43 METEOROLOGICAL OBSERVATIONS AT SEA

164. INTRODUCTION

These Chapters are devoted to expanding the general coverage of Meteorology given so far in this Study, in order to provide the small craft mariner with a practical and comprehensive guide to single-observer weather watch-keeping and forecasting. In particular, these chapters deal with the sensory and instrumental observations that a mariner can make to maintain his knowledge of the weather situation. Although some of the topics covered have been discussed in the earlier chapters on Meteorology, where this occurs it will be found that here they are dealt with in more depth, to enable students to obtain a greater understanding of the subject.

After a brief summary of the history and development of modern Meteorology, § 2 deals with the temperature and stability of the atmosphere – a subject which was not covered in the previous Meteorology chapters, but which it is necessary to understand, in order to give more meaning and purpose to the observation of air and sea temperatures, of atmospheric pressure and clouds. § 3 deals with the practical observation of atmospheric pressure, both by aneroid barometer and barograph, § 4 with the observation and forecasting of wind, § 5 with the observation of humidity and forecasting the likelihood of fog and §6 with the observation of sea and swell.

165. THE HISTORY OF METEOROLOGY

The first treatise on Meteorology was written by Aristotle in about 350 BC, and the weather signs he described were similar to those spoken of today, but for a period of about 2,000 years, no appreciable progress was made in formulating a theory which explained these signs.

True meteorological measurement, as opposed to simple perception of the vagaries of the atmosphere, began in the 17th century with the invention of the barometer and the thermometer. The invention of the barometer in 1643 is attributed to Evangelista Torricelli, an associate of Galileo at Florence, who realised that changes in weather were related to fluctuations in the height of the mercury. By 1645, the idea had spread to France and, in 1663, the Royal Society of London had devised its own instruments, using the same basic principle. A century later, the principle of the aneroid barometer was discovered, but not until 1843 was a satisfactory model produced.

Although the perception of heat and cold must have been more obvious to Man than the weight of the air, the invention of the thermometer virtually coincided with that of the barometer. Early “thermoscopes”, such as that devised by Galileo were defective, in that they relied on the expansion of a gas, usually air, to indicate temperature change, and they had no scales. The barometer revealed the variability of the pressure of air and led to the adoption of the liquid-in-glass thermometer, which was being produced by Florentine glassblowers by the mid-17th century. Of the many scales invented for the thermometer, the most successful were those of D. G. Fahrenheit of Germany and Anders Celsius of Sweden 

(see § 2 of this Study).

The invention of the barometer led to the “weatherglass”, with its inscriptions “storm”, “much rain”, “rain”, “change”, “fair”, “set fair” and “very dry”. Scientists realised the inaccuracy of these barometrical forecasts, but little progress was made until 1820, when the relation between wind and pressure distribution was first defined. Later, in 1857, the value of simultaneous observations of pressure, wind, temperature and weather conditions at a number of places was fully appreciated and the Dutchman Buys Ballot produced his Law, relating wind-direction to pressure (see Study on Basic Meteorology), which holds good to the present day.

In 1860, Admiral Fitzroy, then head of the Meteorological Office in London, which had then been established only a few years, began to collect daily reports by telegraph from various stations, describing the local weather at certain fixed times As a result of these reports, the first weather forecasts were issued to the press. International co-operation was established in 1872 and, with the advent of wireless telegraphy, an ever-increasing amount of meteorological information became available. The value of international co-operation was recognised at an early date and by 1880 the Directors of the national meteorological services had grouped themselves into the International Meteorological Organisation (IMO). For over 70 years, the IMO encouraged the improvement of observation and transmission of data for forecasting purposes. After the second World War, the IMO was reconstituted as the World Meteorological Organisation (WMO), and in 1951 the United Nations recognised the WMO as a specialised agency. Over 130 countries (virtually the entire world) are members of WMO and, thus, allow the free communication of standardised observations, mainly for forecasting purposes.

166. MODERN DEVELOPMENTS IN METEOROLOGY

At an early date, it was recognised that a knowledge of the conditions in the upper atmosphere would be of assistance in weather forecasting, but it was not until the First World War (1914-1918), that aircraft were used for observing upper air temperatures and the pilot balloon became the standard of observing the upper winds. During the Second World War (1939-1945), the development of the radio-sonde and the introduction of a technique for tracking pilot balloons by radar led to a great increase in the amount of information about winds and temperatures in the upper air, particularly over the continents. The radio-sonde (fig. 43-1) is essentially a small radio transmitter with three circuits connected to devices which respond to temperature, humidity and pressure. It is carried up on a free balloon. A small windmill turned by the motion of the balloon through the air, switches in each circuit in turn for a few seconds. Changes in the upper air conditions cause changes in the radio frequencies emitted by the three circuits and these frequencies are measured at the receiving station on the ground and translated into temperature, humidity or pressure, as appropriate. Twice a day, radio-sonde balloons are sent aloft at selected stations all over the world, and the results plotted on special charts called Tephigrams, which enable the skilled meteorologist to estimate cloud heights and amounts, the chances of showers and thunderstorms developing and the maximum temperature likely to be reached. The balloon may take the equipment to heights of between 20 to 30 kilometres (12.5 to 19 miles) before bursting, after which the instruments float back to earth on the ‘parachute’ of their nylon-net radar reflector.

