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
WARM AIR ADVECTION
VT
COLD AIR ADVECTION
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
(thickness) gradient will have strong
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)
• Gradual Slope
• 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
• Temperature (dewpoint) Gradient
• Change in wind direction
– Converging winds at the front
• “Kink” or “trough” in isobars (lower pressure)
• Banded precipitation
Upper-level terminology
• TROUGH: area of lower heights
• RIDGE: area of higher heights
L
H