Contributions to the meteorology of Natal: observations taken at Maritzburg Author(s): Mann, Robert James Source: Foreign and Commonwealth Office Collection, (1878) Published by: The University of Manchester, The John Rylands University Library Stable URL: http://www.jstor.org/stable/60231394 . Accessed: 16/06/2014 17:42 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Digitization of this work funded by the JISC Digitisation Programme. The University of Manchester, The John Rylands University Library and are collaborating with JSTOR to digitize, preserve and extend access to Foreign and Commonwealth Office Collection. http://www.jstor.org This content downloaded from 195.34.78.78 on Mon, 16 Jun 2014 17:42:35 PM All use subject to JSTOR Terms and Conditions
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Contributions to the meteorology of Natal: observations taken at MaritzburgAuthor(s): Mann, Robert JamesSource: Foreign and Commonwealth Office Collection, (1878)Published by: The University of Manchester, The John Rylands University LibraryStable URL: http://www.jstor.org/stable/60231394 .
Accessed: 16/06/2014 17:42
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].
.
Digitization of this work funded by the JISC Digitisation Programme.
The University of Manchester, The John Rylands University Library and are collaborating with JSTOR todigitize, preserve and extend access to Foreign and Commonwealth Office Collection.
http://www.jstor.org
This content downloaded from 195.34.78.78 on Mon, 16 Jun 2014 17:42:35 PMAll use subject to JSTOR Terms and Conditions
The following tables contain the results of observations made at Maritzburg, f- the capital of the colony of Natal, during the years 1860-5. Observations
for the two previous years (1858-9) were printed in externa in the " Fifth Number of Meteorological Papers, published by authority of the Board of Trade "
(1861). The observations for those two years have not, however, been included in the following tables, because a change was made in the position of the observatory (in March, 1859) to about five miles to the westward, rendering the observations for the two periods not strictly comparable. Abstracts, especially relating to temperature and rainfall of
\ these periods, have also appeared in the "Proceedings" of the Society. The observatory was situated in lat. 29° 30' S, and long. 30° 2' E, at an
elevation of 2,095 feet above the level of the sea, and distant in a direct line about 40 miles from it. This elevation was determined by 90 simultaneous observations made with the standard barometer in the observatory, and with a marine barometer of Casella's, furnished by the Meteorological Department of the Board of Trade, and kept at the Custom-house at the port, estab-
•* lished 6 feet above the average high tide. The two barometers were carefully compared by means of a very excellent mountain barometer.
The observations were taken at the hours of 9 a.m., 3 p.m., and 9 p.m., local time. The standard barometer (Negretti, No. 318) was examined by Mr. Glaisher, between May 13 and June 30, 1857, and certified as having
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171 QUARTERLY J0UEXAL OP THE METEOROLOGICAL SOCIETY.
(i correction for index error of + 0-014 in., and for capillarity + 0-013 in. These corrections, and that for reduction to temperature of 32° have been r*
applied, but the observations have not been reduced to sea level. The thermometers employed were a maximum by Negretti and Zambra,
and a spirit minimum of the Rutherford-Negretti pattern. Both were
compared by Mr. Glaisher, and the corrections ascertained. The ordinary thermometers were supplied by the Meteorological Department of the Board of Trade, and had Kew certificates. The errors were insignificant, and corrections were not applied, being, in both instances, within the tenth of a
degree. The thermometers were placed 7 feet above the ground on the •*-
wall of the house under a broad verandah, and only exposed to the cool SE
quarter, sheltered from the hot NW and well isolated from the wall. This situation was adopted on account of the impossibility of getting regular readings in the open during the storm season. Careful comparisons of
temperatures were however made, which left no shadow of doubt that during the hot season the verandah temperatures were the best air temperatures. The heat at this time is so great, that all small structures in the sunshine, such as ordinary stands, get heated through and through, and retain very great radiating power. The monthly means of air-temperature have been <~ obtained by a combination of the means for 9 a.m., 3 p.m., 9 p.m., with the mean of the minimum, as a comparison of other inter-tropical observa¬ tions, has shown that this combination gives a result as nearly as possible equal to the true mean obtained from 24-hourly observations. ^-
The funnel of the rain-gauge was 6 inches square, and was 6 feet above the ground, on an isolated pillar in an open space ; there being no buildings near, and the trees within range all round being kept down to a very low y elevation, which in no instance involved an angle of 25° with the horizon.
The wind direction was recorded by true bearings, and the force estimated
by Beaufort's scale (0-12). The following general remarks on the climate of Natal, at the elevation of
2,000 feet above the sea, are based upon the observations which are summarised in these tables, and will probably be deemed a suitable introduction to them.
The colony of Natal is not very far removed from the tropical latitudes in v. which the southern trade-wind prevails ; that is, it is just outside the region \ where there is a constant and steady movement of the air from the south-east towards the north-west. The general range of the sea coast,—from SW to NE,— is also immediately across this direction of the southern trade. The consequence is, since the land gets more rapidly heated by the sunshine in the day than the water, that a very strong tendency to the production of a sea-breeze is established during the day. But such a breeze is also from the
SE, that is, it lies in the same general direction as the current of the trade-wind.