Above the 30-kilometre (19-mile) level, rocket soundings are made to take instruments to the middle atmosphere (30-100 kilometres 19-63 miles). Once there, the actual procedure of measurement of temperature and winds is very similar to that employed with balloon-borne sondes. The temperature element is ejected from the rocket at altitudes of about 70 kilometres (43 miles); when used by day, corrections of up to 20°C. (36°F) must be made for solar radiation and, to obviate this, many rocket soundings are made at night. Because the air density is so low at these altitudes, the parachute and sensor fall at 200 metres (660 feet) per second, a speed that may cause dynamic heating, which must be allowed for in the final analysis. The wind speed and direction are determined from radar observations of the partially-metalized parachute. There is no pressure sensor on a rocket-sonde pressure is calculated on the basis of the pressure at the 20-kilometre level.

Although the network of radio-sonde stations, particularly in the northern hemisphere, is impressive, the network of rocket-sonde stations is comparatively sparse. This is, of course, due to the relatively recent innovation of this observational technique and the expense involved in each ascent. It is possible that further meteorological rocket development may be rendered unnecessary by the increasing capability of satellites (see below).

Upper-air observation by radio-sonde and rocket-sonde is restricted to temperature, humidity, pressure and wind speed and direction at selected levels. Data on other meteorological phenomena – particularly clouds – are available, with the use of sferic equipment, radar and satellites.

When lightning flashes in a thunderstorm, electromagnetic waves are produced, which are detected on radio-receiving equipment as “crackle” or “atmospherics”, which meteorologists have abbreviated as “sferics”. As these waves are fairly easily detectable for distances of thousands of kilometres, sferics give a convenient way of locating thunderstorms. The most common type of sferic equipment is the cathode-ray direction finder. This unit has two aerials at right angles and, on receipt of the sferic, the direction from which it comes is displayed on a cathode ray tube and the information is either photographed or noted by the observer. A single direction is, of course, of little value, so a sferic network usually comprises from two to four stations 500-1,000 kilometres (300-600 miles) apart. The simultaneous recording of a lightning flash by these stations allows a “fix” on the storm to be made by simple intersection methods and, in thundery weather, the sferics stations usually operate for ten minutes before each hour and plot as many fixes as possible.

The development of radar meteorology has provided information on the dynamics of clouds and upper-air winds and is most valuable in short-term forecasting. In meteorology, radar is used to survey a cloud or a collection of clouds and the energy is reflected by precipitation-sized raindrops or ice crystals in the cloud.

This is displayed in two basic ways on a cathode-ray tube. The horizontal display of “echoes” is shown by the plan position indicator (PPI), usually, a full circle, display with the radar station at the centre, with azimuth markings every 20° and concentric range markers a constant distance apart. A variation on this is the constant-altitude plan position indicator (CAPPI), which provides PPI pictures of echoes at various altitudes other than the surface. The second basic type of display is the range-height indicator (RHI), which gives a section of precipitating clouds along a chosen direction. Both types of display may have a horizontal range of over 150 kilometres (about 93 miles), and the vertical range of the RHI is about 15 kilometres. Although most radar information is transmitted and stored in the form of either verbal descriptions or photographs, research is currently being directed toward the automatic numerical coding of echoes. Radar is also used to track targets attached to balloons or ejected from rockets. The slant range and direction of the target at different times provide a measure of the upper-air wind speed and direction.

Possibly the most important modern development in the past twenty years is the preparation of forecast charts by electronic computers. The whole problem of weather forecasting is basically scientific – a problem of mathematical physics. The laws connecting wind and pressure, density and temperature, etc., are numerical in nature and can be expressed mathematically. If the state of the whole atmosphere were known in sufficient detail at any moment of time, it is theoretically possible to calculate the state at any future moment – that is, to calculate a weather forecast. But the problem is enormously complicated, owing to the vast amount of data involved. To compute a twenty-four-hour forecast, using pencil and paper, would require many years of work, which is absurd. However, with a modern computer, the forecast can be completed in a few hours, using data supplied in the routine synoptic messages.

