Download high and low pressure systems. The belts which most influence us in

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Wind and Weather Although we cannot see the wind but
as paddlers we are always aware of its
effects on us and on the waters we
paddle. A gentle breeze in the face
paddling upwind can be quite refreshing. The same wind seems to disappear when we turn and paddle downwind. This occurs when we are paddling about the same speed as the
wind. The effect can be quite eerie and
in summer we lose the cooling effect.
Wind at higher speeds can make paddling quite exiting and invigorating but
as wind speeds increase paddling can
become quite hazardous.
A knowledge of wind and its effects is
a valuable asset for paddlers comfort
and safety. This article is the first of a
series designed to help paddlers make
sound risk management decisions concerning when and where to paddle.
The science of wind and weather meteorology, is large and complex and
meteorologist are still researching the
causes of weather and trying to
improve forecasts. Clearly then this
article can only give a simplified
overview of the broad principle of
wind.
The big picture
Wind is the movement or flow of air.
The large wind systems occur due to
the different temperatures across the
earth from the tropics to the poles. Air
over the tropics heats up and rises. As
it does so it cools and descends again
and the cycle restarts. This simple pattern is complicated by the difference in
temperature over the oceans compared
with the continents.
As air rises the pressure at sea level
falls. As air falls pressure is increased.
These air pressure is recorded and
mapped by meteorologist and published as the synoptic charts seen on
the TV weather or newspapers. The
maps are made by joining points of
equal pressure called isobars (see fig.
2). The H indicates the centres of high
pressure systems (anticyclones) and L
the centre of low pressure systems
(cyclones or depressions).
In each hemisphere working out from
the low pressure belt around the tropics
(Doldrums) are a alternating bands of
Alan Jones AC Instructor
high and low pressure systems.
The belts which most influence us in
and around Sydney (latitude 33 deg)
cal cyclones. High winds are common
in low pressure systems.
In a high pressure system, air is settling
slowly downwards as it spirals gradually outwards . This gentle subsidence
tends to give rise to clear air and often
sunny days. Sometimes however stagnant air can trap low cloud giving a
murky cloudy day. This gentler air low
usually means gentle winds.
are the Sub-tropical High Pressure belt
(approx. 24 -45 deg) and the Westerlies
and Roaring Forties (approx. 40-50
deg). These belts migrate north and
south following the suns seasonal pattern.
In summer the high pressure belt
moves south over Sydney. In winter
this belt migrates north with sun allowing the westerlies low pressure belt to
move up over southern Australia.
All of the systems gradually move
around the earth from west to east.
Hence we say that our weather comes
from the west. We may get Perth
weather a few days after they do and
New Zealand a few days after us.
The wind and the earths rotation
Air flows flows from high to low pressure but it does not do so directly. The
rotation of the earth deflects the flow
to the left in the southern hemisphere
and to the right in the northern hemisphere. This is known as the Coriolis
force (after its discoverer). In the
southern hemisphere, the net effect is
that air flowing from high pressure to
low is deflected left and circulates
gradually outwards anti-clockwise
around the high pressure cell and
clockwise around the low pressure cell.
Wind speed.
The higher the high and the lower the
low the faster the air (wind) will travel.
Also the greater the pressure gradient
the faster the wind. The closer the isobars are together, the steeper the gradient, the faster the wind.
Fronts
Warm fronts are rare in the southern
hemisphere. Cold fronts are masses of
cold air from the southern polar region
usually moving clockwise around a
low with the whole system moving
west to east. Cold fronts are marked
on the weather map as a thick line with
bards or spikes on its leading edge (see
fig 2).
In summer when the lows are usually
way south the fronts are associated
with the troughs of low pressure
between highs. Two such fronts can be
seem in fig.2, one south of perth and
the other extending from Adelaide and
continuing south through Melbourne.
This front reached Sydney a couple of
days later.
Fronts, wind and rain
A cold front is mass of cold air which
crawls across the countryside like a
wedge, forcing the warm air in front of
it to rise.
At Sydney’s latitude the Coriolis force
is such that the wind approximately
follows the isobars.
Highs and lows
As air spirals into low pressure cells it
has to go somewhere. It rises. As it
does so it cools and water vapour it is
carrying condenses and forms clouds
which often bring rain, hail or snow.
