1. Chris Parkes Rm 455 Kelvin Building
Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 5: Atmospheric Convection
Chris Parkes
Rm 455 Kelvin Building

2. Absorption spectrum of atmosphere
Revision
Radiation
4. Solar Radiation
Wm-2μm-1 μm
Sun:
Incoming
Absorption spectrum of atmosphere
Spectrum of incoming & outgoing radiation
Insolation – daily & annual variation
Albedo
Energy budget
Greenhouse effect
Earth:
Outgoing

3. Convection in the Atmosphere
What drives it?
Hadley cell
- a simple model
A more realistic model of earth’s atmospheric convection

4. Archimedes’ Principle
Upward buoyancy
Archimedes’ Principle
Objects in fluid experience an upward (buoyancy) force equal to the weight of the displaced volume of fluid
Static balloon must have buoyancy equal to its weight
Hot air is less dense than cold air
weight

5. Hot air rises
Air moves in “parcels” – like balloons but without the fabric
A parcel hotter than surroundings will experience a greater buoyancy force than its weight – net force upward – it will rise.
Upward buoyancy
cooler
surroundings
warmer air
parcel
weight
As rises:
Temperature decreases
Pressure decreases
Volume Increases
(see lecture topic 3)
pV=T

6. Column of air being heated
Pressure (mb)
200
Height
Heating
500
800
Initially at same temperature as surroundings
Heating at ground level
Air volume expansion

7. Column of air being heated
Pressure difference at the top leads to outflow
Less weight in column
Lower pressure at surface
Inflow towards low pressure
Convective flow of air
Pressure (mb) H
200
Height
500
800 L
Initially at same temperature as surroundings
Heating near ground level
Air volume expansion

8. Equator (low pressure)
The Hadley Cell (1735)
Tropopause B
Height
North
South A
Ground
Equator (low pressure)
Air column AB expands
High pressure at B
Low pressure at A (w.r.t. surroundings)
Warm air rises.
At B further convection is limited by temperature inversion at the tropopause

9. Equator (low pressure)
The Hadley Cell (1735)
Tropopause C B
Height
North
South
Convection Cell D A
Ground
(High pressure)
Equator (low pressure)
Air moves away from equator
cools gradually becoming more dense (B to C)
Air sinks back to surface (C to D)
Movement of air from high to low pressure (D to A)

10. Equator (low pressure)
The Hadley Cell (1735)
Tropopause C B
Height
North
South
Convection Cell D A
Ground
(High pressure)
Equator (low pressure)
Vertical motion is on average ~10 cm/s (c.f. 10 m/s in cumulus cloud) – caused by change in density and pressure
Horizontal motion is due to pressure difference.
Changes are small ΔP = 50 mb (average P = 1000mb)  5% change

11. Equator (low pressure)
The Hadley Cell (1735)
Tropopause C B
Height
North
South
Convection Cell D A
Ground
(High pressure)
Equator (low pressure)
No air is created or lost
Mass moved per unit time = speed × density
Must be the same for surface and high level winds.
Density lower at higher altitude  high altitude winds are fast

12. Pressure “systems” High pressure Air sinking, generally cooling
Mostly over oceans
Low pressure
Air rising, generally being heated
Mostly over land
Here high and low pressure refer to surface pressure
i.e. top of “low” pressure region has a higher pressure than surroundings

13. Differential heating on Earth
Poles receive same amount of energy over
larger area - less energy density on surface
North
Solar energy
Solar energy
Equator receives a quantity of solar energy over a small area
Equator
Rotating an unit area by 60º
reduce incident radiation by half 
at 60º latitudes only get half of sun’s energy

14. Hadley cells on Non-rotating planet
All area heated, but -
More solar heating at equator creates hotter region
Air rises at equator (inter-tropical convergence zone, ITCZ)
Cooler air from poles moves towards equator to region of lower pressure
In reality on Earth:
Rotation
Day/night difference
Annual variation
Global air movement takes weeks
Cold
Hadley cell
Hot
Equator
Hadley cell
Cold

15. Venus as Hadley Cell Mars Venus: Mars:
Slow rotation (Venus day is 243 Earth days)
weak rotation effect – works like Hadley cell
Dense atmosphere
Efficient transport of heat – equator and poles similar temperature, despite incident radiation angle effect
Mars:
Thin atmosphere
Very little heat transported, poles much colder than equator
Mars

16. Rotation - The Coriolis effect
Apparent deflection of objects from a straight path when viewed in a rotating frame
Apparent “force” pushing outward going bodies to the right and inward going bodies to the left
Rotation of earth means:
Apparent movement to the right while moving on northern hemisphere and, to the left in the southern hemisphere

17. Coriolis Force Merry-go-round Balls path deviates to the right
Ball rolled inwards
Or ball rolled outwards
Would be reversed if anti-clockwise
coriolis force opposite in south / north hemispheres

18. The Coriolis effect

19. The Coriolis effect

20. The Coriolis effect In Hadley cell in northern hemisphere:
upper air moves north  coriolis pushes it eastwards
lower surface air moves south  coriolis pushes it westwards
Simple Hadley circulation cell model breaks down

21. The Three-cell model of Earth’s atmosphere
Direct (Hadley) cell
- from equator to 30º
Indirect (Ferrel) cell
- from ~30º to 60º
Polar cell
Indirect cell driven by the other two

22. The Three-cell model of Earth’s atmosphere
Surface and upper winds have east/west as well as north/south component
East/west balanced such that the whole atmosphere rotates with the globe
Better model needs to include:
North/south & east/west movement
Angular momentum
Differential heating
Effect of land mass
Seasonal changes …
easterlies
jet streams
westerlies
trade winds

23. Smaller scale convection – Sea Breezes
Land heats up quicker than sea
Air above land begins to rise
Sea air moves inland since rising air above land produces lower pressure
Size of effect increases throughout the day
Keep coastal regions cooler than inland
Reverse at night

24. Example exam questions
Q1. State what is a Hadley cell and explain how it works?
Q2. How does differential heating arise on Earth?
Q3. Sketch a diagram to explain the Coriolis effect.
Q4. Explain how sea breezes keep the coastal region cooler than inland.
Next lecture – wind

28. Current in fluid under gravitational field & differential heating
Convection … advection
– mechanism of heat transfer
Current in fluid under gravitational
field & differential heating