# PPT

```EL
14
CAPE
12
HEIGHT (km)
10
8
Te
6
Tp
LFC
4
2
LCL
0
-60
-50
-40
-30
-20
-10
TEMPERATURE (oC)
0
10
20
30
Next Week: QUIZ
• One question from each of week:
– 9 normal lectures + global warming lecture
– Over main topic of lecture and homework
• Multiple choice, short answer, matching, map
question
• Powerpoints:
http://www.aos.wisc.edu/~ahulme/aos101/
AOS 101
Thickness and Thermal Wind
April 15/17
Thickness
• The vertical
distance in meters
between two
pressure levels
Z  z( p1 )  z( p2 )
500 hPa = 5600 m
THICKNESS
= 5600 m – 0 m
= 5600 m
Z
1000 hPa = 0 m
Consider a column…
• Cool the average
temperature of the
column by 20 K
• Air becomes more
dense, mass stays the
same so volume must
decrease
• Air takes up less
space
• COLUMN SHRINKS
500 hPa = 5600 m
COOL
1000 hPa = 0 m
Consider a column…
• Cool the average
temperature of the
column by 20 K
• Air becomes more
dense, mass stays the
same so volume must
decrease
• Air takes up less
space
• COLUMN SHRINKS
500 hPa = 5000 m
COOL
Z = 5000 m
1000 hPa = 0 m
Consider a column…
• Warm the average
temperature of the
column by 20 K
• Air becomes less
dense, mass stays the
same so volume must
increase
• Air takes up more
space
• COLUMN EXPANDS
500 hPa = 5600 m
WARM
1000 hPa = 0 m
Consider a column…
500 hPa = 6200 m
• Warm the average
temperature of the
column by 20 K
• Air becomes less
dense, mass stays the
same so volume must
increase
• Air takes up more
space
• COLUMN EXPANDS
Z = 6200 m
WARM
1000 hPa = 0 m
Summary
• COOL air will result in LOW THICKNESS
• WARM air will result in HIGH THICKNESS
• Thus, the thickness between two pressure
layers is proportional to the average
temperature of that layer
Z ≈ const x Tave
Thermal Wind
• Not an actual wind
• “Blows” along thickness contours with cold (low
thickness) air to the left
• Stronger temperature gradients imply stronger
thermal wind
• Equal to the SHEAR of the wind (i.e. is related
to the observed wind)



VT  VUPPER  VLOWER
VT
V200
V850
COLD
5540 m
VT
5600 m
5660 m
WARM
Veering
Clockwise turning of winds
with height
Backing
Counterclockwise turning of
winds with height
VT
300 hPa
850 hPa
850 hPa
300 hPa
VT
Midlatitude Weather
• Upper-level winds will be much stronger
than low-level winds
– i.e. thermal wind will be very close to upperlevel wind
• Consider a front with cold air to the north
and warm air to the south.
Geostrophic wind into page
Thermal wind into page
PGF
LOW heights
P = 500 hPa
HIGH heights
P = 700 hPa
COOL
WARM
NORTH
Thermal Wind Balance
• Pressure gradient increases with height
– Winds increase with height
• Thus, areas of strong temperature
winds above them.
700 hPa Temperature
500-850 hPa Thickness
500 hPa Height
500 hPa Wind Speed
500-850 hPa Thickness
500 hPa Wind Speed
Cyclone
• Symbols:
• Point in
direction of
front
movement
COLD
WARM
OCCLUDED
STATIONARY
Warm Front
WARM
COOL
Associated Weather (WF)
• Stratiform rain
– long lasting light rain
– occurs on cool side of front
• Temperature increases prior to frontal
passage
• Wind becomes southerly after passage
Cold Front
WARM
COOL
Associated Weather (CF)
• Much Steeper Slope
• More intense (convective) rain
– Thunderstorms for a shorter period
– occurs on warm side of front
• Temperature decreases after frontal
passage
• Wind becomes northerly after passage
COOL AIR
L
LIGHTER RAIN
COLD
AIR
WARM
AIR
HEAVIER
RAIN
Finding a Front