443
T H E SEASONS I N A TICOPICAL RAIN-FOREST
The seasons in a tropical rain-forest (New Hebrides).-Put
1. Meteorology.
R. BAKER,
M.A., D.Phil., a d T. H. HARRISSON.
By JOHN
(Communicated
by Professor E. S. GOQDRICH,
F.R.S., F.L.S.)
(With 8 Text-figures)
[Head 7 N(~vei~iber
19353
CONTENTS.
Introduction ................................................
The usual climate of Hog Harbour ............................
Comparison of the climate of' Hog Harbour with that of other
......................
tropical regions . . . . . . . . . . . . . .
The climate of Hog Harbour durlllg 1
.....................
(1) Methods .............................................
(2) General remarks. ..
...............................
( 3 ) Temperature (in the open and in the forest ; o n the grass
and hi the earth ; the cooling power of the atmosphere)
(4) Light. (Length of daylight atid of sunshine ; ultra-violet
light ; twilight.) . . . . . . . . . . .
.................
(5) Humidity. (Rainfall ; relati
idity ; saturation
deficiency ; absolute liuniidlty ; evaporation.) .
( 6 ) Barometric pressure ..................................
Summary .........................
....................
References . . . . . . . . . .
.....................
.............
Appendix ....................
.............
.
.
Page
443
446
446
430
450
453
463
455
439
461
461
462
463
INTRODUCTION.
'The studies described in this paper will be followed by papers upon seasonal
phenomena in plants and aninials in a tropical rain-forest. The whole
investigation was the main corporate activity of the Oxford University Expedition to the New Hebrides, 1933-4. It was planned and directed by one of us
(J.It. B.), but the whole expedition co-opcrated in carrying out the work. For
simplicity of reference, we are limiting the names of the authors of each paper
to two or three, but we wish to make it clear that all aix members of the
expedition did their shares of the meteorological observations, and we wish
to acknowledge the help of the others (Mr. A. J. Marshall, Mi. T. F. Bird,
Miss I. Baker, and Mrs. Z . Baker).
The expedition was made under the auspices of the Oxford Univesity
Exploration Club, and its meteorological and biological work was financed
partly by grants from the Royal Society, the Percy Sladen Memorial Fund,
444
DR. JOHN R . BAKER AND MR. T. H. HARRISSON ON
the University of Oxford, and New College, Oxford. When planning the
expedition we had the great advantage of receiving very helpful advice on
meteorology from Mr. W. G. Kendrew and Professor P. A. Buxton. Among
the many people who helped us in the islands we must particularly mention
the British and French Resident Commissioners, the Rev. W. Anderson,
Mr. T. R. ff. Salisbury, and Mr. W. Robertson. Many natives with accurate
knowledge of natural history gave willing help. The results of the expedition
are being worked out in thc Department of Zoology and Comparative Anatomy
a t Oxford by permission of Prof. E. S. Goodrich, E.R.S. A general account
of the expedition is given elsewhere (Baker, 1935 a).
The object of the whole investigation was to find how the reproduction of
animals and plants is carried on in a climate which varies little during the
course of the year. The investigation was mainly zoological, the plants being
studied rather as a side-line. I n the northern New Hebrides, in the Pacific
Ocean, there is little seasonal change of climate, and the vertebrate fauna
is sufficiently varied for a general study of the problem to be possible. The
main problems confronting us were these :-(1) What seasonal changes of
climate occur ? (2) Do animals and plants have distinct breeding and
flowering seasons '1 (3) Can the breeding and flowering seasons, if they exist,
be correlated with any particular meteorological phenomena 1 (4) Is any
teleological explanation of special breeding and flowering seasons in such
a climate possible ? I n this paper only the first of these problems is considered.
The meteorological observations were made a t Hog Harbour, which is
situated in latitude 15" 15' S. on the east coast of Espiritu Santo, the largest
island of the New Hebrides (see map, fig. 1). The island is about 75 miles
long and 45 miles wide. At Hog Harbour there live five white people and
between 150 and 200 natives. There are a coconut plantation and native
gardens. The village is only connected with other parts of the island by
narrow tortuous native paths which run through the luxuriant rain-forest.
This covers nearly the whole island except part of the south and south-east
coasts, where there is a number of plantations owned by whites. Native
villages, with their gardens, are scattered here and there throughout the
island, but occupy a very small total area. In some respects i t would have
been better to make the base away from all human habitation, but the
practical difficulty of getting all the equipment and food of an expedition
to such a place would be too great, nor could one afford to dispense with the
help of native collectors when hundreds of specimens were required monthly.
