Lecture 14 – Chapter 12
General Circulation of the atmosphere
• Incoming and outgoing radiation are not balanced at individual latitudes
- This drives the general circulation of the atmosphere.
• The circulation system operates differently in the tropics than outside the tropics
o Tropics weather = relatively stable
o extratropical weather = more variables
• This is because there is a surplus of radiation between 40 *N and 40* S, where
there is a deficit of radiation poleward of these latitudes.
(Left side of diagram) As a result, there must be a poleward transfer of energy to
prevent Polar Regions from getting colder and tropical regions from getting warmer.
(Right side of diagram) Easterly winds in the tropical latitudes gain momentum from
Earth and westerly winds of the mid latitudes lose momentum to Earth.
Momentum is the product of mass and velocity
- Angular momentum: the product of mass, is conserved in a rotating system.
m × v × r
- if we want to conserve momentum, it must be transferred
o ex. Transferred from the easterly winds to the westerly winds to maintain
The general circulation: must transport both energy and momentum toward the
- Tropics: accomplished by simple convection
- Mid-latitudes: accomplished by weather systems in the form or eddies and
waves Descriptions of the General Circulation
Simple Case - (2 assumptions)
• The sun is always overhead at the equator
• Earth’s surface is composed of one surface type
Surface wind form in response to the following Pressure patters:
- Equatorial low
- Subtropical highs
- Subpolar lows
- Polar highs
*Remember the winds are named for the direction they
Trade Winds: steady wind from the tropical region
- Subtropical highs to equator low are deflected by
Equatorial Low: or Intertropical Convergence zone (ITCZ) is where trade
winds from two hemisphere converges
Mid-latitude westerlies: are created between subtropical highs and
- they experience much greater deflection because poleward increases the
strength of coriolis force.
Polar esterlies: are created between polar highs and subpolar lows
*The equatorial low and polar highs are THERMAL in origin
*The subtropical highs and subpolar lows are DYNAMIC in origin
Annually averaged pressure and wind patterns at the Earth’s surface. Alternating
• Annually averaged pressure and wind patterns aloft
• High pressure over the equator
• Low pressure over poles.
• Coriolis produces prevailing westerlies. The real case:
• The assumptions no longer apply.
• The sun is directly 23*N in the June Solstice and 23*S on the December
solstice. The surface is not homogenous and so it takes into account of the
influence of the differences in heating and cooling of land and water on surface
• Location of maximum solar angle shifts with the seasons – ITCZ moves south in
January and north in July. Other pressure systems shift seasonally as well.
• Continents produce uneven heating and cooling.
o Aleutian low & Icelandic low: Thermal lows over warmers continents in
o Canadian High and Siberian high: Thermal highs over cooler continents in
Monsoon: A circulation pattern that leads to very wet summers and very dry
- Thermal lows over the continents in July are important for this process
The tropical Circulation
Haley cell – a thermally direct cell with warm air rising at the equator and colder
air sinking in the subtropics.
- It produces equatorial (thermal) low along the ITCZ. It also produces subtropical
(dynamic) highs at around 30*.
- The clouds created are cumulonimbus clouds and has copious amounts of
rainfall. The ITCZ is also known for being a location of very weak and variable
winds due to lack of strong pressure gradients (doldrums).
- In the center of subtropical highs is another region of light and variable surface winds (horse latitudes)
The extratropical Circulation –
• Mid-latitude westerly – not steady as the trade winds. Westerly on average. This
is because flow is disturbed by the travelling cyclones and anticyclones
responsible for the variable weather on mid latitudes.
IN DETAIL: Just as warm air flows toward the pole from subtropical highs, cold air
flows toward the equator from the polar highs. Remember the flow is deflected
by Coriolis force, thus producing the polar easterlies.
Polar front: ideally represents the meeting of a cold polar air and warm tropical
The circulation Aloft
• Equatorial thermal low becomes high pressure aloft
• Polar thermal low becomes low pressure aloft
• Coriolis force produces upper air westerlies.
• The equatorial to pole temperature gradient leads to a surface pressure gradient
directed from the poles to the equator. The pressure gradient reverses and
strengthens with height.
The subtropical Jet streams
align sinking air and subtropical highs at the surface
• • Conservatio
n of momentum
velocity to greatly increase.
Polar front jetstreams: have more variable paths. Net flow is zonal but
meandering produces meridional components.
