CLOUD ALTITUDES
DURING an extended experience of cloud photography, it was found that
it was quite possible to get pictures which showed the cloud detail
even when the sun was in the field of view. Sometimes the solar image
was reversed, but if the exposure was very short this was not the
case. In such photographs the structure of the cloud was exceedingly
clear and sharply defined quite close to the sun. Indeed, the intense
illumination seemed to reveal minute details of internal arrangement
which could not be detected in similar clouds some distance away.
The methods which had been employed for the measurement of cloud
altitudes elsewhere have already been briefly referred to. Some of
them required two observers, who were equally responsible, each of
them having to direct his apparatus or camera to the same point of the
cloud, and to record the exact direction in which the instrument was
pointed. The instruments, if accurate, were costly, and there were many
opportunities for error in reading the graduated circles which gave the
directions. Moreover, in most of these methods the two observers were
connected by telephone, and had to agree on the exact point towards
which their instruments should be directed; either the exact point
of the cloud, or the precise direction as shown by the mounting of
the camera or other instrument. At Kew some of these sources of error
were avoided by fixing the two cameras with the axes of the lenses
and centres of the plates in a vertical position and exposing the two
plates simultaneously. The Kew observations were not long continued,
and for some years the only measurements in progress were those carried
out abroad, particularly at the Blue Hill Observatory and at Upsala.
The experience gained in photographing clouds in order to record their
forms suggested a way in which many of the sources of error in previous
measurements of altitude could be avoided, especially by simplifying
and reducing the operations at the moment of making the observation.
If two cameras are placed at the opposite ends of a measured base line,
whose direction is known, and if they are both pointed towards the sun,
on making the exposures by electrical means at the same moment, the
position of the image of the sun upon the plate gives the direction in
which the cameras are pointed. It will be in the same direction as seen
from both ends of the line.
Now, if we note the time at which the exposure is made, this with the
date gives all that is required for ascertaining the sun’s position
in the sky, and is, therefore, the only exact observation which need
be made at the time of taking the photographs. Mistakes are almost
impossible, as each plate contains its own record of the sun’s
position, and even if some of the plates should get mixed the images of
the clouds will generally suffice to pair them properly. For general
measurements there is one grave defect in the method, and that is that
it can only be used when the sun and cloud can be got into the same
field of view. But with the higher varieties of cloud this is generally
possible, and it was just these higher sorts about which knowledge was
least certain, and which it was proposed to study.
An initial difficulty was the finding of a level site, flat land being
very uncommon in Devonshire, but fortunately a suitable place was
found in some artificially levelled ground close to Exeter, belonging
to the London and South Western Railway Company. It was a stretch of
ground intended to be covered with sidings, but had not been finished,
and had become overgrown with grass, stunted sallows, and other wild
plants. Being railway ground, it was, comparatively, though by no means
entirely, free from mischievous and inquisitive people. The next point
was a suitable camera. It must have fairly long focus in order to give
a large image, and therefore large displacement; it must be capable
of being pointed in any direction and clamped there; and it must be
capable of standing considerable extremes of temperature and variations
of dampness, as it was intended that they should be kept on the spot in
wooden structures, which served for stands as well as to contain the
apparatus.
The pattern finally decided upon is represented in Plate 58, which
shows one of the cameras pointed up to the sky and standing on one of
the stands. These cameras were to take plates of whole plate size, two
double dark slides of the ordinary pattern being attached to each.
[Illustration: Plate 58.
CAMERA FOR MEASURING ALTITUDES.]
The camera looks rather complicated, but it is really simple. Its
body consists of front and back, each attached to a central part by a
short bellows and sliding on a base board, to which it can be clamped
by screws of the usual pattern. The central part carries trunnions,
such as are used for looking-glasses, which swing in sockets carried
by two upright supports, so as to give the whole free motion in a
vertical plane. In order to be able to fix it firmly at any angle, the
base board of the camera body carries on its underside a thin board
projecting beneath it and forming a segment of a circle whose centre
would be the horizontal axis through the trunnions. The board passes
between the jaws of a small wooden clamping vice in front, which is
carried by the square base to which the uprights are fixed. The whole
is firmly made of well-seasoned pine, and has stood well the hard usage
of half a dozen years.
