Hum for a Blustery Day
Oh the wind is lashing lustily
And the trees are thrashing thrustily
And the leaves are rustling gustily
So it's rather safe to say
That it seems that it may turn out to be
It feels that it will undoubtedly
It looks like a rather blustery day, today
It sounds that it may turn out to be
Feels that it will undoubtedly
Looks like a rather blustery day today
Atmospheric Pressure and Winds
 Atmospheric pressure is the pressure exerted by the
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atmosphere at the surface of the Earth. It is due to the
weight of the air.
Atmospheric pressure is measured using a barometer.
The unit of atmospheric pressure is the millibar (mb).
The average sea-level value is 1013 mb.
Points of equal atmospheric pressure are shown on a
weather map by lines called isobars.
Like this ...
More about Pressure ...
 The layers of the
 Climbers need to take
atmosphere closest to the
ground surface have the
greatest weight acting
upon them, so pressure is
greatest here.
 Consequently, air pressure
decreases with altitude. At
the top of the highest
mountains it is very low.
similar precautions to
surfacing divers to guard
against falling air
pressure.
 Air passengers are
protected from low air
pressure in the upper
atmosphere by
pressurised cabins.
But ...
 Atmospheric pressure also
varies horizontally,
because it is a direct
function of temperature.
 When the temperature
rises, air expands and rises
by convection, and
pressure decreases.
 Conversely, when the
temperature falls, air
contracts and becomes
denser, causing an increase
in pressure.
Pressure and the Weather
 High pressure occurs where air is descending and is
associated with dry weather. This is because air warms
as it descends, leading to the evaporation of most
water vapour.
 Low pressure occurs where air is rising. It is generally
linked to precipitation and windy conditions. As it
ascends air cools, and as a consequence it cannot hold
as much water vapour. The water condenses into
droplets, which become clouds at condensation level.
The general atmospheric
circulation system
 The differential heating of the Earth’s surface is
sufficient to create a pattern of pressure cells. The
movement of air within each cell is generally circular
and, overall, is responsible for the transfer of surplus
energy from equatorial regions to other parts the
Earth.
 A three cell model forms the basis of our
understanding of global circulation.
 The three cells in each hemisphere are known as the
Hadley cell, the Ferrel cell and the Polar cell.
The 3 Cell Model
The Hadley Cells
 The two Hadley cells,
one in each hemisphere,
form the basis of tropical
air circulation.
 They are responsible for
the seasonal changes in
the climate of these
regions which experience
a wet and dry climate.
And just to make it even
more fun ...
Each
Hadley Cell
can be
divided into
4 components!
1) The Inter-Tropical Convergence Zone (ITCZ)
• Between the two Hadley Cells
there is an area of low pressure in
equatorial latitudes which is
known as the inter-tropical convergence zone or ITCZ.
• As the sun is always high in the
sky, the ground heats rapidly by
day and there is much surface
evaporation.
• As the hot air rises in convection
currents, an area of low pressure
develops. This rising air cools and
the water vapour eventually
condenses, giving heavy rainfall.
2nd Component ...
•At high altitudes the air
moves polewards.
•This air usually circulates
as upper westerly winds
around the planet due to
the deflection effect of the
rotation of the Earth,
known as the Coriolis
effect.
•The net effect, though, is
still for the air to move
polewards.
3rd Component ...
• Around 30ºN and 30ºS the
colder air at higher altitudes
begins to sink, or subside,
back to the Earth’s surface.
• As this air descends, it
warms and any residual
moisture evaporates.
• At the surface, high
pressure is created, with
cloudless skies.
• These areas are known as
the subtropical anticyclones.
4th component ...
• On
reaching the ground, some of
the air returns towards equatorial
areas as consistent winds known
as the trade winds.
• These air movements are also
subject to the Coriolis effect and
are deflected to the right in the
northern hemisphere and to the
left in the southern hemisphere.
• As a result they blow from a
northeasterly direction in the
northern hemisphere and from
the southeast in the southern
hemisphere.
•The two trade wind systems
move air towards the equator
where it forms the ITCZ.
The Ferrel and Polar cells
 A second cell called the Ferrel cell occurs at higher latitudes
(between 30º and 60ºN and 30º and 60ºS). This is responsible for
the climate types occurring in the mid-latitudes.
 Here, air on the surface is pulled toward the poles, forming the
warm southwesterly winds in the northern hemisphere and the
warm northwesterlies in the southern hemisphere.
 These winds pick up moisture as they travel over the oceans. At
around 60ºN and 60ºS, they meet cold air, which has drifted
from the poles.
