Air Pressure Experiments

Lessons from Paper Cup experiment:

1. Air pressure is present everywhere

2. Air “tries to” move from an area of higher pressure to an area of low pressure

Lesson from Pop Bottle experiment:

3. Warm air occupies more space than an equal number of molecules of cold air

Wind Power Generation in Southern Alberta

“Don’t try this at home”

The speed and direction of windis determined by three forces:

1. Pressure Gradient Force

2. Inertial Coriolis Force

3. Friction Force

Pressure Gradient Force

H L102.2 99.8

101.4 100.6

600 km

Pressure Gradient Force = 2.4 kPa / 600 km= 0.4 kPa / 100 km

Definition:

The difference inatmosphericpressure per unitdistance

PGF acts at rightangles to isobarsof equal pressure

Where is the PGF forecast to be strongest today ?

Regina orLethbridge?

Solution:Check the spacing of theisobars of equal surfacepressure

Source: http://weatheroffice.ec.gc.ca/data/model_forecast/592_100.gif

The Inertial Coriolis Force

Objects moving in an “absolute” straight linebetween two points on the Earth’s surface aredeflected:

To the RIGHT in the N hemisphereTo the LEFT in the S hemisphere

Why ?

The Earth rotates more quickly at the equator.

Visualizing the Coriolis Force

Source: NASA

The Friction Force

Surface roughness decreases wind speed

Reduces impact of Inertial Coriolis Force

Winds cross isobars, spiralling out ofANTICYCLONES (H), and into CYCLONES (L)

H L

Can you infer wind direction and relative speed from this map ?

weather.unisys.com

Sea level pressure:

AltitudeCorrection

Source: Ahrens (1994)

Weather symbols andwind barbs

Classic Low Pressure SystemIn Temperate Latitudes

SHARPCOLDFRONT

WARM,MOISTSOUTHERLYFLOW

NORTH-EASTWINDS

www.atmos.washington.edu

0600h GMTAPRIL 52003

Cold Front

Arctic high pressure drives cold arctic air behind low

                                                                                                                                                                                             

                           

Warm Front

•Not as steep a division as in a cold front•It takes longer to scour out surface air (warm air rises)

WARM, SOUTHWIND

COOLNW WIND

COLD FRONT

WARM FRONT

HURRICANE ISABEL

The weather pattern last September

Main Low and High Pressure Zones

1. Equatorial Low Pressure Trough

2. Subtropical High Pressure Cells

3. Subpolar Low Pressure Cells

4. Weak Polar High Pressure Cells

Atmospheric Circulation Overview

HADLEYCELL

FERRELCELL

POLARCELL

Equatorial low pressure trough (warm, wet)

ITCZ shifts with season

High solar angle

HeatingConvergence

Consistent daylength

Hadley Cells

1. Warm, moist air rises in equatorial lowCools, condenses, and causes heavy rain

2. Outward flow to subtropical high at high altitude

3. Air descends in subtropical highHeats, compresses and becomes very dry

4. The subtropical high provides the gradient for trade winds and westerlies

eg. Bermuda/Azores and Pacific/Hawaii highs

Strahler and Strahler (2002)

Ferrel Cells

Between subtropical highs and subpolar lows

Poleward transport of excess heat througheddies and migration of lows toward polar front

Strong low pressure develops in a belt aroundAntarctica, near the Aleutians and near Iceland

Lows strongest in winter (shift and diminish periodically, especially in the summer)

Why ? Water much warmer than land in winter leading to lower pressure over oceans

H L

Air tends to be unstable in low pressure (tendency to rise)Air tends to be stable in high pressure (tendency to fall)

(more on stability in next class)

WINTER SUMMER

Generalized Overview of Seasonal Surface Pressure

Average GlobalSurface Pressurein January andJuly

Can you explainthe monsoon seasonof the Indian sub-continent with thischart ?

Polar High Pressure Cells

Tendency for higher pressure near polesthan at the polar front

Anticyclonic flow develops

Weak and variable polar easterlies result(stronger in southern hemisphere)

In northern hemisphere winter, the polar front usually lies over Canada and Russia,(further south than in the summer)

Geostrophic Winds 500 mbar height map

Lower heightswhere air is cold

Airflow parallel to isobars inupper troposphere

Why ?

Combination of PGF and Coriolis force

Source: http://weatheroffice.ec.gc.ca/data/model_forecast/134_100.gif

Source: Ahrens (1994)

Effect of Air temperature on 500 mb heights

Upper Atmospheric Circulation

Jet StreamsA band of wind in the upper troposphere

150 – 500 km wide0.9-2.2 km thickSpeeds may exceed 300 km/h

Polar Jet Stream: Between Polar and Ferrel cells

Subtropical Jet Stream: Between Hadley and Ferrel Cells

Source: http://apollo.lsc.vsc.edu

Source: http://apollo.lsc.vsc.edu

Tropopauseheight

18 000 m

12 000 m

6 000 m

Discontinuity orstep in tropopause height

“Rivers” of strong wind where cold and warm meet

Jet Stream Cross Section

See: www.avsim.com/avwx/avsim_wxus_jetstream.html

Polar Jet Stream

Subtropical Jet Stream

Meanders from 30-70° N or S

Moves more poleward in summer

Influences (and is influenced by) storm paths

Meanders from 20-50° N or S

May occur simultaneously with Polar Jet in NA

Rossby Waves

The polar jet stream follows the Rossby Waves

Rossby Waves are undulations in the upper-airwesterlies extending from the middle to upper troposphere

Form along the polar front

Mechanism of poleward heat transport

(Strahler and Strahler, 2002)

Daytime

Night

Source: Ahrens, 2001

Mountain Valley Breezes

Source: http://apollo.lsc.vsc.edu

Daytime

The sun heats the hillslope, causing air to move up theslope

Night

Night radiation coolsthe slopes

Cooler, denser air moves downslope

Katabatic Winds

•Air cools on a plateau or sloping terrain, becomes more dense and descends

•Winds get faster and faster downslope

•Relatively warm water at base can further increase winds, which can be very strong as a result

•Can occur on large scale (eg. Greenland, Antarctica)

•Also referred to as gravity drainage winds

VANCOUVER LETHBRIDGE8°C 12°C

X X

CoolingAt MALR6°C/km

WarmingAt DALR10 °C/km

CoolingAt DALR10 °C/km

CoolingAt MALR6°C/km

WarmingAt DALR10 °C/km

More sensible heat

Chinook Winds

•Water piles up around equator due to trade winds

•Along western edge of oceans, water spills N and S along shorelines of continents (also downwelling)

•Upwelling occurs near east edge of oceans (west coasts)

Upwelling of cool waters

Result: The water column becomes unstable and mixes vertically in the north. This newly formed water is carried southward at great depths - North Atlantic Deep Water (NADW)

The Thermohaline Circulation

(1) Intensive cooling at the ocean surface in North Atlantic(2) Northward transport of salty surface water from lower latitudes (both increase the density).

Interannual climatic variability atthe global scale

Caused by changing atmospheric andoceanic circulation in the tropicalPacific Ocean

Top La Nina December 1998; Middle Normal December 1993; Bottom El Nino Dec 1997

See http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml