Air Pressure and Wind

Pressure: the amount of force exerted per unit of surface area

Pressure can be increased in 2 ways

1. By increasing density or decreasing the volume2. By Increasing the temperature

Air Pressure and Wind

Key factor: Unequal heating of the Earthtemperature Changes cause pressure changes!

Air Pressure:

air

Exerted in all directions

Standard Sea Level Pressure= 1013.2 mb

Wind: H L

P1 P2

If P1>P2, then the “box” would Move to your right.Therefore air moves fromHigher pressure to lower pressure

Measurement of Air Pressure

Mercurial Barometer

Measurement of Air Pressure

Aneroid Barometer

The Distribution of Pressure

Isobars: Lines of Equal pressure

Pressure Gradient & Pressure Gradient Force (PGF)

On a Surface Map:

-The spacing indicates the strength of the PGF

The change in pressure over a distance

Where is an areaOf high winds?…weak winds???

Pressure Gradient Force (PGF)

Closely spaced isobars indicate: faster change in pressureOver a distancestrong pressure gradient, and thereforeSTRONG WINDS!!

Loosely spaced isobars indicate: slower change in pressure Over a distanceweak pressure gradient, and therefore WEAK WINDS!!

Wind direction

•Creates the wind•“Blows” from high pressure to low pressure

Vertical Pressure Gradient Force (PGF)

•Pressure decreases with height•PGF moves air from high to low pressure

WE KNOW:

SO…….

Why isn’t air always blowing upward?????

Answer: Hydrostatic Balance gravity!

g

VPGF

Wind

Three Forces:

•Pressure Gradient Force (PGF)•Coriolis Effect (CF)•Friction

Wind

Coriolis Force (CF)

•Occurs because the Earth is rotating!•All objects are deflected

•To the right in N. Hem. And to the leftin the S. Hem.

•Only affects wind direction•Affected by wind speed•Strongest at the poles, weakens equatorward

Wind direction

p.106

4 Fundamental Characteristics

WindFriction (F)

•Important to wind in the first 1.5 km at Earth’s surface

•Acts to slow air movement!!!!

Upper Air: Not affected by friction

Upper Air Winds

m

heat m

1. As the column heats up, it expands2. It’s mass stays constant, but it stretched over a longer distance

a. Therefore you have a lower density3. The height of 500mb is higher.

………………………..

……………….500mbwarm

500mbcold

Read pp. 108-110

Upper Air Winds

Therefore, in the upper air we plot heights instead of pressure!!!!!!

Height contours: lines of equal height

Properties of height contours

1. The slope of the height contours is proportional to the horizontal pressure gradient.2. Heights decrease towards colder air3. They are label in decameters (units of 10m)4. They are plotted at an interval of 60m

Can you see the JetStream?Where are the highest winds??

Upper Air WindsForces affecting winds in the upper air

NO FRICTION!!!!

Only the pressure gradient force and coriolis force

Fig 4-12 on p. 109

Geostrophic flow: •air is flowing in a straight line •Air has no acceleration•PGF=CF

Gradient flow:•air is constantly changing direction

•Air has acceleration•The PGF does not balance the CF

Highs and Lows

Low: CycloneHigh: Anticyclone

What happens when the forces we just describedAre applied to pressure centers? Fig.4-14

L H

Upper Air Pressure Systems

Why do winds around a low circulate counterclockwise??

Why do winds around a high circulate clockwise??

V= Wind’s original directionPGF=pressure gradient forceCF=Coriolis forceCCW=counterclockwiseCW=clockwise

L

V

CFPGF

PGF> CFTherefore wind turnsCCW

CF > PGFTherefore wind turnsCW

H

V

PGFCF

Surface Pressure Systems

L

V

CFPGF H

V

PGFCF

F=friction

FFriction slows the windWeakens the CF

CF is outward force in lowTherefore winds will circulateCCW and INWARDS!!!

CF is inward force in highTherefore winds will circulateCW and OUTWARDS!!!

CONVERGENCE DIVERGENCE

Troughs and Ridgesp. 113 fig.4-19

Trough

Ridge

H

L

How the Upper Air is Connected to the SurfaceThe upper air drives the weather at the surface

•The jet stream is a river of strong winds•Moves from West to East•Exists at 200 (summer) or 300mb (winter)•Brings the weather!

Wind slows as it makes this turn and therefore windConverges on the west side of an upper level trough

Wind speeds up as it comes Out of the turn and therefore Wind diverges on the east side of an upper level trough

Chapter 10 p.285Fig. 10-6

p. 286 fig. 10-7

How the Upper Air is Connected to the Surface

Below the area of convergence in the jet stream trough, air sinksCausing a build up of pressure at the surfacetherefore, surface high pressure forms below the west side of an upperLevel trough

Below the area of divergence in the jet stream trough, air rises Causing a pressure drop at the surfacetherefore, surface low pressure forms below the east side of an upper level trough

H L

The upper level trough causes the surface systems to form, strengthenAnd move!