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Lecture 3

EESA09H3 Lecture Notes - Lecture 3: Air Mass, Weather Balloon, Radiosonde


Department
Environmental Science
Course Code
EESA09H3
Professor
Tanzina Mohsin
Lecture
3

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Lecture 3 Notes: Global Wind Circulation
The basic physics behind the formation of winds is pressure differences. As described above, air has a
natural inclination to move from areas of high pressure to those of low pressure. This is modified by
seeal fatos. Eaths otatio eates the Coiolis Effect, which causes the wind to deflect (to the right
i the othe heisphee ad to the left i the southe heisphee. Eaths sufae otiutes
friction which slows down and redirects (funnels) the wind. Land/sea contrast leading to temperature
differences between land and water surface also influences wind. Finally, winds vary with season.
There are three broad categories of atmospheric circulation:
1. Gloal ~ ,s of KMs
2. Synoptic ~ 100s to 1000s of KMs (Lec 4)
3. Small scale < 100 KM (Lec 7- Thermal Winds)
Global Circulation
Observation: There is more energy released in the Polar Regions than is received from the sun and the
reverse is true of Equatorial Regions. We need to understand two theories to grasp this poleward
transport of energy.
Atmospheric Heat Transport
Sensible Heat: heat that e a feel
Latent Heat: stored as water vapor, heat is absorbed when water evaporates and released when water
vapor condenses.
One Cell Theory: there is one large overturning atmospheric Hadley cell where air rises at the equator,
moves poleward and then sinks at the poles. Energy Transportation: Energy is transported as sensible
and latent heat. Latent heat transport occurs when moist equatorial air moves poleward cools,
condenses, and releases latent heat. However this theoy has thee ai poles that dot allo it to
explain phenomena we experience on Earth. Cat explain prevailing wind from the east and it also
at explain prevailing wind in the mid latitudes from the west.
WHY IS ONE CELL THEORY NOT ENOUGH TO EXPLAIN EARTHS PHENOMENA?
1. Rotation introduces the Coriolis effect
2. Tilt causes seasonality
3. Continentality causes land/sea variation due to varying thermal inertia (e.g. Monsoons)
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Three Cell Theory: On the global scale there are three main circulation cells per hemisphere. These are
alled the Hadley ell, the Feel soeties spelled Feell ell ad the Pola ell. These osist of
vertical and horizontal winds. The Hadley cell is characterized by air rising in the equatorial region,
driven by warm surface conditions. In the upper troposphere this air starts to move horizontally towards
the poles. At 30N and 30S, air descends to the surface. At the surface the air returns to the
equatorial region to complete the circulation. This circulation occurs in both the northern and southern
hemispheres. Poleward of the Hadley cell is the Ferrel cell. This cell shares the descending branch of air
at 30N and S with the Hadley cell. At the surface, the air moves poleward until about 60N and S
under the influence of the Coriolis force (veers to the east) and then ascends. The circulation is
completed by a return flow equatorward in the upper troposphere. The Polar cell shares the ascending
branch with the Ferrel cell at 60N and S. In the upper troposphere the air from this cell flows
poleward and then descends at the pole. To complete circulation, air flows equatorward at the surface
to 60N and S. The Feel ell is desied as eig theally idiet eause heat ist etiely
sinking.
If the world did not rotate, this simple picture would be a good model for the winds at the surface of the
earth. However, the earth rotates and this leads to a deflection of winds due to the Coriolis force (take
EESB03H3, Principles of Climatology to find out more about the Coriolis Effect). As a result we have a
surface distribution of winds that are different than this three cell model would initially suggest.
Between the equator and 30N the wind should flow to the south. However, with the rotational
deflection the winds actually flow to the southwest; these are called the Trade Winds. They are the
steadiest winds in the world. They were used in the 15th and 16th century by Europeans to explore
North America (Christopher Columbus used them in 1492).
In the absence of rotation the winds from 30 to 60N should flow north. The rotational deflection
causes these winds to move in a northeast direction. These are called the Westerlies, named for the
direction from which they come (not the direction they are heading). The Westerlies are the prevailing
wind in the midlatitudes and therefore the predominant wind in Toronto. North of the Westerlies we
should get winds flowing south from the pole to 60N. However with the deflection these winds head
to the southwest and are called polar easterlies indicating the direction from which they come from.
The horse latitudes (subtropical high) occur near 30N and 30S. This is a region of little wind
between Westerlies and Trade Winds and corresponds to the descending branch of Hadley/Ferrel cells.
It was named by European sailors; ships in this region did not make much progress because there is no
strong prevailing wind. Thus, they had to jettison some of their cargo which included horses (the
livestock would die off as water and food ran out). The doldrums are a region of little wind between
North and South Trade Winds. This corresponds to the ascending branch of two Hadley cells, also called
the Intertropical Convergence Zone (ITCZ) because of the surface convergence of air from either side of
the equator. The Polar Front is a very active region between the Ferrel and Polar cells, where cold air
and warm air collide. It is the source of the most intense large scale storms in the midlatitudes (more on
this in Lecture 4). It meanders north and south considerably with season. Polar easterlies are winds that
blow south west from the North Pole (northwest from South Pole).
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