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Planetary Atmospheres, the Environment and Life
(ExCos2Y)
Topic 6: Wind
Chris Parkes
Rm 455 Kelvin Building
5. Atmospheric Convection
Hot air rises, expands circulation cell
– Heating at equator, cooling at polesHadley cell
– Coriolis Effect east/west winds disrupts Hadley cell
• Three cell model of
Earth’s atmosphere
– Convection in Sea Breezes
Winds
Horizontal movement of air
Controlled by four main forces:
Pressure-gradient force
Coriolis force
Centripetal acceleration
Friction
The Pressure-Gradient force
Isobar chart
If pressure change by Δp
over a distance of Δs,
then the force is:
1  p 
F  

  s 
where ρ is air density
Isobars = lines of constant pressure
Higher wind speed
Bigger pressure change
Lower air density - high altitude
Weather dominated by High & Low pressure systems
The Coriolis effect (again)
• Rotation of the earth
• Speed is greater nearer the equator then
nearer poles
– Further from rotation axis
• Object (not attached to the surface) moving
from equator towards poles will appear to
deflect eastwards
• Appears as a force the size of which
depends on the Coriolis parameter ( f )
The Coriolis effect (again)
FCoriolis = - 2 m (ω × vr )
magnitude depends on sin(θ )
N
Equator
θ
The Geostrophic Wind
Balance between pressure-gradient force and coriolis force
View from above
L
Pressure
Gradient
Force
1000 mb
Geostrophic
Wind
Coriolis
Force H
1004 mb
•Wind rarely purely geostrophic
but approximately
•Ocean currents also
• As air moves feels perpendicular
coriolis force
• wind directions follow isobars
• “free atmosphere” above ~500m
– where friction can be neglected
• Picture shown for northern
hemisphere – opposite direction
for southern hemisphere
• Velocity depends on latitude:
Latitude (degree)
Speed (m/s)
43
15
90
10
Centripetal Acceleration
High
Low
Coriolis
force
Pressure
gradient
Direction of
centripetal
acceleration
Direction of
gradient wind
Coriolis
force
Direction of
gradient wind
Direction of
centripetal
acceleration
Pressure
gradient
Flow around low (high) pressure system is cyclonic (anti-cyclonic)
Fcent = FPG – Fcor (low pressure)
Fcent = Fcor – FPG (high pressure)
FPG > Fcor
wind speed less than vg (subgeostrophic)
FPG < Fcor
wind speed higher than vg (supergeostrophic)
for same FPG
(PG usually higher for low pressure systems)
Frictional force
View from above
No friction in
• Friction slows down wind
middle of troposphere
near surface
L
Pressure
Gradient
Force
1000 mb
Geostrophic
Wind
Coriolis
Force H
1004 mb
• Decreases effect of
deflective forces (Coriolis &
Centripetal)
• Wind direction points more
towards pressure gradient
• Direction points across
isobars:
– 10º - 20º over ocean,
– 25º – 30º over land
View from above
Friction
Near Surface
L
Pressure
Gradient
Force
1000 mb
Geostrophic
Wind
Coriolis
Force H
1004 mb
Frictional force
• Friction slows down wind
near surface
• Decreases effect of
deflective forces (Coriolis &
Centripetal)
• Wind direction points more
towards pressure gradient
• Direction points across
isobars:
– 10º - 20º over ocean,
– 25º – 30º over land
Frictional force
• Friction slows down wind
near surface
• Decreases effect of
deflective forces (Coriolis &
Centripetal)
• Wind direction points more
towards pressure gradient
• Direction points across
isobars:
– 10º - 20º over ocean,
– 25º – 30º over land
• Driven by:
Global Wind Patterns
– Atmospheric heating
– Planetary Rotation
• Equatorial: East to West
– Surface wind towards equator
Coriolis effect  east to west winds
• Driven by:
Global Wind Patterns
– Atmospheric heating
– Planetary Rotation
• Polar: East to West
– Surface wind towards equator (away from poles)
Coriolis effect  east to west winds
• Driven by:
Global Wind Patterns
– Atmospheric heating
– Planetary Rotation
• Midlatitude cells: West to East
– Surface wind towards poles (away from equator)
Coriolis effect  west to east winds
The Three-cell model & Global wind belts
Features
easterlies
jet streams
westerlies
trade winds
ITCZ, Doldrums
Intertropical Convergence Zone
(ITCZ)
Doldrums
Trade Winds
Mid-latitude westerlies
Polar front
Polar easterlies
Polar Front
The Intertropical Convergence Zone (ITCZ)
Convergence zone of winds from North & South Hemispheres
Region of intense rainfall – violent thunderstorms
Position of ITCZ varies with season
Affected by land masses – more land in Nothern hemisphere
Noticeable “spurs” occur at different times
Mean position ~5º north
The Intertropical Convergence Zone (ITCZ)
Cloud formation near equator indicating the ITCZ
Westerlies, Trade Winds and the Doldrums
Calm region near
equator in ITCZ:
Doldrums
West to East
Westerlies
Trade winds
East to west
Westerlies
West to East
• As explained in three cell mode:
• Trade Winds:
– Prevailing pattern of east to west winds in tropics
• Westerlies:
– Prevailing pattern of west to east winds in mid-latitudes
Air masses
Large parcels of air with almost uniform
temperature; moisture content; lapse rate; stability; visibility
Sources: Stationary for at least a week – from high pressure regions
Air masses
Tropical Continental
Polar Continental
Tropical Maritime
Polar Maritime
Arctic Maritime
Ret. Polar Maritime
Modification: Over ocean - moisture increases; over land - dry
Cold air mass over warm region, heating from below – less stable
Warm air mass over water – more stable
Air Mass Characteristics
Temperature
Humidity
Visibility
Typical weather
Tropical Maritime
Warm
Moist
Poor/fog
Low clouds,
drizzle
Tropical continental
(summer)
Hot
Dry
Moderate
Clear, some
thunder
Tropical continental
(winter)
Average
Moist
Poor
Clear
Polar Maritime
Cold
Moist
Good
Variable, showers
Fronts
Formed at the boundary between air masses
Wind movement causes ripples along boundary
Warm front: warm air advances into cold air region
Cold front: cold air advances into warm air region
Fronts
Jet streams
Regions of very high speed upper
winds (up to 100m/s)
Polar Front Jet due to Temp
difference between tropical and
polar air
Subtropical Jet due to Temp gradient
in upper troposphere
Jet can influence the track of weather
systems
Jet streams
Tropopause high for
tropical air
Induces geostrophic flow (E)
Rising warm air  NE
Two components in E direction add
Very high velocities occur (100m/s)
Stronger in winter when Temp.
gradients are greater
Aviation: turbulence
Pollution: mixes in atmosphere
Weather: can influence tracks of depressions
Example exam questions
Q1. List the forces affecting the movement of air
current.
Q2. Is anti-cyclone stronger than cyclone? Why?
Q3. What is the Coriolis parameter? How does it
vary with latitude?
Q4. Draw a diagram to explain the features of the
global wind belts.
Next lecture – effects of water