These are sometimes referred to as rain
depressions or to our north deep
depressions sometimes become tropi-
The above diagram shows a typical
pattern. As the warn air rises condensation takes place forming large fluffy
cumulus clouds which may bring rain,
hail or snow. Behind the front subsiding air may trap a layer of low cloud
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and coming from the north-east. A
fairly typical pattern in late summer in
Sydney. Since the wind has passed
over the warm northern Pacific Ocean
it picks up moisture. As is reaches the
land which is hotter than the ocean the
air rises forming a mass of coastal
cloud seen over Queensland and northern NSW. Also in the two figures the
cloud masses associated with the tropical lows to our north can be seen.
These will form the monsoons and the
big wet in the north.
Wind speed and paddling
Wind speed is often give in Knots i.e 1
nautical mile/hour. Knots are used
because 1 minute of latitude is 1 nautical mile. One nautical mile is 1.8 kilometres. Conversion is easy, double the
knots and take about 10% off e.g 10
knots is (2 x 10) - 2 = 18 kph.
Source: Japan Meteorological Agency via Bureau of Meteorology, Australia
Australian Region Satellite Image - Wed Feb 05 12:30pm EST - Fig 1.
Paddling speed can be about halved
heading into a 10 knot wind, so paddling back from somewhere in a 10
knot wind will take twice as long or be
very tiring. Paddlers should also note
that wind gusts can be 40% higher than
forecast.
The Beaufort Scale
The beaufort scale uses an index for
wind speed and associated term as
below;
0 Calm 0kn (0kph) Sea mirrorlike
Source: Bureau of Meteorology, Australia
Mean Sea Level (MSL) Analysis - Wed Feb 05 2003 10am EST - Fig 2.
bringing continuous rain.
There is a wide variation associated
with the passage of cold fronts. The
temperature drop is more noticeable in
summer as the difference in land and
sea temperatures is greater. This is
sometimes referred to as a ‘cool
change’ with less likelihood of cloud
and rain. In winter the change in temperature are usually quite small.
Weather map and satellite images.
Satellite images are useful in forecast-
ing the major weather patterns. In the
image for 5 February 2003 above
(fig.1) the Band of cloud associated
with the cold fronts (see fig.2) can be
clearly seen. Note the clear air over
the highs.
Which way were the winds blowing in
Sydney that day? The air spiralling out
of the high in the tasman sea roughly
follows the isobars. The isobars are
fairly widely spaced and the difference
in pressure between the trough and the
high is not great. So the wind is light
1 Light air 1-3kn (1-5kph) Ripples
like small scales without crests
2 Light breeze 4-6kn (6-11kph)
Small wavelets, glassy appearance but
do not break.
3 Gentle breeze 7-10kn (12-19kph)
Large wavelets; crests begin to break;
foam of glassy appearance; some scattered white horses.
4 Moderate breeze 11-16kn (2028kph). Small waves 1-1.5m, fairly
frequent white horses
5 Fresh breeze 17-21kn (29-38kph)
Moderate waves 2-2.5m; many white
horses some spray.
(...and so on to force 12 Hurricane)
Sources:
The Wonders of the Weather, Bob
Crowder. Bureau of Meteorology 2000
Bureau of Meteorology
www.bom.gov.au
web site.
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Wind and Weather 2 -
Alan Jones AC Instructor
Despite this ‘big picture’ system of
winds, the wind sometimes seems to
defy the larger pattern and seems to be
blowing from a different direction.
What other forces are at play?
The major wing patterns are driven by
the differential heating and cooling of
the earth’s surface by the sun. So too
are local wind systems.
Fig.1 High and Low Pressure Systems
Source: Wonders of the Weather,Bob Crowder. Bureau of Meteorology, Australia
As mentioned in the last article the
winds generally follow the isobars at
our latitude. The arrows in Fig.1 show
the general wind direction.
Air travels (wind) clockwise round low
pressure systems (L) spiralling slightly
inward towards the centre. Outside the
tropics these low pressure systems are
called sub-tropical cyclones.
The low to the east of Tasmania ,see
Fig 1, is producing south-easterly
winds in Sydney. Since the air has just
passed over a large stretch of cool
ocean the air is moist, rising in the low
and rising over a warmer land mass
land it probably means cloud or rain in
Sydney. The isobars are relatively
close showing a steepish gradient from
high to low, consequently the air flow
(wind) will be quite fast i.e Sydney
will be experiencing strong southerly
winds along the coast.
Air moves anticlockwise around the
the high pressure systems (H), anticyclones, spiralling outwards slightly
from high to low pressure.
Fig.1 shows a large high pressure cell
over the Great Australian Bight.
Adelaide will have light (isobars far
apart) southerly winds. Air is falling in
the high so it will probably be cold and
dry. Perth will have light N-NW
winds.
