Lecture 2 Notes
1. Outline of this lecture
a. Part 1: Wind Primer
b. Part 2: New air mass classification system – applied to Toronto
2. Part 1. Wind Primer
What is wind?
Wind cannot be seen directly but can be inferred by its effects such as the movement
of objects (trees, grass) and causing objects to become airborne (dust). Wind can be felt
and it can be heard.
Wind results from the differences in air pressure, either vertical or horizontal.
Differences in pressure produce a pressure gradient. In the absence of other forces air
tends to move from high pressure to low pressure. The force that causes this is called the
pressure gradient force.
Because the earth rotates, another force does act on the wind. It is called a
fictitious force because it directly results from this rotation of the earth. It is called the
Coriolis force. In the Northern Hemisphere it acts to deflect the wind to the right and in
the Southern Hemisphere it acts to deflect the wind to the left. A French nobleman,
Gustave de Coriolis derived the mathematical formulation in 1835, although Englishman
George Hadley had described the effect a century before.
Above the surface of the earth (1 km or more) there is a balance between pressure
gradient force and the Coriolis force forming the geostrophic wind. The only way to
achieve a balance between these forces is for the wind to flow perpendicular to the pressure gradient. This is a wind that is not directed toward a low pressure or from a high
pressure, counter to intuition.
At and near the surface, friction plays a role forming a three way balance of
forces. In this instance there is a component of the wind that flows towards the centre of a
low and away from the high pressure centre. This surface convergence of winds at the
centre of a low is critical for the formation of midlatitude cyclones as will be detailed in
3. Global circulation
Although the upper atmosphere (in particular the stratosphere) experiences winds, we
will focus on the winds in the troposphere. 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 several factors.
Earth‟s rotation creates the Coriolis effect, which causes the wind to deflect (to the right
in the northern hemisphere and to the left in the southern hemisphere). Earth‟s surface
contributes 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.
On the global scale there are three main circulation cells per hemisphere. These are
called the Hadley cell, the Ferrel (sometimes spelled „Ferrell‟) cell and the Polar cell.
These consist 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 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.
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 does rotate and this causes 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 15 and 16 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 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. 3.1 Surface Winds
The three cell theory predicts surface winds that are modified by the rotation of the
earth, causing the winds to deflect to the right in the northern hemisphere and to the left
in the southern hemisphere. This is reflected in the Trades, Westerlies and polar easterlies
discussed in the previous section. Also modifying wind structure is land/sea contrast and
Land/sea contrast arises because the land surface has a lower heat capacity than water
and therefore heats more rapidly and cools more rapidly. A warmer surface leads to a
lower surface pressure, and a cooler surface leads to a higher surface pressure. Therefore,
in the northern hemisphere winter high pressures form over land and lows over the
oceans. The opposite occurs in the summer.
3.2 Upper level winds
Upper level winds are not affected by friction from the surface, so the winds at
this level are generally geostrophic. They are also much less affected by the land/sea
contrast, so high and low pressures are distributed more or less evenly along lines of
3.3 Jet streams
At the upper level (5 to 10 km above the surface) there is a current that is especially
noteworthy. Jet streams, regions of intensified flow, occur. The air flows generally from
west to east with occasional meanders. Two of these occur roughly above the divisions of
the Hadley and Ferrel cells (the subtropical jet) and the Ferrel and Polar cells (the polar
front jet). The jet streams play a key role in determining the nature of the surface
conditions, acting as a guide to surface storms. They also play key roles in aviation. 4.1 Air masses
An air mass is a large body of air spanning up to several thousands of kilometers in
the lower troposphere. An air mass is characterized by horizontally uniform temperature
and moisture content (humidity) at any given altitude. These air masses form in a source
region. This is an area of large horizontal extent with relatively uniform surface
properties such as an ocean or a continent. Air mass theory views the weather as a result
the presence and interaction of these large air masses. The air masses are relatively stable
high pressures with low pressure systems (storms or midlatitude cyclones) forming at the
boundary between these air masses. In this view of the atmosphere, there are a number of
relatively stable high pressure systems punctuated by smaller, energetic low pressures, or
frontal systems, occurring in a battle zone between the air masses.
In North America, there are four main types of air masses. Air masses are classified
using a two letter nomenclature. The first letter (written in lower case) indicates whether
the air mas