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Sea temperature
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SEA TEMPERATURE
The routine meteorological requirement is for observation of sea-water tempera-ture taken
from near or just below the surface. The precise depth is not specified but any one of several
methods is regarded as adequate. These methods are:
From a slow-moving ship having a bridge height of up to about 10 metres it is comparatively
easy to draw a sample of sea water on board by almost any form of bucket strong enough to
withstand the water pressure while being towed. A thermometer may then be inserted and the
water temperature measured. Small buckets made of double-skinned canvas or rubber are
very suitable for this pur-pose. Single-skinned canvas buckets are not suitable because any
evaporation from the sides of the bucket would lower the temperature of the water sample.
The problem of obtaining a sea-water sample with a bucket becomes increasingly difficult
as ships' size, speed and height of bridge are increased. Canvas buckets are so light that they
would obviously be unsuitable for a fast ship from a high bridge. Even if not torn away on entry
into the sea, little water would remain by the time it had been drawn up to deck level and the
bucket's life would be very short. A smaller and somewhat heavier bucket made of rubber
reinforced by canvas is now supplied to all UK Voluntary Observing Ships. This bucket is little
more than a closed length of rubber hose and it is suitable for taking sea temperatures in
almost any ship, but a complete solution of successfully using a bucket regardless of the size
and speed of ship has yet to be found. Extensive trials with this rubber bucket have shown
that the temperature of the water sample changes very slowly after it has been hove on deck.
The small rubber buckets described above were originally designed to contain a
thermometer which was lowered and immersed in the sea with the bucket itself. A high rate of
thermometer breakage was experienced and the policy now is to immerse the thermometer in
its sheath into the sample of sea water when the bucket is drawn up on deck. There is in fact
little disadvantage in this: whether the thermometer is immersed in the sea or inserted later, it
will do no more than measure the temperature of the sample at the moment of observing.
Whichever type of bucket is used, it should be swung as far out as possible to avoid the
shallow layers of water close to the hull which have been warmed by the ship itself. Probably
the best way of getting the water sample is to use the bucket as though one were taking a cast
of the hand lead. On entering the water the buck-et should submerge quickly and cleanly. If
drawn along the surface, a fault to which some designs are particularly prone, it will be filled
with spray, possessing some temperature intermediate to that of the sea and that of the air.
On being withdrawn, a thermometer should be inserted into the sample immediately. This
should be done in the shade; direct sunlight, in addition to its direct effect upon the
thermometer, can warm the sea-water sample very quickly.
Individual thermometers are calibrated either for complete immersion into the medium
whose temperature is to be measured, or for contact through the thermometer bulb alone (e.g.
clinical thermometers). Meteorological thermometers are invariably of the former class and, if
not large, would give rise to unacceptable errors when the air/sea temperature differences are
large. In consequence the whole thermometer should be covered by the sea water without
touching either the sides or bottom of the bucket. Devices which hold the thermometer within
the bucket may be available, but otherwise it should be held at the extreme end by finger tip,
without actually letting the fingers (which are a source of heat) enter the sample.
With the large canvas bucket the thermometer should be moved with a slow stirring action.
After immersion for about one minute the thermometer should be withdrawn just sufficiently to
allow the scale to be read, the bulb and as much of the stem as possible being left immersed.
The special sea-temperature thermometer, when supplied, should be used for this purpose,
but almost any meteorological thermometer may be used, including those employed for wet-
and dry-bulb observations. After use, the thermometer should be dried and returned to its box
for storage with the bulb end downwards.
Distant-reading thermometers
There would obviously be many advantages in measuring temperature by means of a distant-
reading instrument while the thermometer bulb was actually immersed in the sea. In its most
simple form such a device would be lowered by cable alongside the ship and readings taken
inboard while it was towed. There are, however, certain difficulties in such a method. It is
difficult to control the depth of such a device or even ensure that it enters the water at all and
does not merely skip along the surface. The strain of towing upon the cable can also be a
cause of error in the electrical measurements, while a freely towed device could damage itself
against the side of the ship.
