thunderstorm)

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

3000 - 8000 feet

3000 - 8000 feet

10000 - 20000 feet

10000 - 20000 feet

20000-40000 feet

CHAPTER 6 - CASE #3

• Weak lapse rate (_________ ATMOSPHERE):

Parcel rises only slightly Result: Stratus

STABLE

Lapse Rate

(weak)

z

COLD

Temperature

HOT

0 - 8000 feet

0 - 8000 feet

10000 - 20000 feet

10000 - 20000 feet

20000 - 40000 feet

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

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)

30000 feet +

Composed of Ice Crystals

30000 feet +

Composed of Ice Crystals