Chapter 6

Pressure, and the forces that explain the wind

gz

p

or

zgp

depthdensitygravitypressure

thus

depthdensityarea

mass

or

deptharea

mass

volume

massdensity

area

massgravitypressure

thusarea

weightpressure

1

**

*

*

*

Hydrostatic balance:

The upward pressure gradient force isequal and opposite to the gravity

area

z

km

mb

s

m

m

kgg

z

p120102.1

23

aneroidbarometer

mercurybarometeraneroid barograph

What is the typical SLP?

How much does it vary ?

Average air pressure in Laramie780

We need to reduce station pressure to a standard height, for instance sea level

Why?

Because winds are driven by horizontal pressure differences

Isobars and pressure patterns

Where are you more likely to find a pressure value of 994 mb? At A or B ?

Becoming acquainted with contouring and frontal analysis

• http://cimss.ssec.wisc.edu/wxwise/contour/index.html• http://cimss.ssec.wisc.edu/wxwise/fronts/fronts.html

Defining patterns on a surface weather chart

• lows and highs• trofs and ridges• saddle

trof

ridge

examine the current weather analysis

What drives the wind?

Pressure gradient force (PGF) and wind

x

pPGF

1

here, x=100 km and p=4 mb

The PGF is directed from high to low pressure, and is stronger when the isobars are more tightly

packed

in reality, winds do not blow from high to low, at least not along the shortest path

… so there must be other force(s)

Coriolis force

Geo.mov

Geostrophic wind balance: a balance between the PGF and the Coriolis force

speedwindv

parameterCoriolisf

fvCF

_:

_:

x

p

fv

or

x

pfv

1

1link

L

Buys-Ballot law

• When you face downwind, the low will be on your left• Vice versa in the southern hemisphere

you (seen from above)

The geostrophic wind blows along the isobars (height contours), counterclockwise around lows (in the NH), and at a speed inversely proportional to the spacing between

the isobars (height contours)

x

p

fv

1

L

in the southern hemisphere, the low is on your right when you look downwind

There is a third force, important only near the ground

Friction slows the wind

Interplay between 3 forces

• Pressure gradient force• Coriolis force• Friction (near the ground)

Check out how they affect the wind!

1000

1004

1008

Guldberg-Mohn balance

~ 10° over oceans ~ 30° over land > 30° near mountainous terrain

Trajectoriesspiral out of a

high,

and into a low

finally, a fourth force: centrifugal force

PGF

Coriolis

CFF

CFF

slower-than-geostrophic wind(subgeostrophic)

faster-than-geostrophic wind(supergeostrophic)

Coriolis

PGF

The jet stream wind is subgeostrophic in trofs, and supergeostrophic in ridges

slow

fast

fast

slow

height

Where does the air, spiraling into a low, end up?

rising motion leads to cloudiness and precipitation

subsidence leads to clear skies

Fig. 10.11

Fig. 10.13

Find the trofs

300 mb height, 9 Nov 1975, 7 pm

Fig. 10.13

300 mb height, 9 Nov 1975, 7 pm

surface low

slow

fast

upper-level divergence,low-level convergence

Today’s surface weather analysis

Today’s upper-air maps

http://weather.uwyo.edu/upperair/uamap.html

http://www.rap.ucar.edu/weather/surface/sfc_den.gifhttp://weather.uwyo.edu/surface/front.html

Upper-level winds,and upper-level charts

Upper level charts are NOT plotted at constant height, eg 18,000 ft. Rather, they display the topography of a pressure surface, eg 500 mb

Pressure Approximate Height Approximate Temperature*

1013 mb 0 m (sea level)

0 ft 15 °C 59 °F

1000 mb 100 m 300 ft 15 °C 59 °F

850 mb 1500 m 5000 ft 5 C 41 F

700 mb 3000 m 10000 ft -5 C 23 F

500 mb 5000 m 18000 ft -20 C -4 F

300 mb 9000 m 30000 ft -45 C -49 F

200 mb 12000 m 40000 ft -55 C -67 F

100 mb 16000 m 53000 ft -56 C -69 F

Approximate conversion of pressure level to

altitude

1000 mb – near sea level

850 mb - ~5,000 ft

700 mb - ~10,000 ft

500 mb - ~18,000 ft

300 mb - ~30,000 ft

200 mb ~ 40,000 ft

pressure at a fixed height (sea level)

elevation of the 1000 mb surface

contours: sea-level pressurecolor fill: 1000 mb height

heightlowhigh 1500 m

1560 m

Pressure decreases with height at about 10 mb every 100 m

New York Bostonsea level

Why do isobar and height contour charts look (almost) the same?

pressure surface

Locate the trofs

Thickness and temperature

thicknessbetween 2 material surfaces(1000- 500 mb)

temperature

L

Pop quiz: why is their a ‘pit’ in the 500 mb surface over Antarctica?

