Home' Afloat : AFLOAT July 2014 Contents 42 AFLOAT.com.au July 2014
Oneof the greatest seafaring endeavours was the passage
from Tonga to Indonesia in an open boat by Captain Bligh
and his crew. Throughout his journal he relates how his
crew were constantly very cold and very wet.
“ We suffer extreme cold and everyone dreads the approach
of night,” Bligh wrote.
Bligh’s journey took him from Tonga, past Fiji, north of Vanuatu
and onto Australia just north of Cooktown. These are areas that
are well within the tropics but Bligh’s crew were constantly cold.
This is despite the average sea temperature for the time of year
they were travelling (April May) likely to have been around the
26°/27° Celsius. The air temperatures were likely to have been
mid to high twenties during the day and low 20s at night. So why
were they so cold?
The current world standard for measuring temperature is a
thermometer in a Stevenson Screen. A Stevenson Screen is the
little white louvered box that can often be seen at airports in
enclosures with other atmospheric monitoring equipment. It was
designed in 1864 by Thomas Stevenson, a Scottish engineer and
father of Robert Louis Stevenson. Thomas was also the builder
of many lighthouses around the UK coastline and published a
paper on wave heights being a square root function of the fetch.
Inside a Stevenson screen there are two thermometers. One
has the bulb of the thermometer wrapped in muslin with a wick
(string) down into a bottle of distilled water. This thermometer
is referred to as the wet bulb thermometer and evaporation from
the wet muslin cools the temperature: the more evaporation the
cooler the temperature.
Between these two temperature readings the relative humidity
and dewpoint can be calculated.
However, human beings are warm blooded mammals that
have mechanisms for maintaining our body temperature. When
we get to hot we start to sweat; the sweat evaporates and cools
If you get too cold several things occur, the hairs on your body
stand out to try and insulate by trapping a layer of air close to
the body. This may have worked many years ago in our evolution
(when we were more hairy) but is not a great solution nowadays.
It is not unusual to see pets fluff up their fur or feathers when
the thermometer heads south.
You will start to stamp your feet and jump around and rub
your body to maintain warmth (basically exercise). Then the
body will take this process over and you will start to involuntary
shiver. The shivering process generates heat but does burn energy.
Finally the body directs blood flow from extremities (arms and
legs) toward the core trunk.
Incidentally, there is a belief that you lose more heat from your
head than other parts of the body. This is not true. However, the
rest of your body is usually clothed and often the head or face
uncovered so in that case heat loss is greater from your head.
In calm conditions (as long as you do not move) the air next to
the body can heat up and insulate the body somewhat. However,
wind takes this heated layer away, the faster the wind the more
efficiently heat is removed; this is often referred to as wind-chill.
Bligh’s party were constantly wet from rain and sea spray. They
also encountered many days of strong winds. The clothing of the
time would have become sodden and acted just like the muslin
on a wet bulb thermometer. The combination of the evaporative
effects and the effects of wind-chill would have made Bligh’s crew
cold despite the relatively high temperatures.
There are many different ways to monitor how heat and cold
BoM @ SYDNEY BOAT SHOW
The Bureau of Meteorology will be at the Sydney International Boat
Show from 31 July to 4 August. The Bureau will be part of the Marine
Safety Precinct at the Sydney Exhibition Centre at Glebe Island.
Come and say hello and meet some Bureau of Meteorology staff,
including some of your forecasters from NSW and yours truly. Come
and see how to get the best from Meteye or just ask that weather
question you have always wondered about.
by Malcolm Riley*
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