reel in the tube to a reel in the probe is a length of fine three-core copper wirewhich 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, aresistance 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 contactsinto the end of the probe tube.

Recorders. The recording instruments consist of a chart recorder to registertemperature against depth, a metal stylus which etches a trace on waxed paper anda cassette recorder which records the sameinformation on magnetic tape.A manuscriptlog is also kept from which the completed XBT message is compiled for relayashore 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 (TropicalOcean and Global Atmosphere Programme).

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CHAPTER 3

GENERAL METEOROLOGICAL INSTRUMENTS

Instruments for the measurement of almost every meteorological element havebeen developed for land stations, with the notable exception of cloud type and amount. These, being purely subjective observations, can only be assessed byhuman interpretation. Unfortunately, many of the instruments designed for use of land stations are unsuitable for operation with reasonable satisfaction on board ship. Special problems arise, not only from the motion of the vessel, its forward movement and exposure to saline spray,but also becauseof the disturbances createdby the ship itself in the surrounding atmosphere.

In consequence, methods suitable to land stations would be at best merely all unnecessary expense at sea, and at worst would soon fail to operate or givemisleading results. Rather than receive inaccurate data which cannot be verified,forecasters and climatologists would prefer to receive no data. For example, a hairhygrograph, an instrument giving a continuous recording of the relative humidity

of the air, relies upon the change of length of a bundle of human hairs for its operation. In addition to the adverse effect of ship's movement upon the light anddelicate pen movement, the existence of salt upon the hairs would seriously affecttheir readings.

Pressure, temperature and humidity have been covered in the preceding chapters.For general interest the standard instruments for measuring the other normallyobserved elements are given below, without regard as to whether they might, in special circumstances. be adapted for shipboard use.

\V I\D SPEED AND DIRECTION

4nemumeterThis consists of cups rotatable about a %ertical shaft or a propeller rotating about a horizontal shaft. When driven by the wind at a speed proportional to wind force,

the rotating shaft drives an electrical generator whose output is itself proportionalto the speed of rotation. A voltmeter may thus be calibrated as wind speed. Anemometers are not normally used aboard merchant ships because of thedifficulty of finding a suitable site and also because of expense. The UK Ocean Weather Ship carries anemometers on the yardarms at a suitable height above the water (23m), which seems to be the site furthest from eddying effects. But evenhere estimates of wind force and direction from the appearance of the sea are regularly made as a check on the instruments. (See Figure 17).

NandWhilst

anemometerestimates of wind force and direction made on merchant ships by observations

of sea state are to be considered the norm, a hand-held anemometer can be usedfor verification or in sheltered waters where the appearance of the sea may be modified by topographic features. The hand anemometer is held with its

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I

Figure 17. Anemometer and wind vane.

axis vertical, at arm's length, with the arm at right angles to the wind direction to avoid disturbance of the airflow by the observer's body. At least two readings ofmean wind speed need to be taken within the overall period of observation, each

reading being taken over at least 15 seconds. The wind speed is shown by a

graduated scale across which a pointer moves. Hand anemometers are calibratedin knots over a range of 0 to 60.

Dr, ital hand anemometerThis is similar to the hand anemometer except that the speed sensing mechanismdoesnot make use of switched contacts. After sampling for 15secondsthe instrumentdisplays the average wind speed digitally for a further period of 10 seconds. The

body and switch of the anemometer are waterproof and support legs fixed to the body protect the cups against damage when the instrument is laid down. The

power is supplied by battery with an indication when battery voltage is low.

TheII end

windvane

direction may be directly observed by the position of the wind vane orremotely read at deck level by an electrical direction transmitter known as a \lagslip for mains power or a Desynn in the case of a battery system operated through gear wheels and a countershaft. Both types may be joined to an anemometer unit so that the two instruments form a single transmitting head.

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VISIBILITY

Visibility recorder or transrnissometerLight from a lamp of known output is transmitted over a prescribed distance parallel to the ground. It is then received by a calibrated photocell. The degree by which the light has been attenuated by mist or fog is thus observable. An alternative system. employing the increased amount of light scattered backwards

towards the light when fog exists, is already in use in Trinity House lighthouses and automatic light vessels. Both methods depend upon optical systems andhence are unsuitable for the marine environment.

