Transcript Circulation, ITCZ, Coriolis & Rossby Waves

Why is the highest annual mean
insolation NOT along the equator?
Atmospheric Circulation
L.O.s:
1. To describe and explain atmospheric
circulation.
2. To know the impact of the Coriolis Force
on the direction of wind.
This picture shows the air rising at the equators where it is
heated (convection), causing an area of low pressure and then
travelling up to the poles losing heat as it goes. At the poles it
sinks again (subsidence), being cold, and forms an area of
high pressure. It then blows outwards from the poles towards
the equator along the surface.
In reality the coriolis force and the uneven distribution of land
masses and seas also play a part in the air circulation. This
means that more than one cell is created.
Tricellular Model of Atmospheric Circulation
The Coriolis Force
If the Earth did not rotate at all then the air would move at
right angles to the isobars, in the direction of the pressure
gradient force.
But in reality the Earth
does rotate and the air
moving towards the pole
appears to be deflected to
the right. This is the result
of the coriolis force.
The coriolis force can be explained by using an example of a
roundabout.
If we consider person a
standing in the middle of the
roundabout and trying to
throw a ball to person B
standing at the edge of the
roundabout.
If A throws the ball straight
at B then by the time the ball
has reached the edge of the
roundabout B has further
round.
To the people on the
roundabout it looks like the
ball has curved round to the
right.
Tasks:
• Add to the diagram
where the areas of
high and low pressure
are.
• Add the following
labels:
ITCZ
STHP/STJS
PFJS
3. Add the following
labels for the major
winds:
Westerlies
SE Trades
NE Trades
The Earth’s rotation means that, in the Northern hemisphere, a
wind blowing towards the north appears to move to the right.
This explains why the prevailing wind in Britain is a southwesterly, not a southerly.
Exam Question
• With the help of a diagram, describe the
global distribution of pressure belts and
explain why this varies seasonally.
8 marks
Pilots in WW2
flying at heights
above 8km found
eastward flights
much faster then
their return
westward
journeys.
Why?
Rossby Waves & Jet Streams
The explanation was found to be a belt of upper-air westerlies, the Rossby
Waves which often follow a meandering path. At any given time there is an
average of between 4 and 6 of these waves circulating the Earth. It is thought
they may be caused by substantial relief such as the Andes, Rockies and Tibetan
Plateau although contrasts in surface temperatures are also thought to play their
part.
Investigation has shown that within these waves velocity is not uniform; within
them are narrow bands of extremely fast-moving air known as jet streams.
Speeds can exceed 230km/hr (enough to carry a balloon around the Earth within
a couple of weeks).
The Polar Front Jet Stream (PFJS) forms the division between the Ferrel and
Polar Cells. Where in the N.Hemisphere the jet stream moves south, it brings
with it cold air which descends in a clockwise direction (giving conditions
associated with anticyclones). When the (now warmed) jet stream moves back
northwards, it takes with it warm air which rises in an anticlockwise direction
(giving conditions associated with a depression). As the usual path of the PFJS
over Britain is towards the NE – this accounts for our frequently wet and windy
weather.
The Sub-Tropical Jet Stream (STJS) forms the boundary between the Hadley
and Ferrel Cells. It meanders less than the PFJS, has lower velocities but
follows a similar west-east path.
Description of ITCZ/STHP
•
•
•
•
What it stands for
Location
Where do winds converge/diverge?
Weather associated:
– Cloud cover
– Air pressure
– Precipitation
– Stability
ITCZ
STHP
• Between the tropics, near or on
the equator (0), equatorial side of
Hadley cell. It moves according to
overhead sun (seasons).
• Trade winds converge near
surface. Divergence near
tropopause.
• Intense heat from overhead sun
creates warm air  rises and
cools rapidly (unstable) =
cumulonimbus clouds
(thunderstorms), very heavy rain.
•Low pressure.
• Tropics, 30 N & S, between
Hadley and Ferrel cells. It moves
according to movement of the
ITCZ.
• Converge near tropopause,
diverging near surface – trade
winds to equator and westerlies to
60 N/S.
• Subsiding dry air = little ppn (if
any), dry conditions, few clouds,
high temperatures.
• High pressure.