This high will move west bring clearer
skies as it does so.
Pressure systems generally move from
west to east. WE may get Perth weather three days later and New Zealand
cops it a couple of days later than
Sydney.
Summer patterns
In summer a belt of high pressure cells
For example the ‘sea breeze’. On a hot
summers day the land heats up much
more than the sea. As a result air over
the land heats up, becomes lighter and
rises. Cooler air over the sea is sucked
onshore to take its place. The bigger
the temperature difference between the
sea and the land, the stronger the wind.
The sea breeze initially blows at right
angles to the line of the coast. On the
east coast the direction is from the east.
However the earths rotation (the
Coriolis effect) deflects the wind to the
left i.e the wind comes from the NE.
This Nor-easter is a common occurrence in Sydney summers. It strengthens as the day heats up and dies off in
the evening as the sun sets.
The opposite affect occurs at night.
The land cools faster and the air over
the sea is relatively warmer and rises
sucking the air off the land. An off
shore breeze. This wind is usually
weaker than the sea breeze as the temperature differential is less.
Systems Interact
Fig.2 High pressure belt
Source: Wonders of the Weather,Bob Crowder.
Bureau of Meteorology, Australia
are generally centred over the south of
the continent with the lows well off
shore to the south (see Fig 2.). The
prevailing winds are generally from
the north east following anticlockwise
around the highs. In winter the high
pressure belt moves north following
the sun. This allows the lows to move
north closer to and over the continent
bringing colder southerly winds.
Sea Breezes
The strength and timing of the sea
breeze depends upon the strength and
direction of the prevailing wind (the
big system). Take for example a high
over the Pacific producing a light NE
wind on a Sydney summer day. Highs
tend to produce clear skies so the land
will get progressively hotter. The sea
breeze will also be from the NE so it
will augment the prevailing Northeaster. This effect is called a ‘Black’
North-easter.
The opposite is true if the prevailing
wind is from the south. This will
oppose the sea breeze from the NE and
reduce wind strength.
Winter Patterns
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Myall Lakes 2003
Annual Camp
To the south of Australia , the Southern
Ocean extends around 3000km to the
Antarctic continent. Parcels of very
cold polar air move northward, warming up somewhat as they pass over the
ocean. The air is still cool when it
reaches Australia. These cold air
masses form the cold fronts which
bring the cold wet winter days.
Southern Ocean cyclones (lows) are
giant storms, often 2000km across, and
as mentioned earlier generally circling
the earth from west to east. This
cyclone belt moves further north in
winter bringing cold southerly winds
and storms to southern Australia.
22 July
A well executed reverse sweep - Cassie 2yrs
A typical winter pattern is illustrated
by the 4 day synoptic charts for 22 to
25th July 2000.
On the 22nd July a high is centred over
eastern Australia and lows in the Bight.
The wind circling anticlockwise
around the high and clockwise around
the lows, and roughly following the
isobars, will be producing westerly
winds over Sydney. Since this air has
just passed over the cold arid central
Australia it will be cold and dry. The
dry air is descending into the high so it
is likely that Sydney is experiencing a
cold clear day. Two cold fronts
(toothed lines) can be seen in the Bight
moving towards Melbourne.
23 July
A cool dude kayaker - Christopher 7yrs
24 July
By the 23rd the high is moving offshore to the east and the lows have
converged. The cold front will be
bringing cloud and rain to Adelaide
and Melbourne. The wind is Sydney
has shifted to the NW and the skies
clear will still probably be clear.
By the 24th July the lows are dominating the weather pattern bringing
cloudy, cold and rain along the Victoria
and NSW coast and possible storms.
The wind will be fairly strong (close
isobars) from the W to SW.
By the 25th July the low is still hanging around south of Tasmania with
southerly winds in Melbourne tending
W to NW in Sydney. Since the wind
has now come from the southern ocean
it will be a cold wind with some moisture. As it hits the continent, which is
warmer than the ocean and rises over
the dividing range it may produce
cloud and rain from the west.
A cool dude canoeist - James 9yrs
25 July
Fig.3 Typical winter synoptic charts
Source:
www.bom.gov.au/info/ftweather/contents.shtml
Sources:
The Wonders of the Weather, Bob
Crowder, Bureau of Meteorology.
Priced at around $40 this is 260 pages
worth of excellent reading and reference from mid-teens to adult. It is written in ‘plain English’ without compromising the science.
Bureau of Meteorology Web site;
Myall views through the paperbarks
Perfect one day, perfect the next. On
again next year mid March, Violet Hill,
Myall Lakes National Park.