There is no focusing screen. Focusing was done with great care once for
all, and then a coat of hard varnish was put over all the adjusting
screws. A small view-finder is attached to one side, and it was by
this that the camera was pointed in the desired direction.
In order to lessen risk of mistake, it was so arranged that the two
slides belonging to one camera would not fit the other. The lenses,
of 18 inches focus, and giving sharp detail all over the plate, were
carefully matched, and the focus adjusted until the images given by
them when placed side by side appeared to coincide exactly. They were
provided with iris diaphragms, which were shut down to an aperture of
a quarter of an inch, and with shutters which could be released at the
same moment by an electric current, acting through the electro-magnet
shown under the lens on the front of the camera.
The shutters were of the kind known as the “Chronolux,” which will give
any exposure from the sixty-fourth of a second up to three seconds. But
it was found in practice that the highest speed was sufficient and gave
satisfactory results. Of course, there was no idea of adjusting matters
on each occasion so as to get the best possible negatives capable of
yielding good prints. Measurement was the object, and if the negative
showed the sun and sufficient cloud detail for the identification of
cloud points, that was all that was wanted. The shutters gave a good
deal of trouble at first. Their sliding parts were made of ebonite, and
when the cameras were left in their stands with an August sun shining
down upon them, everything inside got very hot and the ebonite warped;
but the difficulty was got over by substituting aluminium.
The two camera stands were placed 200 yards apart, and were connected
by a line of telegraph wire carried on short poles. At each end of
the wire an insulated connecting piece was brought down to the camera
stand, and to the batteries and other apparatus. The current which was
sent through this wire by pressing a contact at one end of the line did
not directly make the exposures; but two similar relays were brought
into action, and each of these sent the current from a local battery of
Leclanché cells through the electro-magnet on the camera and made the
exposure.
After development the two negatives showed the image of the sun, not
far from the centre of the field of view, and the cloud whose altitude
was required. Since this was taken from two different points of view,
the negatives were not alike, but the distances between the centre
of the sun’s disc and any special point of the cloud were different.
For instance, if the cloud were east of the sun, with its edge just
apparently touching the solar image as photographed from the eastern
station, then the negative taken from the western end of the base would
show an interval of clear sky between the two, which would be greater
as the cloud was lower.
It often happened that after developing the plates the image of the
sun was lost in a black blur, but it was easy to reduce this part of
the image by local application of a reducing agent[3] by means of
a paint-brush, until the disc became clear enough. Two lines were
then drawn on the negative, one vertical and the other horizontal,
intersecting each other at the centre of the sun’s image. These lines
served as the starting-points for exactly measuring the distance from
their point of intersection to any selected point of the cloud.
The distances could generally be determined to a fiftieth or a
hundredth part of an inch, and their difference was, of course,
dependent upon the direction of the sun relative to the base line and
the altitude of the cloud, but for low level clouds the difference
was sometimes so great that no pair of corresponding points could be
detected, while it was often as much as an inch. With higher clouds
the differences were smaller, but unless the sun was very low in the
sky, either east or west, the displacements of the cloud image were
great enough to give reliable measures. Specimen prints from pairs of
negatives are shown in Plates 59 and 60.
[Illustration: Plate 59.
PRINT FROM A NEGATIVE USED FOR MEASURING ALTITUDE.]
[Illustration: Plate 60.
PAIR OF PRINTS SHOWING THE DISPLACEMENT OF THE CLOUD.]
The processes by which the measurements are worked out are
laborious,[4] and consist of two parts, the first being the
determination of the exact position of the sun from the date, hour, and
latitude and longitude of the place, and the second, the determination
of the position of the cloud. Two points which represent the same part
of the cloud are selected, and their respective distances from the two
lines drawn through the sun are measured as accurately as possible.