CLOL D HEIGHT

Cloud-height balloons. Cloud height may be measured in daylight by determining the time taken by a small rubber balloon, inflated with hydrogen or helium, to risefrom ground level to the base of the cloud. The rate of ascent of the balloon isdetermined mainly by the free lift of the balloon and can be adjusted by controllingthe amount of hydrogen or helium in the balloon. The time of travel between

release of the balloon and its entry into the cloud is measured by stop-watch.Cloud height is then calculated from the rate of ascent and the time taken for theballoon to rise to the cloud base.

ThisCloud

methodsearchlight

is normall onlN Mailable at night. A purpose-built searchlight witha parabolic silvered-glass reflector, mounted in a strong cylindrical case and having

three adjustable feet on the base for levelling purposes, is projected vertically onto the cloud base. The angle of elevation of the patch of light on the cloud base is measured by alidade, sextant or theodolite from a distant point, the optimum distance of separation being about 300 m (1000 ft). The cloud height can then be calculated by conversion of the angle of observation into a height in metres for a baseline of 300 m. On a dark overcast night. cloud heights up to about 3000 nm(10 000 ft) can be measured with acceptable accuracy.

Cloud-hu. +c recorderThis is an automatic device in which the same principles are employed as for thecloud searchlight, in that the angle at which a projected beam of light meets the

cloud is measured over a known baseline. In this automatic instrument, the lightis modulated', i.e. it is interrupted at a known rate and a photocell, replacing thehuman eye, detects only this modulated light even in the presence of daylight. The

cloud-base recorder can thus operate day and night and will provide a continuous record of cloud height from its own operation of the angle of sight measured bythe photocell system. The complete system consists of three units: a transmitter,a receiver and a recorder. The transmitter and receiver are remotely switched fromthe recorder which may be positioned from the operating site by between I km

and 24 kin, depending on the model used. These recorders are being supersededby cloud-base recorders using a laser technique.

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I

UPPER WINDS

Pilot-Bcallooi:. I. rCe, it Small helium-filled balloons, whose rate of ascent is predetermined, are trackedby optical theodolite. From a safety viewpoint, the use of hydrogen as a balloon-

inflating medium is being phased out and is not used on board ship. From the

observed bearings the horizontal movement of the balloon, and hence the wind, can be computed for each layer of the atmosphere through which the balloon

passes. This method is rarely used nowadays in routine practice because of its limitations. The most significant drawback is the presence of cloud but it still serves a purpose at locations where cloud cover is minimal or where there is a specialized requirement at a particular location, e. g. gliding.

Wind-finding radarIn this instance the balloon carries a radar reflector, usually of metallized mesh orsolid reflecting surfaces. The radar provides accurate readings of azimuth, elevation and slant range. In consequence the upper winds may be computed even when theballoon is invisible owing to cloud or fog. The use of radar enables upper winds to be determined on a routine basis to heights of 25 km or more. For shipboard

use, radar poses special problems in that the radar platform needs stabilizing if the required accuracy is to be achieved: this is expensive and difficult and only specialized ships are fitted with stabilized radar platforms. Today's technology

makes it possible to measure upper winds without radar but it requires an additional attachment (a radiosonde) to be flown on the balloon.

UPPER-AIR SOUNDINGS

RadiosondeA device carried by hydrogen- or helium-filled balloons to make observations of pressure, temperature and humidity to heights generally between 25 km and 30km and to transmit these observations to a ground station by radio. (See Figure18). There have been significant advances in the design and construction of suchinstruments in recent years; they are now smaller and lighter and the derivation ofthe results, after being received by the ground station, is carried out by a small computer. This also encodes and sends results in an internationally recognized\\ \10 format. These data are an essential input into the mathematical models upon which modern forecasts are based.

The determination of upper winds without radar is achieved by the addition of some electronics both to the radiosonde and ground station and then utilizing the

ecneral availability of either OMEGA or LORAN navigation signals. The ground , ration, either fixed or mobile, e.g. a ship, and the radiosonde, both receive the

appropriate NAVAID transmissions. The radiosonde retransmits the data appropriateto its movement in response to the upper winds, and from this information the

surface-based computer calculates the upper winds (in practice it is the phase , hitt which is detected). Such a process eliminates the need for a stabilized radar platform but does require the satisfactory reception of the appropriate NAVAID

, i`_nals from a sufficient number of transmitters.

RS 80-15 N radiosonde

Pressure 6. TransmitterBAROCAP%

sensor 7. String and Omega

2. Transducer electronics 8.

antennaUHF antenna3. Omega receiver

4. UHF antenna counter-poise 9. Temperature

radiator sensor

5. Battery THERMOCAP®i10. Humidity

HUMICAPE sensor

Figure 18. Radiosonde instrument.

