Chapter_7

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Chapter 7:
Atmospheric
Circulations

By the end of this chapter you should:

Understand the scales of motion

Be able to give an example of an eddy

Have an understanding of the general circulation
of the atmosphere
Scales of Atmospheric
Motions

Scales of motion – different sizes of circulation
 Microscale – smallest scale of motion. 2 meters or
less and last seconds to minutes
Mesoscale – circulation around a city (20 km). Can
last minutes to hours

Synoptic scale – Circulations around highs and lows
(2,000 km). Can last many days

Planetary scale – Circulations around the entire
Earth.

Scales of Atmospheric
Motions
Scales of Atmospheric
Motions
Eddies - Big and Small



Eddy – a circulation formed downwind from an object
(examples?)
Rotor – rotation formed downwind from a mountain wave
Wind shear –
change of wind
speed or
direction with
height
Thermal Circulations

Thermal circulations
 Circulations brought
on by changes in air
Temperature.
Sea and Land Breezes


Types of thermal circulations
 Sea breeze (scale of motion?)
 Land breeze
Sea breeze front – leading
edge of the sea breeze
Florida sea breezes can have
huge walls of clouds. Make for
great gliding due to vertical
movement

Sea and Land Breezes
Seasonally Changing Winds the Monsoon


Monsoon wind system – “monsoon” means seasonal
Asian monsoon – in winter the land is much colder than
ocean. In summer, opposite is true
Seasonally Changing Winds the Monsoon
Mountain and Valley Breezes


Valley breeze – explain this
Mountain breeze – explain this
Katabatic Winds


Katabatic winds – in general, any wind that flows
downhill
 Perfect scenario is elevated plateau with mountains
around
Bora – katabatic
wind in the Adriatic
Sea
Chinook (Foehn) Winds


Chinook winds – fierce wind that flows on the eastern
slope of the Rockies.
 Compressional heating – main source of the heat
from the Chinook. Air descends very rapidly and
warms
Chinook wall cloud – a bank of clouds forming over the
mountains
Santa Ana Winds



Santa Ana wind
 Warm, dry wind that
originates from the
Desert. Usually needs
a high in the Great
Basin
Compressional heating
Fans huge wildfires
Desert Winds


Dust storms – formed by surface winds that form from
surface heating. Wind picks up dust
Dust devils – Formed from combination of surface
heating, unstable atmosphere, and an obstacle in the
way
General Circulation of the
Atmosphere

General circulation only represents the average
circulation
 Why are the prevailing winds in Hawaii northeasterly
and in san Jose northwesterly?

Cause: unequal heating of the earth’s surface

Effect: atmospheric heat transport
Single-cell Model

Basic assumptions
 Covered with water
 Sun is always over equator
 Earth does not rotate
Hadley
Heat
cell
at the equator
creates a low that
brings heat to the
poles aloft
Single-cell Model

Why is the single-cell model wrong?
Earth has the Coriolis force. In this model, all winds
would be easterly. We know that is wrong
 So let’s spin
the planet.

Three-cell Model
Model for a rotating earth - keep first two assumptions
 Hadley cell is still apparent
 Doldrums – equatorial region where winds are light.
Warm air rises, condenses and moves laterally towards
the poles
 Subtropical highs – convergence of air aloft creates
highs at 30 degrees. Major deserts and horse latitudes.

Trade winds – some of the surface air from the
Subtropical highs move back to the equator and deflect
due to the Coriolis force.
 Intertropical convergence zone – converge of the trade
winds from both hemisphere to get rising air and
thunderstorms

Three-cell Model

Westerlies – some of the air from the Subtropical highs
move towards the poles and deflect towards the east
Polar front – the boundary between the mild air towards
the equator and the cold air near the pole (Subpolar lows)

Polar easterlies – air behind the polar front that is
deflected to the west by the Coriolis force

Fig. 7-21, p. 185
Average Surface Winds and
Pressure: The Real World




Semipermanent highs and lows – two areas of highs and
lows per hemisphere
Bermuda high & Pacific high
Icelandic low & Aleutian low
Siberian high
Fig. 7-22a, p. 188
Fig. 7-22b, p. 189
The General Circulation and
Precipitation Patterns


Where would you expect regions of high and low
precipitation?
Heavy precipitation
 Regions close to ITCZ,
polar fronts, subpolar
lows
Westerly Winds and the Jet
Stream

Jet streams – regions of very fast-moving air in the upper
atmosphere due to high pressure at the equator and low
pressure near the poles
Winds and Upwelling


Upwelling – rising of cold water from below
Wind flow parallel to the coastline is deflected to the right
by what force? Cold water replaces this
El Niño and the Southern
Oscillation



El Niño – a natural event that occurs around Christmas.
Upwelling weakens along the west coast of South
America. Major events can cause great hardship
Southern Oscillation – seasaw pattern of high and low
pressures switching on opposite sides of the Pacific
Ocean
La Niña – cold water episodes opposite El Niño
El Niño and the Southern
Oscillation
Other Atmosphere-Ocean
Interactions



North Atlantic Oscillation
Arctic Oscillation
Pacific Decadal Oscillation
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