Now, a certain distance on the negative corresponds with a definite
angular displacement, and a scale can be constructed showing how much
should be added to or subtracted from the sun’s position to get the
exact position of the cloud. This being done, it is then a simple piece
of trigonometry to deduce the actual height of the cloud above the
place of observation. The work of computation, however, was greatly
lightened by the fact that many of the pairs of negatives showed more
than one layer of cloud; thus Plate 59, which is a fair specimen,
shows three layers, and, consequently, one determination of the sun’s
position sufficed for three distinct results.
For the highest clouds the displacements were, of course, small, and
could only be made with certainty of a correct result within about
three hours of noon. Earlier than 9 a.m., or later than 3 p.m., the
sun was too nearly in a line with the two stations, or too low in the
sky, to give a sufficient displacement of image. A base line of 400
yards instead of 200 would have been better for the high clouds. But,
on the other hand, when low level clouds are viewed from two different
spots their outlines may seem so changed that it may be impossible to
identify a pair of corresponding points, and the same difficulty may
also arise when high clouds are seen through a gap in a lower stratum.
The longer the base line the more frequent and more obtrusive would
this perspective difficulty become, so the distance of 200 yards
between the stations was adopted as a convenient mean.
The method of making the observations was simple. Each observer was
provided with some signal flags, by which the necessary communications
were made in accordance with a simple code. Call the two observers A
and B, and suppose A directed the operations. He watched the sky until
a favourable opportunity seemed to be approaching. He then signalled
to B, and both cameras were turned to the sun, the dark slides were
inserted, the shutters set, and everything made ready. Signals were
then interchanged, to signify that preparations were complete, and when
A saw that the edge of the cloud had reached a suitable position to be
in the same field of view with the sun, the contact key was pressed and
the plates simultaneously exposed. At the moment when this was done the
time was noted. Several observations were thus made in a short time.
Measurements were carried out as opportunity allowed over four
consecutive seasons, from the beginning of April until the end of
October. During the last of the four years, the site had become less
convenient owing to an extension of the railway work, and early in
November the series was brought to an abrupt conclusion by a heavy
gale, which snapped off all the poles carrying the connecting wire. But
by that time 423 measurements had been obtained, the great majority of
which referred to clouds of the cirrus and alto groups.
The general results may be tabulated thus, giving heights in metres:--
-------------------+-------------+---------+---------+---------
| Number of | Maximum | Minimum | Mean
|observations.|altitude.|altitude.|altitude.
+-------------+---------+---------+---------
Cirrus | 58 | 27,413 | 4,114 | 10,230
| | | |
Cirro-stratus | 64 | 15,503 | 3,840 | 9,540
| | | |
„ cumulus | 63 | 11,679 | 3,657 | 8,624
| | | |
Alto-cumulus | 83 | 9,390 | 1,828 | 5,348
| | | |
Cumulus top | 42 | 4,582 | -- | 3,006
| | | |
„ base | 48 | 1,959 | 584 | 1,290
| | | |
Strato-cumulus | 27 | 6,926 | 823 | 2,248
| | | |
Cumulo-nimbus top | 15 | 6,409 | 2,004 | 8,002
| | | |
„ „ base | 15 | 2,286 | 766 | 1,045
-------------------+-------------+---------+---------+---------
These values are not very different, on the whole, from those which
have been arrived at elsewhere, and in making a comparison it must
be borne in mind that there is always a little want of precision in
cloud nomenclature. As a whole, the Exeter maxima are greater than the
foreign ones, and this is very markedly so in the case of cirrus, for
which the American highest record is 14,930 metres, the Swedish record
is 13,376, while the Exeter value is 27,413 metres, or about 17 miles.
But this extreme measurement, and several others unusually large,
were made in one morning, a day of very hot damp weather, when cloud
formed at seven different levels: cumulus at a height of 1·9 miles,
alto-cumulus at 3·9 miles, cirro-cumulus at 4·7 miles, cirro-stratus
(No. 1) at 8 miles, cirro-stratus (No. 2) at 9·6 miles, cirrus at 11·5
miles, and cirrus excelsus at 17 miles. By about half-past one in the
afternoon the sky was completely overcast with dull grey clouds, which
cleared off at half-past four, and at half-past five in the evening the
cirrus had fallen to 7·9 miles, and the cirro-cumulus to 4·3 miles.