The great geographical inclination in the atmosphere to flow toward the -*J north-west, which is hardly lost in the latitudes of Natal, is thus renewed, and reinforced by a powerful secondary natural agency ; and the sea-breeze,
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MANN CONTRIBUTIONS TO THE METEOROLOGY OF NATAL. 17.
^
^
as a matter of fact, predominates over the land breeze in a very remarkable degree.
The observations made at Maritzburg showed that even at that distance inland, the air moves from the sea inward over the land six times more frequently than it moves outward from the land towards the sea. This very remarkable prevalence of the sea-breeze determines the character of the climate of the colony, and fixes the laws of its rainfall and tempera'ure. As a matter of course, more vapour is raised from the sea into the in-drifting mass of the sea-breeze in the hot season of the year than in the coll, and accordingly there is more water in the air to be shed upon the slopes of the land in that hot season, than there is at the opposite period of the year. The summer of Natal is for this reason a season of copious rain, and the winter a season of relative dryness, and on the same account, the summer is a time of abundant and frequent cloud, and the winter a time of preponderant sunshine. The summer is consequently cooler in a material degree than it would otherwise be, on account of the frequent prevalauce of cloud and the abundance of the rainfall; and the winter has its temperature materially raised, from the constant occurrence of clear skies and bright sunshine. The mean annual rainfall at Maritzburg for the six years, 1860-5, was 31-13 in. Of this amount nearly 28 in. came down during the six summer months of the year (October to March), and scarcely more than 2 in. during the four winter months (May to August1. During each of the two intermediate months of April and September, 1-5 in. fell. The average fall of rain for each month of the year, shown by the period of six years' observations under discussion, was :—
Month. 1 In. Month. In.
4-189 4-992 3-256 1-552 1-114 0-251
0-165 °757 1-507 3'529 4-652 5-!7°
March
May
The largest monthly fall in this period was in the month of November, 1860, and amounted to 8-951 in.
The greatest rainfall that occurred in any of the twelve months of the year was':—
176 QUARTERLY JOURNAL OF THE METEOROLOGICAL SOCIETY.
The actual rainfall for each year of the series concerned in these observations was :— *
Year. In. Year. In.
i860 j86i 1862
30-551 22-613 30-022
1863 1864 1865
34-659 37-340 31-610
In this series of years, the difference between the greatest and the least
yearly falls was 14-727 in.; the wettest, 1864, being 20 per cent, above the
average; and the driest, 1861, 27 per cent, below it. The exact number of days on which rain fell, reduced to, and estimated as
an average, were :
Month. No. of Days. Month. No. of
Days.
16 16 14 10 4 1
2 5
10 16 '7 '9
The general process of the rainfall in the wet season is, that a series of three, four, or five wet days occur, and that these alternate v> ith a series of as many dry days. The rainfall on the coast is, on the whole, heavier than the rainfall on the hills at the height of Maritzburg (2095 feet above the level of the sea), instead of the reverse, as is so commonly the case in England. There are two obvious reasons in Natal for this peculiarity—in the first place, more rain as a rule falls on the coast than on the uplands during the dry season of the year. But, in addition to this, every now aud then heavy sea-storms occur, which deluge the coast with a down-pour that exhausts its chief force in its first burst on the land, and then grows rapidly less as it drifts inwards up the land slope.
In fact, rainfall is caused in Natal by two essentially distinct influences. First, by the development of storms, more or less of the character of thunderstorms, which most powerfully affect the inland districts and the uplands ; and, secondly, by the occurrence of sea gales, which fall with most violence upon the coast, and the lands in the close neighbourhood of the sea.
The mean number of thunderstorms for each month of the six years' series was :—
January February March April May June
»7 7-0 6-7 5-° i-8 0-2
Jnly August September October November December
0-5 1-2 4-0 8-o 6-8 9-5
Jr
>-
A
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MANN—CONTRIBUTIONS TO THE METEOROLOGY OF NATAL. 177
The chances thus are that no thunderstorm will be experienced at Maritz¬ burg during the months of June and July ; but that in each of the months of the wet season, extending from October to March, something more than seven thunderstorm days may be looked for, or, in other words, that a thunderstorm may be expected on the average every fourth day. As an actual fact, however, storms generally occur on successive days together, and then leave several days free. In the month of October, it sometimes happens that storms occur for six and even seven days in succession.
The greatest number of thunderstorms that occurred at Maritzburg in each month, for this period, was :—
-i.
H
y
Month.
January February March April May June
Year. No.
1861 11 1863 11 i860 9 1862 7 1862 6 1864 1
Month.
July August September October November December
Year.
i860, 1863, 1864 i860, 1862
1865 1864 1863 1863
No.
