Lecture 147 February 2005

Atmospheric and Oceanic Circulations(continued)

Chapter 6

Credit: www.physicalgeography.net

Wind simply put, wind is the horizontal flow of air

in response to differences in air pressurethese pressure differences are usually

due to uneven solar heating at the surface

wind flows because of

pressure gradient

‘heat rises’

Four forces that determine winds

1. Gravity - pulls gas molecules close to Earth density & pressure decrease with height

2. Pressure gradient force - the difference in air pressure between

areas3. Coriolis force - deflects wind from a

straight line to the right or left depending on hemisphere

4. Friction force - the drag on air flow from the Earth’s surface

Pressure Gradient Force

Pressure Gradient Force and Isobars

if there were no other forces acting on wind, it would flow in straight lines (perpendicular to isobars) from high to low pressure zones

Coriolis Force (just the facts)

• Rotation of Earth acts to deflect any motion

from a straight line

• Deflection is to right (NH) to the left (SH)

• Coriolis “force” act on a right angle to the

motion

• Coriolis Force is NOT a real “force” but is

caused by viewing motion on a rotating

planet

Figure Credit: “Earth’s Climate” by W.

Ruddiman

The Coriolis Force affects air flow in response to pressure gradients in the

atmosphere

geostrophic winds - PGF and Coriolis forces are opposite and balanced

Credit: www.physicalgeography.net

the CF deflects the wind to the right until upper troposphere wind flows parallel to

isobars

~7km

Friction Force

Surface friction reduces wind speed and the effects of the Coriolis force

Friction causes winds to move across

isobars at an angle

The friction force operates only in the bottom 0.5-1 km of the atmosphere,

and it acts opposite to the direction of motion

PGF + Coriolis + Friction Forces

Figure Credit: “Earth’s Climate” by W.

Ruddiman

Show shockwave 27_WindPatDev.swf

Useful things to remember

• Coriolis Force

Acts to the right of motion (left in SH)

Not a real force – matter of perspective

• Geostrophic wind – upper

troposphere/ocean

Horizontal pressure & Coriolis forces balance

Winds go ALONG isobars

• Surface friction – lower troposphere

Enables CROSS isobar flow

Useful things to remember

• Low pressure cells

Less dense –humid

Circulation is counter-clockwise (NH) – cyclonic

Convergent near ground – rising air masses

• High pressure cells

More dense – dry

Circulation is clockwise (NH) – anticyclonic

Divergent near ground – descending air masses

Heating & Humidity in Tropicssolar heating in the tropics expands air and decreases its density - leading to increased

buoyancy

How would this change the average molecular weight of air?

average molecular weight of air is ~29

g/mol

average density of air is 1.3 kg/m^3

what happens to air density if

you add water vapor?

It also gets more humid (adding water vapor)

Convection on your Stove

Convection on Earth

Warm, moist air rises and is replaced by cooler drier air from other sites

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as this air rises, it cools and water condenses out, leading to intense

precipitation

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A satellite (GOES) view of the ITCZ over the eastern Pacific

InterTropical Convergence Zone

the position of the ITCZ tracks the sun (it is in the summer hemisphere) - the location of the

ITCZ determines the rainy season in many tropical countries, especially those in Africa

the horizontal winds within the ITCZ are calm - the doldrums

The C in ITCZ

the intense uplift of air creates horizontal pressure gradients at the surface

Credit: NASA JPL

as a result, winds converge towards the equator from both hemispheres

what about the complete cycle - where does the uplifted air go?

Equator-to-pole cross section of circulation

Hadley cell circulation

this circulation refers to the complete circulation of rising air in the tropics,

descending air over 30 °N and °S, and trade winds converging at the equator

the descending branch of the Hadley circulation brings warm, dry air to the surface

leading to high pressure & reduced

precipitation

Subtropical high-pressure cellsthese cells occur where the tropical air

descends in either hemisphere

Equator-to-pole cross section of circulation

Figure 6.18

Jet Stream

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Show shockwave 04_GlobalWinds.swf

Figure Credit: http://www.geog.ucsb.edu/~joel/g110_w03/chapt10/vorticity/agburt2_10_07.jpg

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Figure Credit: physicalgeography.net

Monsoon Circulation

Asian monsoo

nintense, dry winds flow from the Asian interior in response

to the gradient between the

continental high pressure &

equatorial (ITCZ) low pressure

in summer, the ITCZ shifts north,

reversing the pressure gradient

–

winds flow from the Indian ocean & gain moisture

Asian monsoo

n

Daytime land-sea breeze

results from differential heating of land and sea - not from radiation differences - but

from the different specific heats of land and water

Nighttime land-sea breeze

at night, the land cools more rapidly than the sea and thus overlying air becomes more

dense and has a higher pressure

What time of day would you go jogging in Los Angeles?

WHY?