Download `read` the sky to be prepared for rain, hail or shine

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Climate and sustainability:
weather predictions
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Figure 1 Different cloud types
High clouds
Cirrus
Cirrostratus
Cirrocumulus
(mackerel sky)
7000 metres
Anvil top
23,000 feet
Halo around sun
Altostratus
(sun dimly visible)
Altocumulus
middle clouds
6500 feet
2000 metres
Cumulonimbus
Nimbostratus
low clouds
clouds with
vertical development
Stratus
Stratocumulus
Steady precipitation
Cumulus
Showery precipitation
The altitude of a cloud, and thereby the impact on the weather, can be determined by the form it takes.
Source: Geelong Weather Services and Clyve Herbert.
‘Read’ the sky to be prepared
for rain, hail or shine
Lindsay Smail
geelong weather services
At a glance
What to look for in cloud cover:
High cirrus clouds — as long as this
high cloud remains overhead the
weather will stay dry.
Middle-level clouds — if clouds
start to develop castle turret-like
growths on top, then rainfall or
thunderstorms are developing.
Low clouds — if stratus clouds start
to become thicker and lower, they
often create a dark and overcast
sky and potentially generate
moderate rainfalls.
Vertical formations — a thick
rain curtain with lightning bolts
interspersed can be seen. Clouds
that deflate at the front and build
up at the back foreshadow a
violent wind and lashing
rain squalls.
An understanding of cloud types and the way they are moving in the
sky can be very valuable in predicting daily weather changes.
Checking the barometer movement first and a little local knowledge will
ensure farmers are not taken by surprise by any sudden weather change
during the day.
Farmers with a basic knowledge of the
clouds, the local weather influences and a
reasonable barometer need never be taken
by complete surprise by weather changes as
the day unfolds.
producing the white and grey colours of a
cloud or fog.
All clouds are white but some seem dark
and even black due to their thickness and
shading from the sun by other clouds.
Clouds tend to develop differently at
different levels of the atmosphere. This fact
means it is possible not only to categorise
them but to use them in weather prediction
(see Figure 1).
Changing weather
Cloud formation
Clouds form as water vapour in the air is
evaporated upward. As this moist air rises it
cools and expands until it reaches the
temperature where condensation or dew
point occurs. When the water vapour starts
to condense, tiny droplets appear in the air
42 Farming Ahead July 2008 No. 198 www.farmingahead.com.au
No cloud stays the same for
more than a few minutes.
One type or shape can give way to another
quite rapidly and there often will be many
transitional types and shapes in the sky all at
the same time.
Cloud types which can signal a change in
the weather are high cirrus, middle level,
low and vertically growing clouds.
Climate and sustainability:
weather predictions
High cirrus clouds
Vertically growing clouds
Tornado
Photos: Geelong Weather Services and Clyve Herbert
If an air mass is saturated at a high altitude, white
and thin cirrus clouds will form, composed of tiny ice
crystals blown by the wind. These can be feathery
shapes or even the very picturesque ‘mackerel sky’.
If the cirrus becomes extensive or appears as
cirrostratus — long layers of cirrus — or is moving from
a westerly direction, it can indicate the approach of a
frontal system, usually within 24 hours.
Those tiny ice crystals will have been blown to higher
levels from rainfall and storm clouds sometimes
hundreds of kilometres to the west or south-west.
Cirrus clouds
Cumulus mediocris clouds
Mammatus or bulging
pouch clouds
As long as this high cloud remains overhead the
weather will stay dry. If a front is coming, the cloud
pattern will change and usually become lower in
the sky.
Middle-level clouds
There is a lot of variation in middle-level — mostly
altostratus and altocumulus — clouds.
The altostratus cloud appears as a sheet of grey or
blue–grey, sometimes thickening in blobs and creating
areas of light rainfall. The altocumulus cloud often
arrives before a cold front.
Do not be put off by the cotton puff or wavy
appearance of altocumulus cloud; light rainfall is
possible from this set-up because the atmosphere is
becoming more unstable. In other words, the
rounded nature of the clouds shows that the whole
atmosphere is becoming turbulent — an important sign
of likely rainfall.
Altocumulus clouds
Altocumulus castellanus clouds
An indication that rainfall or thunderstorms are
developing is if altocumulus clouds start to develop
castle turret-like growths on top. As the temperature
warms, especially if accompanied by falling
barometric pressure, there is a high chance of rainfall
later in the day.
All middle level clouds are important to watch
because the direction from which they are being
blown will tell if there could be a front behind them.
Usually in non-tropical Australia this will be somewhere
between north-west and south-west. The surface wind
can be quite different but the oncoming front will
change all that.
Low clouds
Stratus, stratocumulus and nimbostratus are all forms
of low clouds.
If the middle-level clouds start to become thicker
and lower, they often are transformed into
nimbostratus, creating a dark and overcast sky and
potentially generating moderate rainfalls.
Stratocumulus clouds
Nimbostratus clouds
Sometimes a low stratus cloud has almost the same
appearance as a nimbostratus but it does not produce
any rainfall at all, or maybe just a light drizzle.
Stratocumulus, one of the most common forms of
cloud, can often form under a temperature inversion,
where they are trapped because the air is warmer at a
higher level. But mostly they are disappointing as
rain-bringers unless they manage to grow vertically
during the day.
Contact Lindsay Smail is director of Geelong
Weather Services.
(03) 5241 5332
[email protected]
www.geelongweatherservices.com
Cumulonimbus clouds
Cumulus and cumulonimbus clouds
form when condensation occurs in
bubble-like parcels of air, or thermals,
hence their rounded shape. But they are
flat-bottomed because this is where the
dew point is reached.
Soft-looking small clumps are called
cumulus humilis and these can grow into a
line of cumulus mediocris, which can often
occur along a cold front.
Up until this point there is nothing to
suggest the weather will deteriorate too
radically. But if there is a deep layer of
unstable air the cloud will continue to grow
upward with turrets or cauliflower tops.
The growing cumulus congestus clouds
are now becoming potential storm clouds
with thunder and lightning and can be up
to several kilometres high.
When the humidity is high and powerful
updrafts continue to increase the vertical
development, eventually the gigantic
cumulonimbus is produced. These can be
recognised by the flat, black base which,
in a severe storm, could be lowering
further, or by a large anvil on top which
spreads outward as it reaches higher levels
of the atmosphere where temperatures
start to increase. This whitish anvil is
composed of ice crystals that extend in the
direction the wind is blowing.
When watching a cumulonimbus cell or
multicell develop from a distance, a thick
rain curtain with lightning bolts
interspersed can be seen. There is often a
deflation at the front and regeneration at
the back of the storm; this indicates
immense instability and ferocious up and
downdraft, which could result in violent
wind and lashing rain squalls on the
ground. People underneath the cloud can
see an eerie green colour that warns
of hail.
A mammatus structure sometimes
occurs underneath the anvil. This situation
also has the potential to cause heavy
rainfall and wild squalls despite its
beautiful, peaceful appearance. If the
structure starts to rotate, it can develop a
funnel, a waterspout or even a tornado,
the most disastrous of all storms.
Farming Ahead July 2008 No. 198 www.farmingahead.com.au
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