Indeed, the fact that one of us was already well known to the natives of Hog
Harbour influenced our choice of a base.
Meteorological observations were carried out by the expedition a t Hog
Harbour during a period of a year and a day, from 5 September 1933 to
5 September 1934 inclusive. I n considering the effect of the climate on breeding
seasons, one must consider not only the climate during the special period
in which the biological observations were made, but also the usual climate
THE SEASONS IN A TROPICAL RAIN-FOREST
F-.
.
.
445
446
DR. JOHN R. BAKER AND M R . T. H. HARRISSON ON
of the region studied, to which the organisms may be regarded as having
adapted themselves. It is convenient t.0 consider the usual climate of Hog
Harbour, and its climate during 1933-4, separately.
THEU S U A L
CLIMATE O F
HOGHARBOUR.
Mr. T. 0.Thoma?, formerly owner of the coconut plantation a t Hog Harbour,
kept records of the rainfall over a period of 23 years (1904-26), and has kindly
put his figures a t our disposal. As regards temperature, we have figures of
da,ily maxima and minima for three periods of twelve months, namely, the
observations of Mr. Thomas during 1926, of Mr. Thomas and one of us during
1927, and of this expedition during 1 9 3 3 4 . It must be mentioned that in
1926 and 1927 the temperature was taken in a shelter of palm-leaf thatch.
This shelter, being open on all sides but, giving excellent insulation from the
direct rays of the sun, gave reliable readings. During 1933-4 the observations
were made in a Stevenson screen. The usual climat,e of Hog Harbour, as
deduced from these observations, is shown in tabular form below. The mean
temperature of each month is the mean of the daily maxima and minima :Jan. Feb. Mar. Apr. May. June.
Mean temp., "C. . . . . . . . . . . . . . . 27.3 26.9 27.0 27.0 26.4 25.8
Mean daily rainfall (mm.) . . . . . . 11.44 11.54 10.31 11.20 10.98 5.75
July. Aug. Sept. Oct.
Mean temp., O C . . . . . . . . . . . . . . .
Mean daily rainfall (mm.) . . . . . .
Nov. Dec.
24.9 25.2 25.4 25.9 26.7 27.2
5.70 4.59 5.53 6.24 10.55 8.55
These figures are represented graphically by means of a hythergraph in
fig. 2 B. The hythergraph is made by plotting the mean temperature of each
month as ordinate, and the mean da,ily rainfall of each month as abscissa, and
joining the point,s so obtained in the order of the sequence of the months. The
general features of the climate of any place are well shown in this way.
There is little variation in temperature during the year, the hottest month
(January) being only 2 O . 4 C. (4O.3F.) hotter than tthe coolest (July). There
is no dry season, for during the least wet month there is about twice as much
rainfall as in the wettest month over most of England. Five consecutive
months in the year (June, July, August, September, and October) are, however,
considerably less wet, as well as a little cooler, than the rest. The wettest
month (February) receives on the average about two and a half times a s much
rain as the least wet (August).
OF THE
COMPARISON
CLIMATE OF
HOGHARBOUR
WITH
THAT
OF OTHER TROPICAL REGIONS.
The whole of this invesbigation was planned and undertaken because the
New Hebrides have an unusual climate which presents special biological
problems. It is necessary, first, to show how unusual the climate is.
THE SEASONS IN A TROPICAL RAIN-FOREST
447
I n temperate regions there is often little tendency for the rainfall to be
seasonal, and the seasons are demarcated by great changes in temperature.
I n the tropics the seasonal changes in temperature are small, but there are
usually great seasonal changes in rainfall. There appears to be no land
surface in the tropics in which there is no seasonal periodicity in rainfall.
In order to obtain a general view of this subject, an analysis was made of the
figures for all tfhe 113 tropical rainfall recording stations, whose mean rainfalls
for ten or more years are given in ' World Weather Records ' (Simpson and
others, 1934). The number of times that the wettest month was wetter than
26
'
2s
'
4
a,
Jepf
Fig. 2.-Hythergraphs
showing the climate of Hog Harbour during 1933-4 ( A )
and over a period of years (B).
the driest month was found for each of the stations. This was done by simply
dividing the amount of rainfall (millimetres or inches) in the driest month
into the amount in the wettest month It was not thought necessary to make
a correction for the varying number of days in the months. The resulting
figure may be called the ' seasonal index ' of rainfall. The seasonal index for
Hog Harbour is 2.5. Among all the 113 stations, distributed over the whole
of the tropics, that are mentioned in ' World Weather Records ', only three
have a lower seasonal index than this. These three are Pontianak (Borneo)
and San Juan (Porto Rico), which both have a seasonal index of 2.2, and
448
DR. JOHN R. BAKER AND MR. T. H. HARRISSON ON
Padang (Sumatra) with an index of 2 4 . Only nineteen of the stations have
an index less than 5.0. Of these nineteen, eight are in the East Indies, three
in the West Indies, three in the Pacific, three in South America, one in the
Atlantic, and one in Africa.
