General Circulation of the atmosphere

AOS 101 Weather and Climate
Lisha M. Roubert
University of Wisconsin-Madison
Department of Atmospheric & Oceanic Sciences
Today’s Lab
• Today we will be using the tank lab to simulate the general
circulation of the atmosphere.
• By replicating certain atmospheric conditions in the tank we
will have in this tank a system analogous to the atmosphere,
which we can use to simulate atmospheric processes.
Today’s Schedule
Lecture on General Circulation.
Explanation of the experiment.
Instructions for Lab Report due next class.
General Circulation
• The worldwide system of winds,
which transports warm air from the
equator towards the
higher latitudes, is called the
general circulation of the
• Since the pioneering studies of the
atmosphere, scientists have tried to
explain why surface winds blow
from the east at subtropical
latitudes and from the west in the
mid latitudes, and why the trade
winds are so steady compared to the
• Studies of the general circulation
seek to describe the statistical
properties of large scale
atmospheric motion in a global
General Circulation
• The general circulation of air is
broken up into a number of cells,
the most common of which is called
the Hadley cell.
• Air heated at the equator rises and
spreads out to the north and south.
• The air cools and then sinks in the
region of the subtropics. Some of
this sinking air is then transported
further poleward and the rest is
transported back to the Tropics.
• However, Earth is rotating so there
will be some deflection of the air
due to the coriolis force.
Coriolis Force
• The effect of the Coriolis force is an
apparent deflection of the path of an
object that moves within a rotating
coordinate system.
• The object does not actually deviate
from its path, but it appears to do so
because of the motion of the
coordinate system.
Coriolis Force
• Due to coriolis force an air mass
will appear to be deflected
because the Earth underneath
the air mass is rotating.
Coriolis Force, Easterlies and Westerlies
• Coriolis force is zero at the
equator and gets stronger as we
move towards the higher
• In the Northern Hemisphere air
deflects to the right. In the
Southern Hemisphere air
deflects to the left.
• The Coriolis force is the reason
why the upper level, poleward
traveling air (wind) is westerly;
whereas the equatorward
traveling, surface winds are
easterly. In the tropics, these
easterlies are known as the trade
Why are the Westerlies faster than the
• To answer this question we first need to know about
momentum. There are two types of momentum:
▫ Linear Momentum-If an object is moving in a straight
line it has:
Momentum=m × v
Why are the Westerlies faster than the
• Angular momentum: An object that is
rotating has an angular momentum:
Ang. M=m × v × r
r=radius or distance from the axis of
• A rotating undisturbed object will
conserve its angular momentum. This
Ang. M= m × v × r= constant
• When the distance from the rotation
axis (r) decreases, the rotation velocity
(v) increases to conserve angular
Why are the Westerlies faster than the
• The air in the westerly winds
is closer to Earth’s axis of
rotation than the air at the
equator so to conserve
angular momentum it’s
velocity will increase and we
will have fast moving air from
west to east.
General Circulation Tank Lab Experiment
• Today we will be using the tank lab to simulate the circulation
of the atmosphere. Specifically to simulate the Hadley Cell.
• The tank will be rotating slowly-less than 2 revolutions per
• Ice is set up in the middle so that we have a temperature
gradient that will drive motion.
• Dye will be used to observe fluid motion and permanganate
will be used as a tracer.
Lab Report-Due next class
• Title
• Objective: describe the purpose of the lab
• Hypothesis: use your knowledge to make a prediction of
what is going to happen.
• Methodology: describe the set up of the lab and explain
the steps performed in the experiment
• Results: what happened in the experiment, what did you
see, etc
• Discussion: discuss what happened in the tank relating it
to what happens in the circulation of the atmosphere.
(physical explanation)
• Conclusion: Summarize the findings of the lab and give
your opinion about the lab

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