Download Jargon busted in weather maps

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Climate and sustainability
Weather tools
100E
FreecalL 1800 677 761
110E
120E
130E
140E
1012
1016
1012
20S
Low
pressure
centre
L
1012
1008
30S 1004
1000
H
1012
1008
Ridge of
Higer Pressure
1000
1004
1008
40S
H
Strong
southwesterlies
996
980
975
50S
80E
1020
Lindsay
996 Smail
1016
1004
Consultant
1008 1000 992
992
1000 1008
1004 996 984
110E
120E
130E
140E
90E
100E
Forecasts: A typical winter weather map
gives enough information for a forecast of
several days ahead.
30S
1004
1000 40S
996
Cold front
20S
High pressure 1020
centre
H
Jargon
busted in
weather
maps
10S
Isobar for 1012
hectoPascals
10S
Source: Bureau of Meteorology.
150E
50S
L
988
150E
984 988 992
160E
Lindsay Smail of Geelong Weather
Services unravels the jargon of weather
maps to make ‘reading’ these important
published charts easier.
Everybody recognises the TV weather
map but, despite all its fancy colours and
graphics, it still remains a mystery to some.
The weather map, or synoptic analysis
chart, as the meteorologists call it, shows
the current weather and what can be
expected in the next few hours or even days
(see Figure 1). But be careful to check
the date and time first. Maps that are more
than a few hours old will need that fact taken
into consideration.
In Australia, south of the
Tropic of Capricorn, most
weather systems tend to
move from west to east.
At a glance
When interpreting weather maps,
take note of the date and time,
even if they are only a few hours
old, and take the time difference
into consideration.
What farmers like to see at the
right time is a cooling lowpressure system because that
means rain is on the way.
While a high means little chance
of rain, this could change if
a low occurs above the
high-pressure system.
So, while it might not be true to say that
Melbourne gets Perth’s weather one week
later, it is often the case that the system
causing Perth’s weather on one day could be
over Victoria in about 5–7 days.
Deciphering the map
The first feature people notice about
the map is the wiggly lines, or isobars.
These lines are drawn by a person or
computer connecting together all places that
have the same barometric pressure.
A barometer measures the pressure of the
air in hectopascals — how heavily the air
is pressing down. The places where the
barometric readings are identical are then
joined together by the isobars. On old
barometers, the measurements were in
inches of mercury.
The higher up a hill or in a plane, the
more the air pressure decreases. But so
that the isobar pattern does not end up
showing only where the mountains are, all
pressures are corrected to what they would
be at sea level.
40 Farming Ahead October 2008 No. 201 www.farmingahead.com.au
Reading between the lines
What the maps end up with is a series of
amoeba-like shapes across it with ‘H’ or ‘L’
inside them. ‘H’ (high) means an area of
higher pressure than the surrounding air.
It is composed of heavier, or sinking air.
There are several useful pieces of
information that can be obtained from
seeing a high on the weather map.
Firstly, the air tends to be stable or gently
sinking downward. This means there will
be little chance of rain because, for rain to
occur, the air has to be cooling and this is
usually achieved by rising air. So, a high
pressure centre (such as the one over northwestern New South Wales in Figure 1)
indicates an area of fine, dry weather, with
few clouds. This situation could extend
for several hundred kilometres around
the centre.
Some rain could still fall where a high is
shown: drizzly showers along a coastline,
or a low-pressure system could occur above
the high.
Are windy conditions forecast?
The high also indicates wind direction.
In the southern hemisphere, high pressure
air circulates anti-clockwise around the
centre, following roughly the isobars and
tending to spread outward. This is due to
the earth’s rotation.
On the map pictured, light north-westerlies
are blowing at Adelaide around the high.
These will be fairly light, perhaps gusting at
20–30 kilometres per hour. Wind speed is
shown by the distance between the isobars,
indicating the pressure gradient.
At Port Hedland, Western Australia,
conditions are likely to be calm, but at
Melbourne, the winds are likely to be
increasing in strength as the isobars become
packed closer together.
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Weather tools
The cool down
A low (L), on the other hand, indicates
lighter, or rising air. If there is sufficient
water vapour available and it is forced to
rise, it will start to cool.
If this cooling is sufficient, or if something
else assists the cooling, like the existence
of a cold pool of air higher up, then
precipitation, usually rain, will result.
Precipitation occurs because, as air cools, it
is less able to hold water vapour and lower
temperatures will force the molecules to
condense, or form a cloud. Tiny visible
droplets form around a speck of salt or dust
in the air and grow larger until gravity
eventually takes over, producing rain.
Winds around the low
move clockwise in the
Southern Hemisphere.
On the map, the tightly packed isobars
around a low south of Tasmania are causing
strong, cold south-westerly winds and
probably rain across Victoria and Tasmania.
Running hot and cold
Cold and warm fronts are useful features
of the weather map. A cold front is the front
edge of an air mass that is cooler than the
air it is moving toward.
In Figure 1, the cold fronts are shown
as black barbed lines which generally
move east and are associated with lows.
Air behind the cold front (to its west) will
be relatively colder than that ahead of it
(to its east). At the surface, the difference
could be five degrees Celsius or more during
August, and much higher in summer.
Warm fronts are uncommon but do occur
sometimes in southern Australia.
Waiting for rain
Both cold and warm fronts are likely to
cause rain, if there is sufficient moisture.
Atmospheric instability, condensation and
precipitation are caused when a cold front,
acting like a gigantic bulldozer, forces cold
air underneath the warmer air ahead of it.
This lifting process can lead to rain, if
enough water vapour is present.
Under these circumstances, the low
pressure centred near Perth, as shown in
Figure 1, is likely to be bringing cold, wet
and windy conditions, with the wind
swinging around, after the low has passed,
to blow from the south. As the low moves
toward Victoria, conditions are likely to
become more wintry at Adelaide and more
rain or showers could result from the rising
air, particularly as it cools when forced up
and over the Mt Lofty Ranges.
Troughs and ridges
To complete the picture, a word is necessary
about troughs and ridges. A trough is an
area of lower pressure, sometimes shown as
a broken line if clouds are evident. A trough
will not usually cause as severe or noticeable
a change as a front but, because the pressure
dips lower, it is still often likely to bring
gusty winds or showers.
A ridge, on the other hand, is an elongated
isobaric pattern of a high (such as that
Glossary
Isobars
Hectopascals
High
Low
Pressure gradient
Condensation
Cold front
Atmospheric instability
Ridge
Trough
Lines on a map joining places of equal barometric pressure at sea level.
Units of pressure used on a barometer and weather map.
Heavier, sinking air that tends to be drier, bringing generally fine weather.
Lighter, rising air that tends to contain more water vapour per unit of volume than a
high, making the atmosphere unstable.
The difference in pressure between two places, resulting in strong winds if isobars are
close together and gentle if further apart.
Invisible water vapour cools to form a visible cloud of tiny droplets.
The leading edge of a mass of colder air.
The likelihood of air to continue to rise and cool once the process has started.
Precipitation is the likely result.
An elongated area of high pressure.
A line of lower pressure, sometimes associated with a cold front.
shown over the eastern part of the Great
Australian Bight in Figure 1). More settled
weather is usually the result. All these
weather patterns need to be interpreted
according to a knowledge of local conditions
— altitude, location as well as the distance
of water bodies, hills and valleys.
Contact Lindsay Smail is director of Geelong
Weather Services, an operational meteorology
and consultancy business.
(03) 5241 5332
[email protected]
www.geelongweatherservices.com
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Farming Ahead October 2008 No. 201 www.farmingahead.com.au
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