The following table shows how rare it is for a tropical country t o have a low
seasonal index of rainfall :Seasonal Index
(i.e. no. of times the wettest
month is wetter
than the driest).
No. of tropical stations
whose rainfall is recorded in
' World Weather Records '.
............................
............................
5.1-10.0 ............................
10~1-20~0............................
20-1-40*0 ............................
40.1-80.0 ............................
More than 8 0 . 0 . . ......................
0-2.6
2*f3-6*0
3
16
24
20
13
9
28
113
It might be thought that the somewhat uneven distribution of rainfallrecording stations materially decreases the value of the preceding remarks,
but this is not so. I n the above analysis Africa is only represented by one
station with an index less than 5, and therefore it is worth while to study
tropical Africa a little more closely. I n his book on the climate of Africa,
Knox (1911) gives a large diagram of the seasonal changes throughout the
tropical regions of the country. 30 of the 33 stations which he mentions have
less than 25 mm. of rain in the driest month and may therefore be regarded
as having a regular dry season. Of the other three stations, the least seasonal
has an index of 4.4. It might be thought that parts of the Belgian Congo
would have relatively unvarying climates, but Gasthuys (1924) has shown that
all the seven stations which he studied are markedly seasonal in rainfall.
Five of the stations have a regular dry season, while at the other two the
distribution of rainfa,ll is distinctly seasonal, although there is not any
prolonged absence of rain.
It has already been mentioned that the mean temperature of the hottest
month a t Hog Harbour is, on the average of three years, 2 O . 4 C. (4O.3F.)
greater than that of the coolest month. Although this is a small difference,
it is not an exceptionally small one for a tropical country. If we take all
the stations mentioned above whose seasonal index of rainfall is less than
5.0 and whose temperature changes are known, we find that the differences
between the hottest and the coolest months, in degrees Centigrade, are as
follows :-0-9, 1.3, 1.4, 1.4, 1*8,2*7,3.1,
3.2, 3 4 , 4 . 1 , and 4.9. A t Hog Harbour
the surrounding ocean ensures a relatively small seasonal change in temperature,
despite the considerable distance (15" 15') from the equator.
449
THE SEASONS IN A TROPICAL RAIN-FOREST
It must not be imagined that such an unseasonal climate as that of the
New Hebrides is found in all regions of the world where a typical rain-forest
is developed. Thus Allee (1926), in his study of the rain-forest of Panama,
showed that, although the temperature varies little during the year, there
is a very great annual cycle in rainfall, which changes almost regularly from
a minimum of about 1.1 inch in March to 16.4 inches in November. The mean
relative humidity is low (about 75 per cent.) in March and high (about 90 per
cent.) in November. There is, further, a regular annual cycle in cloudiness.
Kenoyer (1929) also speaks of the ' marked seasonal distribution ' of rainfall
in this rain-forest. McLean, again, in his study of the climate of the tropical
rain-forest (1919), speaks of the ' decided periodicity ' of that of South Brazil
in temperature, rainfall, relative humidity, and number of clear days. He
states that the cool months are regularly dry and the hot months wet.
It is clear that the climate of Hog Harbour is an unusually unvarying
one, even for a region where a rain-forest is developed. It remains to enquire
what parts of the world present an equally or more unvarying climate. The
little island of Nauru, in the Gilbert group in the Pacific, may have the most
unseasonal climate of any land-surface in the world, for the mean temperature
of the hottest month is said to be only OO.6 C. greater than that) of the coolest
(Kendrew, 1930), and the seasonal index of the rainfall works out a t only
about 2.5, while the situation, within half a degree of the equator, ensures
that the length of day scarcely varies. Jaluit Island, in the Marshall group,
has an even smaller range of temperature (OO.4 C.) and a lower seasonal index
of rainfall (about 1 4 ) , but the greater distance from the equator (nearly 6" N.)
makes the length of day slightly variable. Nevertheless, small isolated islands
aurh as these would be unsuited, from the poorness of their fauna,for any large
investigation of the effect o€an unseasonal climate on breeding seasons. Certain
places in the East Indies would perhaps be even better suited for a study of
this sort than the northern New Hebrides, and i t is hoped to visit one of them
in this connexion. I n particular, the vicinities of Pontianak in Borneo and
Padang in Sumatra seem almost ideal. The former is almost exactly on the
equator and the latter less than 1" away from it, so that the seasonal temperature
change must be very small, and that in length of day almost non-existant.
