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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:

1. (a) by obtaining a sample by bucket;
2. (b) by thermometer immersed in the sea or in proximity to the sea;
3. (c) by engine-room intake temperature;
4. (d) by Met. Office supplied electronic distant reading equipment with hull- mounted sea sensor;
5. (e) by Expendable Bathythermograph (XBT).

Bucket method

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

Figure 16. Distant-reading sea temperature plate

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).