Home' Afloat : AFLOAT June 2014 Contents Take monthly with water June 2014 47
*Malcolm Riley is the Public and Marine Officer for the Bureau of
Meteorology in Hobart. He has worked in all States with the exception
of Qld and is a Master V. He gives education courses on Marine
DARWIN TO AMBON
Start: 1100hrs 23rd August, 2014
Darwin Harbour, Northern Territory, Australia
Finish: Ambon Harbour, Maluku Province, Indonesia
Race or cruise across the tropical Timor and Banda Seas.
Enjoy NT and Indonesian hospitality.
Various divisions for Monos & Multis
Entry fee $100/boat + $50/crewmember includes:
CAIT - Cruising Application for Indonesian Territory
Exemption from Indonesian Customs Bond
Functions and race/rally organization
68 Frances Bay Dr, Darwin, NT 0800
Proudly organised by
Fog is covered in the Afloat article published in March 2012:
Weather_by_Malcolm_Riley_ - _ Fog#.U2_14mfYExE
There are two main types of visibility reduction involving
smoke. The first is direct smoke from a bushfire burning at the
time: this may reduce visibility dramatically but often only over
a small area.
There can be widespread visibility reduction from smoke that
is trapped in the lower levels of the atmosphere: this is often
called smog. Visibility in smog in Australia usually reduces down
to a few kilometres so most of the time you can still see vessels
in ample time for safe navigation.
In severe cases of smoke/smog (more common in some
large northern hemisphere cities) visibility can be reduced to
a matter of metres. I was recently in the South China Sea and
the visibility was reduced to a couple of kilometres due to the
smoke from adjacent countries. We went days without seeing
the horizon. One day visibility was so low that a ‘fog’ signal was
required for a period.
Dust storms are fairly rare but not unknown in Australian
Usually the dust is the result of a strong wind event such
as a cold front, severe storm or tropical cyclone moving over a
dry/drought area and keeping the dust airborne and blowing it
over the water. In extreme cases visibility can be much reduced.
I have never been in a dust storm on a ship but I would imagine
the radar would not be that effective in a dust storm due to the
suspended particles in the atmosphere.
There are several different types of precipitation, the type of
cloud present and the temperature of the atmosphere determine
which type of precipitation occurs.
Drizzle falls from low level stratocumulus cloud that forms
under an inversion. Drizzle droplets are ver y small and can reduce
visibility markedly. The visibility in light drizzle is usually over
1,000 metres but in heavy drizzle this can reduce to less than
Rain falls from layered middle level clouds that generally
form between two and six thousand metres. Rain is usually found
in the vicinity of low pressure systems cold and warm fronts.
Rain will usually fall over a large area and can range from
very light (a few spots in the dust) through to heavy sustained
falls giving large totals and very much reduced visibility.
Showers and Thunderstorms are formed from cumulus
type cloud. The heaviness of the “shower” activity depends on
the vertical extent of the cloud. The greater the vertical extent
of the cloud the greater the likelihood of heavier precipitation.
Severe thunderstorms experienced in NSW would typically reach
between 15 and 20 kilometres high. Very heavy precipitation from
a large shower or thunderstorm will reduce visibility drastically,
especially if it contains hail.
Snow or Blizzard is not usually an issue in coastal Australia
but in other parts of the world can reduce visibility to mere metres.
The loss of visibility can usually be offset by the use of radar
(if you have one). However, in very heavy precipitation and large
waves the scattering of the radar “energy” can reduce the radar’s
effectiveness. Heavy rain and waves will reduce the range of the
radar. It is likely that you would pick up a large vessel on radar but
smaller or less reflective vessels (or coastlines) may be hidden
by the rain and/or sea clutter echoes.
There is a sea clutter and a rain suppression control on most
radars. However, there comes a point where the rain and sea
clutter suppression can remove echoes from smaller vessels
(that do not have much radar profile) entirely.
There is no finite numbers for safe speed of a vessel. However,
Rule 6 of the Collision Regulations highlights the factors for
determining safe speed and the very first one is visibility. h
There is a humorous mnemonic to determine safe speed at sea
that I was taught when I was studying marine navigation.
VD Makes Little Willies Drip
V = Visibility,
D = Density (of traffic),
M = Manoeuvrability (of the vessel),
L = Lights (Background lighting, trying to find a Navigation Light
against the lights of a major city),
W = Weather,
D = Draught (of the vessel and depth of surrounding area).
Links Archive AFLOAT May 2014 AFLOAT July 2014 Navigation Previous Page Next Page