The seasonal indices of rainfall are 2.2 and 2.4 respectively, as compared with
2.5 a t Hog Harbour. Pontianak, with 2.2, has the smallest seasonal index of
all the 113 tropical stations analysed above. Although there is a number
of places in the East Indies with an unusually unseasonal climate, one must
avoid the error of supposing that these islands as a whole possess such a
climate. In addition to the eight East Indian stations already mentioned,
in which the seasonal index of rainfall is less than 5.0, there are six others
with indices of 5.9,7.1, 84,11.9, 35.5, and 262, and one in which there was no
rainfall at all in ten successive Septembers, but an average of 265 mm. in the
January of the same years. The tendency towards great seasonal changes in
rainfall culminates in the monsoon-forest of East Java, where teak and the
LINN. J0URN.-ZOOLOGY,
VOL. XXXIX
33
450
DR. JOHN R. BAKER AND MR. T. H. HARRISSON ON
great majority of the other trees are regularly deciduous during the dry season
(Schimper, 1903).
The climate of Hog Harbour may now be compared with that of other places
in its vicinity. Probably the other islands of the northern New Hebrides
and Banks Islands, nearly all of which we have visited, have a n essentially
similar climate, though along the west coast of Espiritu Santo there is a dry
period in the year. The southern New Hebrides and New Caledonia have
a distinctly seasonal climate as regards both temperature and rainfall. Fiji
(Suva) has a climate similar in general to that of Hog Harbour, with a slightly
higher seasonal index of rainfall and a greater temperature difference between
the hottest and the coolest months (4O.1 C. instead of 2 O . 4 C.). Samoa (Apia)
has a smaller temperature difference
but a much greater seasonal index
of rainfall (5.6) than Hog Harbour. The Solomon Islands, to the north-west,
resemble the New Hebrides. Guppy (1887) states that the meteorological
data a t his disposal ' do not warrant the conclusion that one season is wetter
than another '. This statement, however, does not seem to be fully borne
out by figures which he gives a t the end of his book ; but it is clear that,
as a t Hog Harbour, there is no dry season. The more recent figures (Robson,
1933) from Tulagi show that there is about 3.3 times as much ra.in in the wettest,
month (February) as in the driest (December).
The conclusion to be drawn from this short survey of tropical climates is
that there are few places in the world which are less seasonal than the vicinity
of Hog Harbour. Over much the greater part of the tropics the seasons are
as well marked by changes in rainfall as they are by changes in temperature
in the temperate regions. Hog Harbour is therefore of particular interest,
to the student of seasonal biology.
( l o e l ) ,
THE CLIMATE OF HOGHARBOTJR
DURING 1933-4.
(1) Methods.
During the twelve months (September 1933 to August 1934) in which the
breeding and flowering seasons were studied, daily records of climate were
made. Two meteorological stations were set up, one in an area of several acres
of cleared ground belonging to the Presbyterian Mission and the other in the
rain-forest. These will be called the sunshine station and the forest station
respectively.
The sunshine station was situated on almost level ground about a quarter
of a mile from the sea and about 40 feet above sea-level. Care was taken
to expose all instruments in accordance with the instructions given in ' The
Meteorological Observer's Handbook ' (Anonymous, 1926). A Stevenson screen
was used for the exposure of maximum and minimum thermometers. The
bulbs were more than three inches from the insides of the top, bottom, and
sides of the screen. A grass minimum thermometer was exposed nightly.
Rainfall was measured by an %inch rain-gauge, which was read daily a t about
452
DR. JOHN R . BAKER AND M R . T. H. HARRISSON ON
any direct sunlight filters through the dense foliage, it was not necessary to
use a Stevenson screen. A simple screen with single louvres was attached
to a tree a t the standard height of a Stevenson screen. I n this screen a
maximum and a minimum thermometer and wet and dry bulb thermometers
were exposed. The wet and dry bulb thermometers were read a t 2 p.m. daily.
