Transcript 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-”