Rossby waves or long waves are associated with polar front jet streams.
• Strongly influence mid latitude weather (influences: temperature advection and
- extra tropical
Lecture 15 – Chapter 13
Describe the characteristics of an air-mass source region and distinguish
between five major air mass types (Chapter 13);
Describe and account for air mass paths in North America and explain how
these masses change as they move (Chapter 13); Describe weather changes that can occur when air masses move into a region
Distinguish between the four types of fronts and describe their structures and
weather changes that occur during their passage (Chapter 13)
Air masses are large bodies of air with similar (horizontal) temperature and
humidity characteristics. Reflect energy and water transfer between Earth’s
surface and air layers in contact with the surface. The best source regions are
areas that have uniform surface properties stretching over thousands of square
kilometers. They must be relatively flat (land or water but not both). Winds must
be light because air must remain stagnant for up to two weeks. Good source
regions are therefore associated with large, semi-permanent anticyclones of the
general circulation, polar high and subtropical highs. Mid latitudes are not good
• Air over warm surfaces becomes warm
• Air over moist surfaces become humid
• Air over cold surfaces loses heat
Air masses form as:
- Heat is transferred between the surface and the air
- Water vapour is transferred between the surface and the air
Air-mass source regions are
- Thousands of square kilometers in area
- Of uniform surface type - Usually associated with semi-permanent high-pressure cells
The path an air
and, in turn, they are modified by their new surroundings.
For example, a cold air mass moving
southeastward from northern Canada in winter will warm as it travels. As a
result, it may cause large temperature drop in southern Ontario but it will not
have a strong effect on temperature by the time it reaches Florida.
An air mass that is colder than the surface over which it is travelling will be warmed
from and made unstable. On the other hand, an air mass that is warmer than
the surface over which it is travelling will be cooled from below and become
Lake- effect snows: snowfall that occurs
downwind of large, frozen lakes
As cP air moves south it can pick up moisture.
When it picks up a lot of moisture from large,
unfrozen bodies of water (Great Lakes) it produces lake-effect snows. As the cP air
flows over these lakes from north or the west it is warmed below and picks up
moisture. Warming makes the air unstable and the instability leads to the
formation of clouds and release moisture as snow. These snows are unique in
that they cause areas just downwind of the Great Lakes to have the greatest
snowfall accumulations of the region.
Pineapple express – mT air from the Pacific (Hawaii) reaches North America at
least once a year during fall or winter. Very heavy rainfalls and snowmelt
Dry line: a boundary separating warm
moist air from warm dry air
• Transition zone between cT (dry)
and mT (moist, less dense) air.
• mT air rises over cT air –
• Not a true “front” because
temperatures are similar on both
sides of these lines.
Following changes are associated with the passage of fronts
- Increase or decrease in temperature
- Increase or decrease in dew-point temperature
- An occurrence of clouds and precipitation
- Decrease in pressure
- A shift in wind direction
• Boundaries between airs of different properties – most commonly different
• Uplift of less dense air mass (clouds, precipitation, drop in pressure)
• Wind direction shifts clockwise
• Temperature and dew point temperature are likely to change as front moves
Stationary front: no significant movement
Frontogenesis: the process of increasing a temperature gradient e.g. surface convergence,
differential heating / cooling
Frontolysis: weakening or dissipation of a front e.g. surface divergence, differential
heating / cooling Weather associated with Cold Fronts
Cold Fronts have
cold air advancing
and replacing warm
- can produce
and the possibility of tornadoes.
Weather associated with Warm Fronts
Warm Fronts have
cold air retreating
and warm air moving
Gentle lifting of
sleet or snow
Occluded Fronts: front separates a cold air mass from a cool one, and low
pressure center from warm air in a mid-latitude cyclone
• Occur if a cold front catches up and overtakes a warm front
• Occlusion – closed off
• Coming together of cold, cool and warm air masses. Looking at the picture above.
A) The occluded front separates the two coldest air masses at the surface, and it
lies between the low-pressure center and the warm air mass.
B) A cross-section along the line AB showing where the warm front and cold front
would be located on the surface
C) A cross section along the line CD showing where the occluded front and the
TROWAL would be located on the surface
8Warm air is lifted up forming a trough above the surface. Air behind the cold front
meets the air ahead of the warm front.
Remember it as… TROWAL: TROugh of Warm air Aloft
Occlusion: Process that gr