The appropriate tables for still air were used in the calculation of relative
humidity. An earth thermometer, whose bulb was situated 30 cm. below the
surface of the ground, was read daily at 2 p.m. A kata-thermometer was
suspended from a wire running between two trees, a t such a height that its
bulb was one metre above the ground. The standard model, not the tropical
model, was used, since it is a better indicator of the cooling power of the
atmosphere on warm-blooded animals. Since the temperature in the forest
never reached 95" F., there was never any necessity to substitute the tropical
model. The kata-thermometer was read once dry and once wet every day a t
approximately 2 p.m. An umbrella was held over the instrument when
the readings were made in the rain. The instrument was always warmed
up and allowed t o cool once before the first reading was made. The time
of cooling was measured by stop-watch. During the f i s t two months two dry
readings and two wet readings were made consecutively, but the results showed
that this was a waste of time. The daily first readings of the dry instrument
during the month of October averaged 100.7 seconds. The average of
both readings for the same period was 101.1 seconds. The difference being
negligible, the second reading was discontinued. I n hot climates, especially
in such situations as forests, in which draughts are seldom felt, repeated readings
are unnecessary on account of the slowness of the cooling, which allows sufficient
time for a fair sample of the conditions to exert their effect. A spherical-bulb
Livingston atmometer (evaporimeter), with valve, was exposed among the
vegetation of the forest, the middle of the bulb being 40 cm. above the ground.
Evaporation being extremely slow, readings were made only a t the end of
each month. Distilled water was used. At the beginning of each month
a new evaporimeter, with a bulb that had been cleaned and dried, was substituted. Dr. Livingston's directions were carefully followed throughout.
The results are expressed as the number of C.C. evaporated in a given time,
multiplied by the ' factor ' inscribed on the bulb. The product is in ' Livingston
units ', which are not absolute units. but which are nevertheless the accepted
measure for comparative purposes. (See Livingston, 1935.) The utmost
care was taken not to disturb the undergrowth a t the forest station more
than was absolutely necessary to get a t the instruments. The path t o the
station was intentionally made crooked, so as to avoid the possibility of a direct,
draught.
Most of the meteorological instruments used were made by Messrs. CaselIa,
and we wish to acknowledge our satisfaction with them. Each thermometer
except the grass minimum was tested shortly before or after the expedition,
and all except one were found to have no error or negligible errors in the
THE SEASONS IN A TkOPICAL RAIN-FOREST
453
relevant part of the scale. The exception was the ground thermometer
(by another maker), which had a small error which is allowed for in the
figures given in this paper.
(2) General remarks.
The daily readings were averaged monthly, and the climate for the year is
shown in the Appendix in the form of monthly means.
Fig. 2 A is a hythergraph showing the general features of the climate during
the year September 1933 to August 1934. The temperatures are the monthly
means of the maxima and minima taken in the Stevenson screen at the
sunshine station. 'It will be realised that, a hythergraph showing a single year's
climate will naturally tend to be less regular than one showing the mean climate
of several years. The hythergraph shows that, as usual, the months of June,
July, August, September, and October were less wet and slightly cooler than
December, January, February, and March ; but April and May, usually very
wet months, were not nearly as wet as usual. Further, the total rainfall
for the year was above the average. From 5 September 1933 to 4 September
1934, inclusive, the total rainfall was 3659 mm. (144.1 inches). During the
period 1904-26 inclusive, the mean annual rainfall was 3106 mm. (122.3 inches).
( 3 ) Temperature (in the open and in the forest ; on the grass and in the earth ;
the cooling power of the atmosphere).
The temperatures under various conditions during 1 9 3 3 4 are shown in fig. 3.
(It is unfortunate that the breakage of a maximum thermometer resulted in no
maxima being recorded in the Stevenson screen from the middle of May to the
middle of July. Thus there are no figures for maxima in the sunshine station
during June, and the figures on which the means for May and July are based
are incomplete.) The general conclusions to be drawn from all the temperature
records taken together are these. December to April was on the whole
slightly hotter than the rest of the year. After April the temperature fell
off and reached a minimum in August. Four of the six thermometers made
August the coolest month, and the other two July. Thus there was a definite
but slight seasonal change in temperature. The mean temperature in the
screen in the open only varied by 2"-1C. between the hottest and coolest
months, about one-eighth of the variation a t English stations during the same
period. The minima in the forest were closely similar to the minima in the
screen a t the sunshine station, but the forest maxima were always much
below the maxima in the screen. High temperatures were never reached
in the forest. I n April the mean maximum temperature was only 27"*6C.,
and this was the month with the highest maxima in this situation. Under
no circumstances, however, was the climate ever anything but warm. The
lowest single reading of the minimum thermometer in the screen at the
sunshine station was 17O.3 C. on 16 June. Even the grass minimum, fully
exposed to the sky a t night, never gave low readings. In August, the coolest
454
DR. JOHN R. BAKER AND MR. T. H. HARRISSON ON
month, the mean minimum on the grass was 20".2C. The earth attained
its maximum temperature in April and its minimum in August.
