Chapter 11- Global Wind Systems
Review- When do clouds form?
Low Pressure? High Pressure? Upper Level? Surface?
The important criterion is rising air, not pressure.
Clear skies
Clouds
Sinking air
Rising air
L
H
Wind
V
Lake
Wind speeds up
Less dense
More dense
Here, a surface low gives fair weather. Often a surface low gives clouds and rain. (See Fig. 7.13) In all
cases, rising air gives clouds, regardless of pressure!
Fig. 11.2 represents a model of atmospheric circulation assuming:
1) No land
2) No tilt of the Earth’s axis, however the Earth does rotate.
This is a depiction of surface pressures and wind directions.
General Features:
1) Heat at equator causes air to rise
- Surface low forms, clouds and rain.
- Surface air flows in from higher latitudes.
- Coriolis force causes NE winds in the Northern hemisphere and SE winds in the Southern
hemisphere, respectively.
2) Cold at poles causes sinking air.
- Winds flow away from the poles on surface.
- Coriolis deflects these winds eastward.
3) Another cell forms between 30° and 60° latitude that opposes the polar and Hadley
(subtropical) cells.
- This Ferrel cell has northward flowing surface winds in the Northern Hemisphere
(Southward winds in the Southern Hemisphere) and,
-
Coriolis causes westerlies in the Northern and Southern Hemispheres.
Easterlies above and below equator are called trade winds.
Winds are light near equator and subtropical high (+/- 30°) – doldrums and horse latitudes.
ITCZ – trade winds converge.
Fig. 11.3 – The real world.
Average sea-level pressures and winds.
Observations:
1) Semi-permanent highs and lows.
2) Thermal (seasonal) cyclones (lows) and anticyclones (highs).
3) Seasonal movement of ITCZ and other features.
4) Sinking (stable) air near +/- 30° associated with clear weather, shifts with seasons.
Why are winds light near highs and lows?
H
L
H
∆P
d
d
P
∆P
distance
Generally,
∆P
is small near highs and lows and largest in between highs and lows.
d
Fig. 11.8 Upper level pressure and wind.
1) Semi-permanent lows moved westward (Why?) – Coriolis?
2) Mostly westerlies aloft – small easterly flow equatorward of highs.
3) Contours closer in winter
steeper pressure gradients, higher winds.
Also, recall from Chapter 9,
PGF =
m ∆P
ρ d
As you go higher in altitude, air density decreases ⇒ PGF increases so wind blows faster at higher
altitudes.
N
Westerly winds have v > 1000 knots
Easterly winds have v < 1000 knots
Equator on surface v = 1000 knots
v
As viewed by stationary observer
above earth
S
“Calm” air would be fixed relative to the surface of earth, so to the stationary observer, would be
moving at 1000 knots.
Note: Speed at surface of earth gets smaller toward the poles because you go less far in 24 hours.
Example: Standing 1foot from the North Pole, you would move 2π feet in 24 hours!
However, westerly winds move faster towards the east than the earth’s surface does and so appear to us
on earth to move toward the east.
Winds that appear to us to move towards the west are actually moving toward the east to a stationary
observer!
Jet Stream: Fast moving air currents, long and thin.
Rising air flattens out at the tropopause because the atmosphere in the stratosphere is very stable (an
inversion).
Jet streams are caused by steep pressure gradients aloft. (See Fig 11.10)
At locations where adjacent cells meet (Hadley, Ferrel), warm air meets cold air and strong
temperature and pressure gradients develop.
Jet streams are also helped by angular momentum, “L”.
Smaller r
L = mvr
larger v
As air aloft moves from the equator to the poles and is deflected east by coriolis force, r is also
decreasing, which speeds wind up.
Why do winds follow curvature the curvature of the earth as they blow over long distances?
N
PGF
FG
wind
equator
As the earth curves away from the wind, PGF decreases, so FG > PGF and FG –PGF provides the
centripetal force required, curving the wind in an arc.
It is northward and southward blowing winds that are responsible for pumping momentum away from
the equator.
These north and south winds are generally associated with highs and lows.
Winds blowing away from the equator are turned into westerlies by coriolis force.
Equator
Winds blowing toward the equator are turned into easterlies by coriolis force.
Equator
Ocean currents tend to follow air currents. Closed ocean current loops are called gyres (the same root
as gyration).
Large, high pressure cells dominate the world’s major bodies of water.
Winds blow clockwise and outward in the northern hemisphere and counterclockwise and outward in
the southern hemisphere around these highs.
Air
High
High
Isobars
Northern Hemisphere
Southern Hemisphere
Water responds to these more-or-less constant winds, through the frictional force.
Air
f a,w
Wind
f w,a
Water Surface
Newton’s 3rd Law: For every action there is an equal and opposite reaction.
fw,a
fa,w
Result: Water is pushed in direction of air. However, coriolis force is around, too: in northern
hemisphere, water deflects to the right and in the southern hemisphere, water deflects to the left.
High
High
Water
Northern Hemisphere
Southern Hemisphere
As you go below the surface of the water, the layers above push each lower layer, so water direction
deflects more from wind direction with depth.
Fig. 11.17 Ekman Spiral.
wind
N. America
S. America
Up welling
Up welling
Water
wind
Water
El Niño and the Southern Oscillation.
Falling air,
Clear, Dry
Rising air, Rain
High
Low
Peru
Indonesia
Warm Water –
South Equatorial
Current
“Normal”
Conditions
For some reason, the high and low pressures weaken, or even reverse
sets off a chain of events that effect weather worldwide.
Cool Water –
Peru Current
Upwelling
El Niño Event. This
Since ~ 1975, El Niño events have been more frequent, longer lasting and stronger. We don’t know
why.
Weather in Florida: 1997-1998 – very wet winter followed by 1198 very dry summer ( La Niña).