1. The General Circulation of the Atmosphere

2. Upcoming Schedule Friday, October 19: begin Chapter 10.
Monday, October 22: finish Chapter 10 + office hours
Wednesday, October 24: Second midterm exam. The exam will cover Chapters 6, 7, 8, 9 and 10.
Closed book
No textbooks, calculators or cheatsheets.
Alternate (anti-social) seating
Bring a picture ID to the exam.
Remember to:
Write the 5-digit test code in lines 76-80
Write your name on the exam sheet and sign it
Turn in both the exam sheet and the answer sheet.
Friday, October 26: begin Chapter 11.

3. RECAP

4. What we need to know for today
Pressure gradient force: from High to Low pressure
Coriolis force (effect):
Results from the rotation of the planet.
Maximum at the poles and no effect at the equator.
Acts perpendicular to the direction of motion: changes the direction of the wind but not the wind magnitude.
In the NH deflects the wind to the right.
In the SH deflects the wind to the left.
Winds aloft
Balance between the pressure force and the Corriolis force.
The wind is parallel to the isobars.
Surface winds
Balance between the pressure gradient force, the Coriolis force and the air friction.
The wind crosses the isobars (from High to Low pressure).

5. Surface Winds-a balance of three forces
In the boundary layer (~1km thick) friction is important!
Friction is acting opposite the direction of the velocity -> friction reduces the wind speed -> the Coriolis force becomes weaker -> it cannot balance the pressure force.
The wind starts to blow across the isobars towards the low pressure
The angle between the direction of the wind and the isobars is on average 30 deg (Buys-Ballot’s law). It depends on the topography.

6. Average Wind Structure
The direction and the magnitude of the winds at a given location can vary significantly during the day, and from day to day.
The general circulation (GC) refers to the average (the prevailing) winds on a global scale (around the world).
The GC of the atmosphere is the result of the uneven heating of the Earth’s surface.
It is impacted by the Earths rotation.
The GC transports and redistributes energy from one region to another (warm air towards the poles and cold air towards the equator).

7. The Single Cell Model
This is a very simplified model based on the following three assumptions:
The Earth’s surface is uniformly covered with water (no differential heating of the land and the oceans)
The sun is always directly over the equator (no seasonal variations of the winds).
The Earth does not rotate.
No Coriolis effect.
The only active force is the pressure gradient force.

8. Thermal circulations Due to uneven heating of the surface. Example:
South area heats up, North area cools
Warmer southern air aloft moves north towards low pressure
It then cools and sinks
Surface pressure to the North increases
Surface wind from N to S
The surface air warms up and rises.
The process continues

9. The Hadley Cell
It is driven by the uneven heating of the Earth’s surface by the sun - thermally direct cell: warm air rises, cold air sinks.
One Hadley cell in each hemisphere.
The equator is warmer than the poles.
Warm moist air at the equator rises upwards
It expands, cools, and saturates, the water vapor condenses and forms clouds.
It creates low surface pressure
in the tropics.
At the poles we have cool, dry,
sinking air that creates high surface
pressure in the polar region.
The PGF (pressure gradient force) drives
the surface winds from the poles towards
the equator.
The winds aloft close the cell by blowing
from the equator towards the poles.

10. The one cell model does not work!
It is obviously wrong: predicts northern prevailing winds everywhere in the NH
What is wrong with the model? It is too simple!
The rotation of the Earth will deflect the winds to the right in the Northern hemisphere and to the left in the Southern hemisphere.
This will result in surface winds blowing:
From the East (easterlies) in the NH
From the East (easterlies) in the SH
This will result in winds aloft blowing:
From the West (westerlies) in the NH
From the West (westerlies) in the SH

11. Observing global winds
Intertropical convergence zone
Observing global winds
from space

12. Winds Aloft
Warm air above the equator and cold air above the polar regions
Higher pressure at the equator, lower pressure both to the north and to the south of the equator
The pressure gradient force is towards the poles, sets the air
in motion
The Coriolis force
NH: to the right
SH: to the left
The wind turns right in the NH and left in the SH, becomes parallel to the isobars
Westerly winds aloft in both the NH and SH.
Easterly winds at the surface in both the NH and SH.

13. The Three Cell Model Keep two of the assumptions, relax the third:
The Earth is covered with a continuous ocean
The sun is always directly over the equator
The Earth rotates -> Coriolis force!

14. Three cell model: the Hadley cell (0-30 deg)
Thermally direct cell: warm air rises, cool air sinks
Intertropical Convergence Zone (ITCZ)
A.k.a. equatorial doldrums
Warm air, weak PGF, light winds, cumulus clouds and thunderstorms
Air rises up to the tropopause, then laterally toward the poles
Deflected east due to the CF
Winds aloft in NH: from southwest
Subtropical highs (anticyclones)
Equatorial air cools, sinks, warms up, clear skies -> major deserts
Air converges (follow the meridians on a globe) – high surface pressure
Horse latitudes: small PG, weak horizontal winds -> sailors get stuck
Surface winds in NH: from the northeast (Trade winds)

15. Three cell model: the Ferrel cell (30-60 deg)
Thermally indirect cell: cool air rises and warm air sinks
Some of the sinking air in the horse latitudes heads toward the pole
Deflected east by the CF
Surface winds in NH: from the southwest (westerlies)
At the polar front the westerlies encounter cold air moving down from the poles
Air is forced to rise, some of it returns to the horse latitudes, completing the Ferrel cell, the rest heads for the pole
Upper air winds in the Ferrel cell: from the northeast.
William Ferrel
William Ferrell

16. Three cell model: the polar cell (60-90 deg)
It is a Hadley type of circulation.
Surface winds: from the north east (polar easterlies)
Upper winds in NH: from the southwest
Summary: two major areas of Low pressure (ITCZ and subpolar low), and two of High pressure (poles and subtropical highs)

17. The converging/diverging regions
ITCZ (Intertropical Convergence Zone ) - Equator
Low surface pressure with small PG and weak horizontal winds.
Upward motion of warm moist air. Results in convective cloud towers
Subtropical highs (the horse latitudes) – 30N; 30S
High surface pressure
The upper air is sinking, warms up and the relative humidity is very low.
Weak winds, clear sky, dry climate – large deserts at these latitudes.
Subpolar lows (polar front) – 60N, 60S
A converging zone at the surface. Air moves up and results in strong storms.
Weak winds
Polar highs – 90N, 90S

18. Winds and pressure in the real world
Semi-permanent highs and lows: persist throughout the year, correspond to converging/diverging upper air masses.
Bermuda, Pacific highs; Icelandic, Aleutian lows
Seasonal highs and lows (continents heat/cool faster)
Winter: Siberian high, Canadian high
Summer (thermal lows): Southwest US, Iran
January
July
Subtropical highs
Subtropical highs

19. The General Circulation and Precipitation Paterns
Converging surface flows:
Low surface pressure
Uprising air
Heavy precipitation
Diverging surface flows:
High surface pressure
Sinking air
Dry climate

20. Winds and Pressure Systems Aloft
The wind system aloft differs from the surface wind system. It is close to a geostrophic flow.
There is no significant friction with the ground.
The three cell model does not work that well in the middle latitudes.
The winds aloft are stronger than on the ground.
In the winter the gradients are bigger -> the winds are stronger.
July
January