The cooling power of the atmosphere, as determined by the kata-thermometer daily a t approximately 2 p.m., is shown graphically in fig. 4. For
comparison, readings were made also a t Oxford during July 1934 and *January
1936 in a strip of wood in the Oxford University Parks, composed mainly
of sycamore-maple, elm, and yew. Two readings were made here daily of both
wet and dry bulb kata,-thermometer a t 2 p.m. G.M.T., and the means are shown
on the graph. The readings are joined by broken lines to aid the eye in making
S
O
N
D
J
M
J
J
A
FIQ.3.--Monthly mean temperatures at Hog Harbour, 1933-4.
a comparison with the conditions at Hog Htlrbour.
(Two or more readings
daily a t approximately the same time are necessary in a cool climate, in which
the cooling time is short). It is immediately obvious that' the cooling power
of the atmosphere is very constant at Hog Harbour and varies widely a t
Oxford.
July 1934 was unusually hot at Oxford, and approached the coolest month
a t Hog Harbour in dry kata-thermometer readings ; but the high humidity a t
Hog Harbour made a wide difference between the wet kata-thermometer
readings at the two places, a difference that is exemplified by the failure of
*
THE SEASONS IN A TEOPICAL EAIX-FOREST
\
FIG.4.-The mean cooling power. of the atmosphere at Hog Harbour (1933-4)
and at Oxford ( 1 9 3 4 4 ) .
45.3
456
1JR. JOHN R. BAKER AND MR. T. H. HARRISSON ON
one's sweat to evaporate a t Hog Harbour. By both wet and dry katathermometer the coolest months are July and August. The trade winds, which
blow steadily a t that time, are probably partly responsible for the higher
cooling powers of the atmosphere recorded in these months, though naturally
only a little of their effect is felt in the forest. Buxton (1927) attributed the
seasonal change that he found in kata-thermometer readings in Samoa mostly
to variations in the amount of wind. His observations, however, were made
on a verandah exposed to the wind.
During one week (19-25 December 1933, inclusive) repeated readings of
the kata-thermometer were made throughout t,he day and night a t the forest
station, in order to find tJhe diurnal variation. We wish to acknowledge the
special co-operation of Miss I. Baker a,nd M i . T. F. Bird in this work. Sixteen
I
,
Mrd
mgh?
1
4
b
8
9
I0
n,d
2
, + a 5 6
7
8
O'SY
10
IIN
ld
night.
FIG.5.-The mean cooling power of the atmosphere in the forest at
Hog Harbour, 19-25 Dee. 1933, inclusive.
readings of dry and wet bulb kata-thermometer were made daily, a t 2, 4, 6,
8, 9, and 10 a.m., midday, 2, 4, 5, 6, 7, 8, and 10 pm., and midnight. The
means of the seven days are shown in the graph (fig. 5 ) . 2 p.m., the time of the
daily reading, is the time at which the cooling power of the atmosphere is a t
its lowest. The cooling power increases smoothly thereafter and reaches its
highest point, or a figure very close to it, a t 10 p.m. The dry kata-thermometer
gives almost the same reading from 10 p.m. throughout the night until 6 a.m.,
while the wet one, being especially sensitive to draughts, is less steady. By
8 a.m. the cooling power of the atmosphere is already approaching the minimum
for the day. The diurnal range of cooling power is small (only 1.61 millicalories
per square cm. per sec. by the dry kata-thermometer). It is hoped that
-
5
0
fi
/J
J
f
PI
A
M
J
J
----(
Fra. &-Mean daily hours of daylight and of sunshine at Hog Harbour
and at Ross-on-Wye.
A
458
I)&.
JOHN It. BAKE% A N D M R .
T. H. HARRISSON Oh’
figures will be obtained in forests in other parts of the world for comparison
of the annual and diurnal change. The kata-thermometer, by integrating
the effects of temperature, humidity, and air currents, provides one of the
best means of stating the action of climate upon organisms.
(Length of daylight and of sunshine ; ultra-violet light ; twilight.)
(4)Light.
The longest day at Hog Harbour is about 13 hours 2 minutes and the shortest
about 11 hours 14 minutes. A comparison of the seasonal change of length of
day a t Hog Harbour with that a t an English station is shown in fig. 6. The
sun passes overhead a t Hog Harbour on 14 November and 8 February.