The system evolved by the Met. Office and installed in new buildings and modem ships
places the thermometer inside the hull, measuring the sea temperature by conduction through
the ship's side plating, the principle being that steel is such a good conductor that it transmits
the temperature of the surrounding sea water. The
thermometer, which is in the form of a small, thin, printed electrical resistance circuit little
bigger than a postage stamp, is fixed to the inside of the hull of the ship at a point a metre or
so below the normal water-line. The system which requires the whole plate to change
temperature with that of the sea, has a long time lag, and is thus unaffected by short-period
roll or pitch, but would be invalidated if the position of the thermometer were raised above sea
level by change in loading. The system demands cabling to the place where temperatures are
to be read, normally the bridge, and installation is best carried out during the construction of
the ship.
Engine-room intake temperatures
The temperature of the engine-room intake water can be taken as a measure of seawater
temperature either by thermometer or by thermograph. To an extent dependent on the
individual ship, the accuracy will be questionable although the method is very convenient and
may well be the only one possible (in the absence of the hull thermometer described above)
when the bucket method cannot be used because of rough seas, too great a ship speed or a
bridge too high above the water. The errors arise from the varying depth from which the water
is drawn as the ship rolls or pitches and the risk of pre-heating as the water passes through
pipes at or close to engine-room temperature or through oil and water tanks on the inside of
the hull. A sample of the intake water may be drawn off by tap, the subsequent procedure
being that described in the bucket method above, or the temperature measured by a
thermometer installed within the intake pipe. In the latter case the thermometer will usually be
inserted in a pocket formed within the pipe, and the main problem which then arises is of
assuring good thermal conductivity. Digital readings of sea temperature in the engine control
room can be relayed by telephone or electronic means to the bridge.
BATHYTHERMOGRAPH
This is an instrument used on board ship for obtaining a sea temperature profile from the
surface down to a predetermined depth. Mechanical models consist of a bronze torpedo-
shaped instrument which is lowered and recovered by means of a winch, length of wire veered
being dependent on ship's speed. The record is etched onto a smoked glass slide by a stylus
attached to a bimetallic temperature sensor.
After recovery the slide is removed from the instrument, marked with sounding details,
placed in a holder against a graduated scale and read off: discontinuities and turning points
are then added before the slide is lacquered to preserve the trace.
EXPENDABLE BATHYTHERMOGRAPH
As the name implies this uses a non-recoverable probe. The equipment used can be
considered in three sections:
Expendable Probe. This section is about 5 cm in diameter and 35 cm in length. It consists
of a plastic protective cap, a probe in the shape of a small mortar bomb, a retaining pin and a
plastic tube with a reel and protected contacts. Connecting the reel in the tube to a reel in the
probe is a length of fine three-core copper wire which is veered when the probe is released,
the twin reel system producing a steady rate of descent. On removing the retaining pin at the
time of launching, a resistance thermometer housed in the hollow nose of the probe
continuously transmits temperature readings via the copper wire.
Launcher and Attached Cable. A free-running cable enables the launcher to be positioned
at various locations near the ship's side, where a successful sounding can be made. This
cable connects the launcher to a junction box from which a fixed cable runs to the recorder.
The launcher is a gun-like device with pistol grip, into the breech of which the probe is loaded;
a cocking mechanism forces contacts into the end of the probe tube.
Recorders. The recording instruments consist of a chart recorder to register temperature
against depth, a metal stylus which etches a trace on waxed paper and a cassette recorder
which records the same information on magnetic tape. A manuscript log is also kept from
which the completed XBT message is compiled for relay ashore by terrestrial or satellite
communication.
A completed series of soundings identifies the movement of subsurface currents, the onset
of thermoclines and long-term climatic change; the ships mainly involved in taking XBT
soundings are warships, research vessels, weather ships and those involved in dedicated
scientific projects such as TOGA (Tropical Ocean and Global Atmosphere Programme).