- because it is much colder there than over Australia and other surrounding places

- because of the ozone hole

- because there is less sunshine

- I give up

calm

calmcalm

calm

Jet streamis due to the cold pool below

(circumpolar vortex)

L

Jet stream

• why does it exist?• why does it vary in strength?

The jet stream is the result of a horizontal temperature gradient

… and thus a thickness gradient

thickness = 20.3 * Tmean

thickness is in meters between 1000 and 500 mb

Tmean is the layer-mean temperature in Kelvin

1000 mb height (m) 500 mb height (m)

• Where is the 1000-500 mb thickness lower? Where is it higher?

• Where is the colder airmass – where is the warmer one?

5800

5400

5600

5200

5000

100

150

near the ground: weak PGF, weak wind near 18,000 ft: strong PGF, strong wind

pret

ty fl

at

pre

tty

steep

1000 mb height (m) 500 mb height (m)

• at A: Z500-Z1000 = 5850-150 = 5700 m

• at B: Z500-Z1000 = 5100-100 = 5000 m

5800

5400

5600

5200

5000

100

150

B

A A

B

Calculate thickness at A and B

… answer: the lower atmosphereis less thick at B up north

5100

700 mb mean temperature (C)

indeed, it is colder where the air is less thick

A

B

Relation between wind and temperature ...

Key : colder air is less thick, therefore upper level winds will blow cyclonically around cold

pools

For instance, look at the pole-to-pole variation of temperature with height (in Jan)

Around 30-45 N, temperature drops northward, therefore westerly winds increase in strength with height

The N-S temperature gradient is large between 30-50N and 1000-

300mb

Therefore the westerly wind increases rapidly from 1000 mb up to 300 mb

warm coldcold

JJ

‘thermal wind’

The increase of wind with height parallel to the isotherms, cyclonically around cold

pools

Illustration : compare the 300 mb height over the northern hemisphere ...

… to the temperature

700 mb

Now explain why a jet stream is found above a frontal zone

wind speed (kts)

The jet stream is there because of low-level temperature differences

polar front jet(PFJ)

Pop quiz: why is the jet stream stronger in winter?

• because the north-south temperature gradient is larger

• because cold air is lighter and can be blown around easier

• because there is less sunshine

• because there are fewer thunderstorms that act as obstacles to the upper-level flow.

Pop quiz: why is the jet stream stronger in winter?

because the north-south temperature gradient is larger

because cold air is lighter and can be blown around easier

because there is less sunshine

because there are fewer thunderstorms that act as obstacles to the upper-level flow.

Change the equator-to-pole temperature gradient, and see what happens to the jet stream!

Pop quiz: according to climate change models and observations, the arctic is warming up faster than low latitude

regions. What does this imply about the strength of the jet stream and the intensity of storms spawned by the jet stream?

• they weaken

• they strengthen

• it can go either way

• I give up

Summary

• There are four key forces driving the wind:– pressure gradient force (to start the motion)– Coriolis force– friction (only near the ground)– centrifugal force

• As a result the wind blows counterclockwise around lows (in the NH)– friction makes the low-level wind spiral into lows– the centrifugal force slows the wind in trofs, and speeds it up in ridges

• Weather changes (as we know it) is the result of passing jet streams, with– rising motion & clouds ahead of a trof, with a low at the surface– sinking motion & clear skies upstream of a trof, with a high at the surface– the deep vertical motion is due to changes in wind speed in the jet, as the wind in

trofs (ridges) is slower (faster) than expected from geostrophic balance

• The jet stream tends to occur above regions with a large temperature difference– The jet blows counterclockwise around cold pools (in the NH)

Let’s cover chapter 7 (global winds) and skip chapter 8 (air-sea interaction)

then we ‘ll do chapter 9 (air masses and fronts) andchapter 10 (mid-latitude weather)