The mean daily number of hours of bright sunshine during 1 9 3 3 4 a t an
English station (Ross-on-Wye) and a t Hog Harbour is a,lso shown in fig. 6.
One sees at a glance that the distribution of sunshine is strongly seasonal in
England, while a t Hog Harbour no obvious rhythm is detectable. In the
year under consideration (and there are no data for any other year) April was
the sunniest month and July the cloudiest. It must be remembered, however,
that April is usually a very wet month, and that April 1934 was exceptional
in not being very wet. It is improbable that there is a tendency t o any regular
seasonal rhythm in hours of sunshine.
Ultra-violet light was measured during certain months only. The mean
number of units of ultra-violet light daily were as follows :-September, 6.4 ;
October 6.7 ; December 7.6 ; February 3.5 ; June 2.1. (Very low figures
were obtained during July and August ; but ILS the quartz tube was cracked
a t the time, we have disallowed them.) The method of measuring ultra-violet
light by the fading of the acetone methylene blue solution is open t o criticism,
but the low figures obtained in February and June are probably sigm6cant.
The low figure in June may be caused by the light having to filter a greater
distance through the air during this month t,han during any other, the sun’s
rays being then more oblique. During February the sun passes directly
overhead, and the low amount of ultra-light is to be ascribed, partly a t least,
to the cloudiness during this month. In Samoa, which is in nearly the same
latitude, February is the cloudiest month in the year (Buxton, 1927).
The mean times of disappearance of the white spot,, indicating t,he end of
twilight, are shown in the table :Mean time of‘
Month.
Dec. 1933
Jan. 1934
Feb. 1934
Mar. 1934
Apr. 1934
May 1934
Aug. 1934
......
......
......
......
......
......
......
Number of
observations.
4
6
4
10
10
7
10
sunset on the
nights of
observation.
p.m.
6.24
6.32
6.30
6.13
653
5.38
5.47
End of twilight.
Mean duration
of twilight
(minutes).
p.m.
7.16
7.09
7.04
6.46
636
6.41
6.31
62
37
34
33
43
63
44
THE SEASONS IN A TROPICAL RAIN-FOREST
459
It is clear that the period of twilight is always short and varies irregularlg
during the year. It is hoped that comparable observations will be made
in other parts of the world.
( 5 ) Humidity. (Rainfall ; relative humidity ; saturation deJiciency ; wbsolute
humidity : evaporation.)
The rainfall has already been mentioned in the general remarks on the climate
during 1933-4, and illustrated in the hythergraph, fig. 2 A. It, will be
recollected that April and May, which are usually very wet months, were
relatively dry.
460
DR. JOHN W. BAKER AND MR. T. 11. HARRISSON ON
The relative humidity was measured daily in the forest a t 2 p.m. It was
During the other months it
varied irregularly between 87 per cent. and 91 per cent., the latter being the
figure during five of the months. During one week (19-25 December 1933,
inclusive) it was also measured daily a t 4 a.m. and the following figures were
obtained :-98, 96, 95, 96, 98, 96, 100.
Prof. P. A. Buxton (1931) has stressed the significance of the saturation
deficiency of the atmosphere in biological investigations. Saturation deficiency
may be expressed as the number of grams of water that a cubic metre of a given
sample of air is capable of absorbing, and this depends upon the existing
temperature and relative hurnidit#y. It, is an index of the drying power of
at its lowest (85 per cent.) in April and May.
FIG.8.-Barometric pressure at Hog Harbour, 1933-4.
the atmosphere. One of us (Baker, 19356) has designed nomograms for the
rapid calculation of saturation deficiency, which were used in the preparation
of fig. 7. I n this graph the mean monthly saturation deficiency in the forest
at Hog Harbour a t 2 p.m. is compared with that a t two selected stations during
the same period (September 1933 to August 1934). These stations are
Ross-on-Wye, England, and Bellary, South India. The former was chosen
to show how greatly and how regularly the drying power of the atmosphere
varies during the year, despite the small seasonal changes in rainfall. Bellary
was chosen as typical of monsoon areas, with their enormous seasonal change
in saturation deficiency. Unfortunately there is no standard time of day for
reading wet and dry bulb thermometers. At Bellary the readings were made
a t 8 a.m., and a t Ross-on-Wye a t 1 p.m. Despite these differences in time,
general tendencies during the year may be observed. The Hog Harbour
graph shows no definite seasonal tendency, though the two exceptional months,
April and May, show up as significantly drier than the others. The rest of
the year is remarkably uniform in saturation deficiency.