Artificial Satellites are devices carrying scientific instruments which circle the Earth outside the atmosphere in regular orbits for long periods. Their principal use in meteorology is the taking of photographs of cloud systems from very great heights and the collection of information on heat radiations from the earth, sea, clouds etc. Both photographs and information are transmitted to Earth and can be reproduced in a very short time by special receiving instruments. Before the days of weather satellites, forecasters relied for their knowledge of the existing clouds on the patchwork of observations made at ground reporting stations. A satellite picture, however, can show the whole cloud system of a depression with its fronts and is p especially useful over the oceans and sparsely-populated land areas where observing stations are widely separated. The first weather satellites were the TIROS series, which orbited as shown in fig. 43-2(a); their biggest single contribution lay in the early detection of hurricanes which breed in areas near the Cape Verde Islands, the Caribbean and the Gulf of Mexico, well away from shipping routes and islands. Before the advent of satellites, hurricanes went undetected for days. The TIROS satellites suffered from poor earth coverage (10%-25%) in daylight and no coverage at all by night. The later NIMBUS satellites (fig. 43-2(b)), give 100% earth coverage, by being put into orbit over the Poles. .They rotate with the Earth and they send infra-red pictures of earth and cloud features by night, as well as TV pictures by day; automatic picture transmission (APT system) makes satellite information available to any station in the world which has the necessary equipment and is within range of the satellite. The latest series of weather satellites is typified by the ATS-l satellite (fig. 43-2(c)), launched into a synchronous orbit 22,300 miles above the equator over one spot in the Pacific, and rotating with the Earth as if the two were connected rigidly together. It can photograph the entire Pacific Ocean at 20-minute intervals during the hours of daylight.

Both NIMBUS satellites and ATS (Applied Technology Satellites) provide continuous surveillance of the globe, the former by having a polar orbit and being Earth-orientated and the latter by being geosynchronous. A satisfactory global coverage could probably be achieved by using a combination of three or four geosynchronous and one or two polar-orbiting satellites, with the former relaying pictures received from the orbiting satellites to those points of the Earth, not in direct view of the latter.

It is the development of satellite observation capability which is likely to provide the greatest future advance in meteorological measurement techniques. Much effort has been expended on developing the Global Horizontal Sounding Technique – a system whereby satellites track constant-altitude balloons as they are carried around the Southern Hemisphere, but this technique is at present under review, because of the short life of the balloons. More encouraging are the derivations of wind speeds from cloud movements recorded by geosynchronous satellites and of temperature profiles in the atmosphere below the vehicle (which may render rocket sondes obsolete). These developments are of paramount importance because they will allow the automatic and frequent observation of the parameters vital to the working of the most recent mathematical models of the atmosphere. Perhaps the time will soon come when a global model of the atmosphere will exist, which is constantly being updated by a stream of data from an adequate observation system. When that time arrives, a measure of true understanding of the behaviour of the atmosphere will exist.

167. FACSIMILE WEATHER CHART RECORDERS

Equipment is available for receiving radio signals and automatically printing weather charts, prepared and transmitted from Weather Centres. This equipment is called a Facsimile Weather Chart Recorder and the meteorological service is called Weather Fax for short. This technique has been used by commercial shipping for many years, but miniature equipment is becoming available for use onboard small pleasure craft and the equipment has already been used by serious competitors in trans-Atlantic and Round-the-World yacht races, as far back as 1972.

The facsimile recorder, connected to a suitable HF radio receiver, will, by the operation of a switch, reproduce, unattended, in a few minutes, on moist electro-sensitive recording paper, an exact copy of the weather map, as drawn in a meteorological centre ashore. There are, at present, over 70 Weather Fax transmitting stations in service worldwide, most of these being in the Northern Hemisphere, details of these services being contained in the Admiralty List of Radio Signals, Vol 3. For example, the United Kingdom Meteorological Office (Bracknell) transmits Weather Fax as follows: –

Surface Analysis Map    

 – every six hours

Surface Forecast Map for 24hrs.   

– every six hours 

Surface Forecast Map for 48hrs. and 72hrs.  

– daily

Facsimile charts, when studied in conjunction with the shipping forecast and synopsis received on normal broadcast channels, can be of immense practical value in keeping up to date with the weather situation and anticipated developments. A series of weather maps, taken at regular intervals, shows the movements and changes in surface-pressure systems and provides a useful aid in forecasting. Furthermore, with the aid of such maps, one is better able to understand the reasoning behind a forecast which has been received. A specimen of a surface prognostic FSXX (Surface forecast weather map for 24 hours) broadcast by facsimile from Bracknell at 0600 on 25th August, 1964, showing the predicted situation at 0600 on 26th August, is shown in fig. 43-3.