The observation of phenomena | Previous Top Next

Introduction

The seaman has unusual opportunities for observing natural phenomena of all kinds. This can be made an interesting hobby, and the observer may be lucky enough, sooner or later, to make a rare, or even unique, observation, which if carefully observed and recorded, will contribute to scientific knowledge. The comparative frequency or rarity of certain phenomena is indicated in this and the four following chapters, as far as our present knowledge goes. Phenomena of unknown origin are occasionally seen at sea and these should be carefully observed and recorded.

It is however not only the rare observations which are of value. All meteorological phenomena, whether optical or general, are directly related to the state of the atmosphere and weather prevailing at the time, and their recording in the Additional Remarks section of the Ship 's Meteorological Logbook helps to complete the information given by routine observations. Also, there is probably still a good deal to be learnt about many of the more common phenomena, including their frequency and geographical distribution, for which purpose it is obvious that all observations made in any part of the world should be put on record.

Hints are given in these chapters on the observations or measurements which are necessary if the phenomenon is to be correctly identified. Observations are much more valuable if accompanied by drawings or sketches, in black and white or colour, or by photographs. If there is not room in the log, the observations and sketches can be attached to it.

The more interesting and unusual observations and illustrations are published in The Marine Observer. Notes and sketches on phenomena which are outside the scope of the Met. Office are always sent on to the relevant authority for examination and comment.

Methods of observation. Some optical phenomena such as coronae and iridescent cloud appear to be very near the sun or moon. Those near the sun may not be seen at all unless the eyes are shaded from direct sunlight. Apart from this, optical phenomena such as haloes, coronae, etc., viewed in the daytime, when the sky is often very bright, are more easily seen if the amount of light entering the eye is reduced, and sometimes a very faint halo etc., can only be seen if this is done. The sky may be viewed through neutral-tinted glass of a light tone, such as the lightest of those on a sextant, or the reflection from black glass may be used, if available, or from a piece of ordinary glass painted on one side with black enamel or backed with black paper. If a pair of ordinary sun-glasses of suitable colour is available, this is the best method of all. Yellow-brown, not too deep, has been found to be very satisfactory. Glasses of this colour have the power of slightly increasing contrast, so as to show distant land more distinctly on a misty day. The natural colour of any phenomenon is, of course, modified by these. The same methods also give a better view of clouds, of the details in a bright cloud mass, or of the very faint extensions, near the limit of visibility, of cloud in a blue sky.

There is a useful tip for seeing any very faint light at night, which is near or just beyond the limit of direct visibility. Do not look directly at the object, or where you suspect it to be, but fix the attention on a point a little way above, below, or to the side of it. Then view the spot 'out of the corner of the eye'. Light will thus be seen that would otherwise be invisible or, if it is directly visible, it will appear brighter by this process of 'averted' or 'oblique' vision. This applies to very faint light of every sort, whether concentrated in a point or diffused, such as faint terrestrial lights, faint stars, comets' tails, all zodiacal light phenomena and the fainter parts of aurorae.

In the case of phenomena of considerable duration, it is best to make notes of the various appearances as they are seen to come into view, or of other changes, carefully recording the times throughout the progress of the phenomenon. This is preferable to trusting to the memory afterwards. Rough sketches can also be made at the time and subsequently worked up into finished drawings or sketches. If colour is to be used, notes of the various colours should be made at the time.

It is desirable that observations be accompanied by sketches and/or photographs in colour or black and white, whenever possible. These will often show detail that cannot be put into words. Sketches may be made either in ink or in pencil; in some cases delicacy of shading or fine detail is better rendered in pencil. When chosen for reproduction in The Marine Observer, the sketches will be redrawn. Although as full a report as possible is desirable, photographs taken by any members of the ship's company or passengers are acceptable, especially for publication. All that is asked for the record is the date, UTC, latitude and longitude or geographical position, identification of the object and names of the observers and photographers. Copyright of published material, including photographs, remains with the originator.

An accurate account of the size and relative positions of the main features of what is seen is the prime requirement. Angular measurements are necessary in many cases for the identification of the phenomenon, as explained in the subsequent chapters. These are best incorporated in the written observation, unless the accompanying sketch is purely a diagrammatic one.

Phenomena such as haloes, rainbows and waterspouts may be photographed, giving a sufficiently short exposure, such as would best show cloud detail. The best results, particularly in the case of coloured objects, can only be obtained by the use of colour film or panchromatic film of suitable speed with a suitable colour filter over the lens. The same remarks apply to mirage, which has very rarely been photographed, though there appears to be no reason why satisfactory results should not be obtained.

OBSERVATIONS BY RADAR

With radar sets working on 3 cm or 10 cm having a suitable form of presentation, echoes are obtained from rain up to distances of 40 n. mile or more. In this way showers, fronts and thunderstorms may be located and warning given of their approach. Echoes from cloud have been reported, but these are probably due to rain or drizzle within the cloud and not to the cloud particles themselves.

Objects at ground level or sea level are normally visible on the radar screen at distances a little beyond the visible horizon, owing to refraction. In certain conditions, however, much greater ranges are obtained. This occurs most frequently over the sea, and is due to a temperature inversion near the surface and/or a fall of humidity with height which causes reflection or abnormal refraction of the rays.

The reverse effect, i.e. a smaller degree of refraction (or sub-refraction) than is usual can occur owing to a very pronounced temperature lapse-rate and/or an increase of humidity with height. Sub-refraction however is neither a very marked nor frequent phenomenon.

Ordinary meteorological fronts are not a major cause of abnormal radar ranges. Due to absorption of the radio energy, very heavy rain may tend to mask a radar target behind the rain area; this effect is unlikely to be significant on a wavelength of 10 cm but it may become important at shorter wavelengths.

The use of radar as a means of detection of ice should be borne in mind. In normal meteorological conditions, echoes from most bergs may be detected at a useful range, but in certain meteorological conditions sub-refraction may occur and normal detection ranges be appreciably reduced. It has also been found that at times, even under favourable conditions, a very poor echo has been obtained from quite a large berg, the inclination of the slope presented to the observer apparently having an effect upon its reflecting properties, which is of as much account as the length or height of the berg. Bergy bits, growlers or pieces of pack ice, especially if smoothed by weathering, may pass undetected in strong sea clutter, even if they are large enough to sink or damage ships. On the other hand, in conditions where sea clutter is well marked, the cessation of such echoes may indicate the presence of pack ice.

Radar can therefore only be considered as an additional aid to the navigator in the detection of ice but it must be clearly understood that an absence of indication on the screen does not necessarily mean the absence of dangerous ice in the neighbourhood of the vessel.