THE SEASONS IN A TROPICAL RAIN-FOREST
46 1
The absolute humidity in the forest at Hog Harbour is always high and does
not change much during the year. Curiously enough, it reached its highest,
(23-0 grams per cubic metre) in the month (April) in which the rainfall was
least., and its lowest (19.8 grams per cubic metre) in July. The dampness of
the atmosphere is reflected in t#herapid decay of leather objectJs,the sticking
up of unused envelopes, etc.
No definite seasonal cycle can be traced in the amount of evaporation in
the forest,,though it varied considerably from month to month. (See Appendix.
I n April high winds temporarily destroyed the foliage protecting the evaporimeter from the direct rays of the sun, and the readings of that month have
therefore been discarded.)
(6) Barometric pressure.
It is a remarkable fact that of all the meteorological readings that we took,
the most definitely seasonal are those of barometric pressure (fig. 8). The
readings were made a t 1.45 p.m. daily. With the exception of only one month
(February), there was a regular decrease of pressure from September to March.
Thereafter there was an increase until August, without any month having a
lower pressure than the month before it. The difference between the months
of highest a,nd lowest pressure (September and March) was, however, only
0.23 inch. In the absence of any experimental evidence to the contrary,
it seems likely that this seasonal change is too small t o have any appreciable
influence on organisms.
SUMMARY.
(1) In connexion with an investigation of the reproduct,ion of organisms
in a climate which varies little during the year, meteorologica.1 observations
were made from September 1933 to August 1934 a t Hog Harbour, Espiritii
Santo, New Hebrides (15' 15' S. latitude). Special &ention was paid to the
measurement of the climate in which the animals actually live, namely in
the rain-forest.
(2) I n general, the climate a t Hog Harbour is hot and wet, from .June to
October, and slightly hotter and considerably wetter from November till May.
There is no dry season, the least, wet month having, on the average, about
twice as much rain as the wettest month in England. The year under investigation conformed t,o the average except. that, April and May were less wet than
usual.
(3) The mean temperature only varied by 2".1 C. (3O.8 P.) between tjhe
hottest and coldest months. I n England during the same period the seasonal
change was about eight times as great.
(4)The distribution of rainfall is exceptionally unseasonal for a tropical
region, since the wettest, month receives on the average only 2.5 times as much
rain as t,he least wet. An analysis of meteorological data from the whole
of the tropics shows that such a low figure as this is very rare.
462
THE SEASONS IN A TROPICAL RATN-FOREST
(5) The cooling power of the atmosphere, ELS measured by the kata-thermometer, was greater in July and August than in the other months, but the
difference was small.
(6) There is no obvious seasonal distribution of daily hours of sunshine,
but there appears to be much less ultra-violet light in oertain months (e.g. June)
t,han in others.
(7) The saturation deficiency of the atmosphere was almost uniform
throughout the year, except that it was higher during April and May. The
amount of evaporation varied considerably from month to month, without
disclosing any distinct seasonal tendency.
(8) Barometric pressure varied only very slightly during the year, but its
variations were very regular and indeed constitute the most regular seasonal
change observable in the climate.
REFERENCES.
ALEE, W. C. I M G . I+Ccology,vii, p. 273.
ANONYMOUS.1946. ‘ The MeteorologicalObserver’s Handbook ’. London : H.M. Statinnery
Office.
I ~ A K E R ,J. R. 1934. Nature, cxxxiv, p. 139.
1938 a. Geog. Journ. lxxxv, p. 209.
- l935b. Journ. Sci. Instr. xu, p. 214.
HUXTON, P. A. 1927. ‘ Researches in Polynesia and Melanesia ’. London School of
Hygiene and Tropical Medicine.
- 1931. Bull. Ent. Res. xxii, p. 431.
GASTHUYS.P. 1934. Bull. Agric. Congo Belge, xv. p. 376.
GUPPY,H. B. 1887. ‘ The Solomon Islands and their natives ’. TAontlon.
KENIIREW,
W. G. 1930. ‘ Climate’. Oxford.
KENOYER,
L. A. 1929. Ecology, x, p. 401.
KNOX,A. 1911. ‘ The climate of the continent of Africa’. Cambridge.
rAIVINUSTON, B. E.
1935. Ecology, xvi, p. 438.
MCLEAN,R. C. 1919. Journ. Ecol. vii, p. 6.
RORSON,
R. W. 1933. ‘ Handbook of the Western Pacific’. Sydney.
SCHIBIPER,
A. F. W. 1903. ‘ Plant-geography upon a physiological basis’. Oxfortl.
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