Download Chapter 27

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
condenses to form water droplets (liquid).
= Liquid water droplets
 When condensation occurs, the air parcel is ”saturated”
(RH=100%)
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
 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
Density of
Surrounding
Air
Density of Main Air Parcel
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
Temp. of
Surrounding
Air
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 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
…at a given altitude to…
b. temp. of surrounding parcels
(i.e., the atmosphere)
which cools at a constant rate
changes on a daily basis; is measured
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 CASE #1
UNSTABLEATMOSPHERE):
 Steep lapse rate (_________
Parcel rises to tropopause
COLD
Result: Thunderstorms!
Temperature
HOT
CHAPTER 6 CASE #2
 Steep lapse rate near ground, weak aloft:
Parcel rises to a point, then stops Result: Cumulus
COLD
Temperature
HOT
CHAPTER 6 CASE #3
STABLE ATMOSPHERE):
 Weak lapse rate (_________
Parcel rises only slightly
COLD
Temperature
Result: Stratus
HOT
CHAPTER 6 SUMMARY OF CLOUDS
 Cloud type reveals:
…how quickly temperature is decreasing w/ height
 CUMULUS
Unstable Atmosphere
High Lapse Rate
Deep, Vertical Development
STRATUS
Stable Atmosphere
Low Lapse Rate
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)
CLOUD
NOMENCLATURE
Height Cloud Base
Stable
Unstable
<8000 feet
STRATUS
CUMULUS
10000-20000 feet
Add prefix “Alto-”
>20000 feet
Add prefix “Cirro-”
With rain?
Add “Nimbo-”