If this one day’s observations had been omitted, the Exeter maximum
would only have been little more than 1000 metres above the record from
across the Atlantic, but 1000 metres is a height worth noting.
While the Exeter maxima are all rather greater, we find the minima
for cirrus, cirro-stratus, and cirro-cumulus are rather less than at
the foreign stations; that is to say, that clouds are formed over
Devonshire both at lower and at higher levels than seems to be the case
in Massachusetts or Sweden. It seems probable that this is due to a
greater humidity on our western coasts, such as we should suppose would
be the case from their position and the prevailing winds and ocean
currents. If so, we should expect the great convection clouds to be
larger. Thus, at Exeter, out of only fifteen examples of cumulo-nimbus,
the top varied from 2004 metres to 6409, with an average base level
of 1045. At Upsala the maximum was 5970 and the minimum 1400, with an
average base level of 1400. The mean thickness of the Swedish clouds
was only 1400 metres, while that of the Devonshire specimens was more
than 2000 metres.
Again and again, during the progress of these measurements, it was
found that the greatest altitudes and the richest development of the
higher varieties occurred towards the end of a spell of fine calm
weather, when convection had had free play day after day. A slight
fall of the barometer, only the hundredth part of an inch, would
usually, under those circumstances, bring about abundant formation
of high clouds, frequently of the undatus kind. All the cumulus
clouds, by which we mean to include alto-cumulus and cirro-cumulus,
are most frequent when the levels of condensation are rising, while
the stratiform clouds are an indication of no vertical movement or
of active descent. Pure cirrus is indicative rather of movement in a
horizontal direction, and may occur when the condensation levels are
stationary, or when they are rapidly changing either way.
In broken weather the natural movements of the atmosphere and of its
vapour are masked and disturbed by the strong eddies brought by the
cyclonic systems. It not unfrequently happens that the region of
disturbance does not reach up to the level of the highest cirrus, or,
what is more probable, the cyclonic system leans so far forward that we
may have in its rear the upper clouds floating quietly far above the
comparatively shallow region of disturbance, while in front the upper
part of the storm system projects above undisturbed air.
The frequent appearance of cloud almost at the same time at more than
one level is at first rather difficult to understand, but it will be
noticed that when this occurs the barometer almost invariably falls.
Now, if we suppose that the air is nearly saturated at more than one
level, and that the whole is then bodily relieved of some of the
superincumbent mass, so that the barometer falls, the mass of air will
at once swell up, being cooled from top to bottom simultaneously, and
wherever it is damp enough cloud will be formed.
The converse is equally true. If we have cloud at several levels, and
the whole is compressed by the addition of more air above, which is the
case when the barometer rises, that compression will be accompanied
by the generation of heat and the consequent disintegration and
disappearance of the clouds.
FOOTNOTES:
[3] Ferricyanide of potassium and hyposulphite of soda.
[4] From the declination of the sun corrected for variation and from
the known latitude, the meridian zenith distance is calculated.
From the Greenwich time, the longitude, and the equation of time, the
hour angle is obtained.
Now, if H be the hour angle, D the reduced declination, and M the
meridian zenith distance, the sun’s altitude may be calculated by the
formula--
log versin H + L cos lat. + L cos D-20 = log _n_,
where _n_ is a natural number, and
_n_ + vers M = covers alt.
Again, to find the azimuth--
vers sup. (lat. + alt.)-vers polar dist. = _m_,
where _m_ is another natural number, and
log _m_ + L sec. lat. + L sec. alt.-20 = log vers azim.,
reckoned from the south.
Hence the position of the sun is ascertained for both negatives.
By actual measurements on the plates and reference to a previously
constructed scale the position of the cloud as seen from each camera
is next determined, and the angle subtended by the base line at a
point X vertically beneath the cloud is calculated. If A and B are
the stations, and _a_ and _b_ the angles from them respectively, the
distance AX is given thus--
log AX = L sin _b_-L sin AXB + log AB,
and the height _h_ of the cloud above X is given by--
log _h_ = log AX + L tan alt.-10.