34 35

With the degree of automation now achieved and the ability to determine upperwinds, merchant ships on passage can undertake this type of work. This is known

as the Automated Shipboard Aerological Programme.

.-1irtomated Shipboard Aerological ProgrammeASAP is a method of obtaining upper-air information from a merchant ship on passage. The system, as conceived by the Canadian Atmospheric EnvironmentService in 1982 and now used operationally, utilizes a fitted 20-foot containersecured to the deck abaft the bridge. The container is divided into two sections,one section housing the computerized ground equipment and the other an automated balloon launcher. Power is supplied from the ship's alternators and transformed to the required voltage; the computer and other units receive power through aninverter with battery back-up.

The method of operation is comparatively simple: the balloon is inflated withHelium inside a fibreglass drum which is attached to a hydraulically or electrically operated lifting mechanism. The sonde package is attached to the balloon by way

of a small door in the drum; the latter is then raised through an opening in thedeckhead of the container, tilted and the balloon ejected by air from an attached compressor. Signals from the sonde are received by aerials rigged above thebridge and relayed to the computer, which processes the information and codes itinto a standard format. The coded message is then transferred to a Data Collection

Platform which transmits it via satellite at pre-fixed time-slots.Since the introduction of ASAP to UK Voluntary Observing Fleet ships in

1987, the only major change made to the original installation has been the addi-tion of a portable balloon-launcher: the balloon is filled in the launcher and

released by hand on the lee side of the ship, thus obviating the need for course

alterations in unfavourable wind conditions. The present containerized version «ill eventually be replaced by a distributed system in which the receiving equipment will be housed on the bridge and all balloon launches made from portable launchers

secured at suitable locations on the afterdeck. Although the UK has now ceased ASAP operations, several other countries operate similar systems; two ASAP

units deployed on board ships of the French VOF utilize this configuration. In mostcases a professional meteorologist is provided on hoard to operate the ASAP system.

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Part II Non-instrumental Observations

CHAPTER 4

WIND, WEATHER AND VISIBILITY

IntroductionNon-instrumental observations are very important and being estimates, they aredependent upon the personal judgement of the observer. This judgement is theproduct of training and experience at sea, together with practice in making theobservations. To acquire a technique of observation, adherence to the officialinstructions is essential. The aim of these instructions is not only to outline asatisfactory method of making observations, but also to impose a standard proceduresuch that two observers, despite differences in training, will make approximatelythe same observation in similar circumstances. The assumption that observations are comparable, or made according to the same procedure, is the basis of synopticmeteorology or of a study of climate.

Observations from ships are of special importance to the forecaster not only because they enable him to complete his charts over the oceans, but also because

weather sequences at sea are simpler than those on land. They are therefore morecharacteristic of the air masses and hence more useful in the air-mass analysis thatprecedes the preparation of forecasts. Numerous instances occur in which the presence or absence of adequate ship reports has made all the difference between good and bad weather forecasts. An observer should never forget that his individual effort, his particular observations, may supply just the information required toresolve a forecasting problem hundreds or thousands of miles away.

The making of meteorological observations at sea is attended by manydifficulties that are unknown to the observer on shore. It is in overcoming themthat the experience and training of the mariner are important. These difficulties largely result from the movement of the ship and the absence of landmarks.

Wind force and direction .Wind force is expressed numerically on a , cale from I) to 12 (see note to Table

10). This scale, which originally defined the wind force in terms of the canvascarried by a frigate, was devised by Captain, afterwards Admirals Sir FrancisBeaufort in the year 1806 for use in vessels of the Royal Navy. Since AdmiralBeaufort's time, however, so many changes had taken place in the build, rig, andtonnage of seagoing vessels that in 1874 Beaufort's scale was adapted to the 1'1111-rigged ship with double topsails of that period. With the passing of sail, thisspecification meant very little to those who had no experience in square-riggedships, and the practice arose of judging wind force from the state of the seasurface.In 1939 the International Meteorological Organization, now the WMO, agreed tothe use of a sea criterion by which the wind force was judged from the appearanceof the sea surface. This specification, brought into use in 1941, is shown on pages 38 and 39. Photographs showing the appearance of the sea corresponding to eachBeaufort force are given on pages 40 to 47.

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