 The warmer air from the tropics is lighter than the dense, cold
polar air and so it rises as the two air masses meet. This uplift of
air causes low pressure at the surface and the resulting unstable
conditions result in the mid-latitude depressions,
characteristically experienced in the cool temperate western
maritime (CTWM) climate ... Yes, that’s us folks!!
What happens next?
 On reaching the troposphere, some of this rising air
eventually returns to the tropics as part of the Ferrel
cell circulation; some is diverted polewards, as part of
the Polar cell.
 On the surface at the north and south poles,
descending air from the Polar cell results in high
pressure. Remember that winds always blow from
areas of high pressure to areas of low pressure.
 In both the northern and southern hemispheres they
are pulled towards the mid-latitude low-pressure belt,
which occurs at around 60ºN and 60ºS.
Let’s look at this again ...
If only that was the whole story ...
 The three cell model does not allow for the influence
of depressions/anticyclones or high level jet streams in
the redistribution of energy.
 So ... more recent approaches, known as wave theory
models have now been developed to explain the
behaviour of the upper air westerly air streams (Rossby
waves) and jet streams.
A strange phenomenon ...
 It is known that in the upper atmosphere winds blow
around the Earth in a westerly direction.
 Pilots first noticed these when they were blown off
course when flying north to south, and they also found
that they could travel much more quickly than
expected when flying from west to east!
Oops
Rossby Waves
 Rossby waves follow a wavy
 The waves occur between four
undulating, pattern as they
travel around the Earth’s upper
atmosphere.
 The reason for their existence is
not entirely clear but some
people believe that they are due
to the upper air flow being
forced to divert around the great
north-south mountain ranges of
the Rockies and Andes in the
northern and southern
hemispheres respectively.
and six times in each
hemisphere and they can
stretch from the polar
latitudes to the tropical
latitudes.
 Once a wave motion has
begun, it is perpetuated
around the planet.
 The waves have considerable
variation in amplitude during
the year.
Rossby
Waves
and the
Jet
Stream
Jet Streams I
 Within the upper westerly winds are bands of extremely
fast moving air (up to 250 km/hr) called jet streams.
 A jet stream can be hundreds of kilometres in width but
with a vertical thickness of just one to two thousand
metres.
 On average they are found at altitudes of 10,000 metres.
 They are the product of a large temperature gradient
between two air masses which have markedly different
temperatures. There are two main locations of jet streams:
Jet Streams II
The polar front jet stream
(PFJS)
The sub-tropical jet stream
(STJS)
 This is a westerly band of wind,
 This is another generally
associated with the meeting place of
cold polar and warm tropical air high
above the Atlantic Ocean, somewhere
between latitudes 40 and 60º N and
40 and 60ºS.
 The precise location of the jet stream
varies, but airplane pilots seek to ride
in it when going from west to east, and
to avoid it when flying from east to
west.
 It marks the division between the
Polar and Ferrel cells, and helps to
explain the formation of mid-latitude
low pressure weather systems, or
depressions.
westerly band of wind and is
associated with the pole-ward
ends of the Hadley Cells at
approximately 25ºN and 35ºS.
 However, in summer above West
Africa and Southern India this
jet may become easterly.
 This is due to temperatures over
the land in these areas being
higher than over the more
southerly sea areas.
Oceanic circulation
 The large-scale movement of water within the oceans is part of
the horizontal transfer of heat from the tropics to the Polar
regions and is responsible for around 20% of the total transfer of
heat within the energy budget.
 Each ocean has its own particular circular pattern of currents
(called a gyre) that are produced as masses of water move from
one climatic zone to another, but they all share a pattern which is
similar as they are all initiated by the same factors.
 Ocean currents are largely set in motion by the prevailing surface
winds, associated with the general atmospheric circulation. They
allow heat to become more equitably distributed throughout the
world’s major climate zones. The direction of water movement is
also deflected by the Coriolis force.
Ocean Currents:
The World’s major ocean currents are particularly
dominant along the western sides of the ocean
basins and currents that are less well-defined and
relatively weak are on the eastern sides.
And finally ...
 Heat is transferred by warm ocean currents, such as
the North Atlantic Drift in the Atlantic Ocean, from
the low to high latitudes.
 This warming influence is particularly dominant
between latitudes 40º to 65º where winds blow
onshore, on the western sides of continents and is
confined to the winter season.
 Cold ocean currents generally have less effect upon
temperatures because they usually lie under off-shore
winds. One exception is the Labrador Current off the
East coast of North America.
That really is all ... But do check out this
website!
http://oceans.greenpeace.org/en/theexpedition/news/trashing-ouroceans/ocean_pollution_animation