The sea-gale rainfalls in Natal are almost invariably associated with a high barometric column, whilst the thunderstorm rains are as commonly connected with a low barometer. The reason for this is, however, perfectly obvious. The sea-gales occur with a very strong and very strenuously maintained southerly wind, which, like the northern wind of the opposite hemisphere, means a cold, condensed, and therefore relatively heavy, state of the atmosphere. The thunderstorms, on the other hand, immediately follow the periods of largest rarefaction from heat, which are connected with a relatively light state of the atmosphere. If the movements of the column of the barometer from day to day are traced down upon paper as a series of successive " waves," it is almost invariably found that sea gales correspond to the crest of these waves, and thunderstorm rains to the hollows of
depression which lie between the crests ; or, perhaps, more exactly to the
points where the wave just begins again to rise after the lowest dip of the
trough. There is an almost constant diurnal oscillation of the mercurial column
of the barometer, amounting on an average to 0-078 in. It is seen
distinctly on thirteen days out of fourteen, and the exceptional days are
simply those in which the barometer is rising so rapidly under a larger and more general influence, that the diurnal oscillation is more than compensated by it, and in that way masked.
But, apart from these diurnal fluctuations, it may be further affirmed that the mercurial column is never stationary in Natal. It is continually moving boldly up or boldly down from day to day.
These larger oscillations are brought about by the bodily and progressive tra-->s"e.ence of masses of air from place to place. The upper and lower strata of the air flow in antagonistic currents, but of these two currents the
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178 QUARTERLY JOURNAL OF THE 1IEIE0R0L0GICAL SOCIETY.
lower comes from a cold zone of the earth and is comparatively heavy and dense, while the upper comes from a warm zone of the earth and is comparatively light and rare. When these two currents are at a normal or average relation to each other, the barometer shows a certain average or mean indication of pressure. But if this medium condition is disturbed—if one current encroaches unduly upon the other—the indication of tho barometer is changed in a corresponding degree. When the upper north-west current eucroaohes on the lower, the column of mercury in the barometer goes down. When the lower south-east current encroaches upon the upper, the column of mercury rises.
Whenever the predominance of the north-west upper current prevails in Natal to the extreme degree, so that it actually reaches the surface of the ground and for the time displaces the lower south-easterly current, it is felt as a dry hot wind which parches everything it touches. This hot land-wind, when it occurs, is strong. Most commonly it is very strong, amounting almost to a hurricane. It is always irregular and fitful, expending itself in short bursts and gusts, and sweeping dense clouds of dust over the country as it continues. The hot wind blows most frequently in the months of August, September, October, and November, and therefore it is, in some degree, ruled by the march of the season.
The following table gives the mean, 9 a.m., 3 p.m., 9 p.m., and the highest and the lowest readings of the standard barometer at Maritzburg, for each year of the series:—
/
f
Mean of Years. 9 a.m. and
3 P-m. 9 a.m. 3 p.m. 9 p.m. Highest. Lowest.
In. In. In. In. In. In. i860 27-908 27-945 27-870 27-948 28-393 27-429 1861 •909 •947 •870 •952 •393 •392 1862 •860 •900 819 •902 401 •419 1863 •899 936 •862 •947 •362 •369 1864 •917 •955 •879 •954 •423 •470 1865 27-922 27-959 27-884 27-962 28-431 27-342
The mean pressure of the atmosphere during the six years was 27'903 in. The highest reading during the period was 28-431 in. ; the lowest reading 27-342 in. The extreme range during the six years was, therefore, 1-089 in.
The thermometer very rarely rises above 85° in the shade in Natal, even in the midst of the summer season, unless a hot wind is blowing. It then mounts to somewhere between 85° and 97°, according to the strength of the scirocco. In midwinter a north-west hurricane occasionally prevails without raising the thermometer above 85°, but this is rare. The amount of humidity indicated by the dry and wet bulb thermometers, when a hot wind is blowing, varies between 25 and 52 per cent, of moisture. But on rare occasions, it is
I
4
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The highest air-temperature recorded during this six-year period was 970-6. This occurred during a hot wind on December 30th, 1865. The lowest temperature was 29°. The extreme range in six years was thereforo 68°-6. The mean temperature derived from this series was 63°-3.
The air-temperature in Natal is subject to frequent and sudden vicissitudes, and these are more marked at Maritzburg and amongst the hills than they are in the lower coast region. The great changes of temperature also occur principally between one day and the next, in the summer season ; but between successive days and nights in the winter season. The mean temperature of the day jumps fitfully up and down during the season of summer. The mean temperature of the day, on the other hand, moves evenly along in the winter season, but there is then considerable difference between the temperature of day and night, and the daily range of temperature is accordingly large. The reason for this remarkable and interesting peculiarity is the frequent intervention of the cooling influence of dense cloud and rain in the season of summer, and the almost uninterrupted presence of clear skies in winter, allowing the full influence of sunshine to tell upon the ground by day, and the full power of radiation to come into play between sunset and sunrise.
V-
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