Download More about the Weather

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 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 X 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 undulating,
 The waves occur between four and six
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.
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)
 This is a westerly band of wind, 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.
The sub-tropical jet stream
(STJS)
 This is another generally westerly band
of wind and is associated with the poleward 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