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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

April 15/17

Thickness and Thermal Wind

Thickness

• The vertical distance in meters between two pressure levels

1000 hPa = 0 m

500 hPa = 5600 m

THICKNESS = 5600 m –

0 m = 5600 mZ

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 1000 hPa = 0 m

500 hPa = 5600 m

COOL

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 1000 hPa = 0 m

500 hPa = 5000 m

COOLZ = 5000 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 1000 hPa = 0 m

500 hPa = 5600 m

WARM

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 1000 hPa = 0 m

500 hPa = 6200 m

WARM

Z = 6200 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)

V850

V200

VT

VT

5660 m

COLD

WARM

5600 m

5540 m

Clockwise turning of winds with height

Veering Backing

850 hPa

300 hPa

850 hPa

300 hPa

VT

VT

Counterclockwise turning of winds with height

WARM AIR ADVECTION 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

upper-level wind

• Consider a front with cold air to the north and warm air to the south.

COOL WARM

P = 500 hPa

Geostrophic wind into page Thermal wind into

page

LOW heights

HIGH heights

NORTH

PGF

P = 700 hPa

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

COOL

WARM

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

COOL

WARM

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

LLLIGHTER RAIN

HEAVIER

RAIN

WARM AIR

COLD AIR

COOL AIR

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

LL

HH