Scale of Atmospheric Motion
•
Microscale: meters
• Turbulent eddies
• Formed by mechanical disturbance or
convection
• Lifetimes of minutes.
•
Mesoscale: km’s to 100’s of km’s.
• Local winds and circulations
• Land and sea breezes, mountain and
valley
winds,
thunderstorms,
tornadoes.
• Lifetimes of minutes to hours
• Synoptic scale:
1000’s of km’s
100’s to
• Circulations around high
and low pressure systems.
Eg: Monsoon
• Lifetimes of days to
weeks.
• Global scale:
systems
ranging over entire globe
Scale of Motion
Atmospheric Wind System
• The circulations of air are driven by differential solar heating.
•
Tropics receive much more solar radiation than Poles
because of
•
•
•
Different in the sun`s angle
Tilting of the earth’s axis
Local reflectivity of the surface
• Uneven heating creates high and low pressure zones.
• Low pressure zone
• High temperature.
• High water vapor content (humid).
• Atm pressure decreases
• Air becomes less dense
• High pressure zone
•
•
•
•
Low temperature.
Low water vapor content (dry).
Atm. pressure increases
Air becomes more dense.
Global air pressure due to uneven heating
• High surface pressure in cold
high latitudes
• Low surface pressure in warm
low latitudes
• Recall that air moves from
high pressure to low pressure
areas
• Surface winds blow from north
and south to equator
• Upper winds blow from
equator to north and south
A single cell atmospheric circulation
• Proposed by George Hadley
(1735)
• Assumes that
• Earth`s surface is only
covered by water
• Sun always over equator
• Earth does not rotate
However earth does rotate !!!
The real global circulation from satellite
images
So a single cell model does not represent the real
atmospheric circulation of our earth
3-cell model of atmospheric circulation
Polar cell
Ferrel
cell
Ferrel
cell
Polar cell
• Proposed by William
Ferrel (1865)
• Divided into 3 cells in each
hemisphere
• Hadley cell (0-30o N/S)
o N/S)
•
Ferrel
cell
(30-60
Hadley
• Polar cell (60-90o N/S)
cell
3-cell model of atmospheric circulation
Hadley cell
• Air rises near equator & descends at
30o N/S
• Rising air produces a band of cloud
along the equator known as Intertropical convergence zone (ITCZ)
• Sinking air produces a major desert
along 30o latitudes known as subtropical highs
• Surface winds blow from 30o N to
equator & deflected to the right:
NE trade winds
• Surface winds blow from 30o S to
equator & deflected to the left: SE
trade winds
3-cell model of atmospheric circulation
Ferrel cell
• Air rises near 60o N/S & descends
at 30o N/S
• Rising air along 60o N/S known as
sub-polar lows
• Sinking air along 30o N/S known as
sub-tropical highs
• Surface winds blow from 60o to 30o
& deflected to the right (N) and left
(S): westerly trade winds
3-cell model of atmospheric circulation
Polar cell
• Air rises near 60o N/S & descends
at 90o N/S
• Rising air along 60o N/S known as
sub-polar lows
• Sinking air at 90o N/S known as
polar highs
• Surface winds blow from 60o to 90o
& deflected to the right (N) and left
(S): easterly trade winds
The Real Global Circulation
•
The Coriolis force, continents, mountains, and ice
fields alters the general circulation.
•
Greater heating over Equator causes air to rise and
sink in subtropical regions (30 oN/S).
•
The circulation will develop:
•
A semi low-pressure belt that travels northward
and southward.
•
A semi high pressure belt when air descends
(becomes compressed and denser) in subtropical
regions (30 oS/N latitude)
• The descending subtropical air spreads along earth’s
surface both back towards:
• Equator (trade winds – 0o to 30o S/N).
• Higher latitudes/subpolar (westerly wind belts 30o to 60 oS/N.
• The rising westerlies air (from subpolar – semi low
pressure belt) spreads along earth’s surface both flow
towards:
• Polar (polar easterly wind belts - 60o to 90o S/N)
and to
• Subtropical (semi high pressure belt - 30o to 60o
S/N).
• Semi-permanent Highs and Lows persist throughout
large periods of the year.
• Winter - highs form over land; lows over oceans.
• Summer - highs form over oceans; lows over land.
• The Inter-tropical Convergence Zone (ITCZ) shifts
toward south in January and toward north in July
ITCZ (DOLDRUMS)
World Deserts (along ± 30o latitudes)
Winter
Summer
Precipitation pattern
Global wind pattern and the Oceans