1. science
2. weather

JOURNAL OF CLIMATOLOGY, VOL. 8. 283 -296 (1988)
55 1.553.21:551.577.32(548)
THE ONSET OF THE SOUTHWEST MONSOON OVER KERALA:
1901-1980
R . A N A N T H A K R I S H N A N A N D M. K. S O M A N
Indian lnslirute of Tropical Meleorology. Pune 41 1005, India
Receioed 6 A U ~ U S1987
I
Rc,r>ised28 OcIoher 1987
ABSTRACT
Utilising daily mean rainfall from dense rain gauge networks, the dates of onset of the southwest monsoon over south and
north Kerala have been derived on the basis of objective criteria for the years 1901 to 1980. These dates have been
compared with the onset dates as per records of the India Meteorological Department. Statistics of the onset dates are
presented. The mean onset date for south Kerala is found to be 30 May and for north Kerala 1 June with a standard
deviation of about 9 days in both cases. The sharp increase in rainfall that heralds the onset of the monsoon is highlighted
by superposed epoch analysis. The prevailing notion that rainfall from pre-monsoon thunderstorms progressively
increases and merges with thc monsoon rainfall is shown to be not valid.
KEYWORIIS
Southwest monsoon Onset dates Kerala.
1. INTRODUCTION
The date of onset of the southwest monsoon over Kerala is a crucial date in the meteorological calendar of
India as it marks the beginning of the rainy season for the country. Although the onset of the monsoon is
associated with changes in the circulation features in the lower and upper troposphere, a sustained increase in
rainfall at the observatory stations over Kerala and the island stations over the southeast Arabian Sea is an
essential feature of the monsoon onset. It is difficult to quantify this precisely and so the experience of the
forecaster plays a key role in declaring the date of monsoon onset in individual years.
Some two decades ago a critical study was undertaken to examine the monsoon onset dates as per records
of the India Meteorological Department (IMD) from 1901 onwards in relation to the daily rainfall from 1
May to 30 June at seven observatory stations over Kerala and the adjoining sea area (Ananthakrishnan et al.,
1967). Based on this study some broad criteria were drawn up for operational use. It should be noted that in
general, the forecaster works with a measure of caution. The post-mortem study of the onset dates in relation
to daily rainfall distribution showed that in many years the onset dates should have been somewhat earlier
than the declared dates; the discrepancies were appreciable in some years.
Apart from the small number of observatory stations, Kerala has a dense network of rain gauge stations
which have been in existence from the end of the last century. It was considered to be of interest to examine
the behaviour of mean daily rainfall of Kerala utilizing the data from this network in relation to the monsoon
onset. The aim was to arrive at a uniform set of onset dates based on objective rainfall criteria since the
existing dates of onset as per IMD records are based on subjective estimates by several forecasters over the
years. In recent years there have been attempts to forecast the dates of monsoon onset using regression
techniques based on past data (Kung and Sharif 1980,1982). For such studies it is desirable to have a uniform
set of onset dates free from subjectivity.
2. MEAN RAINFALL SERIES AND OBJECTIVE CRITERIA FOR MONSOON ONSET
The rain gauge network over Kerala utilized in the present study is shown in Figure 1. Latitudinally the state
of Kerala extends from 8" 15" to 12" 50". For the present study we have considered south Kerala and
01961748/88/030283- 14$07.00
the Royal Meteorological Society
0 1988 by
284
R AKAKTHAKRISHNAN A K D M. K SOMAN
13
li
ii
Id
4
0
Figure 1. Rain gauge network over Kcrala
north Kerala separately, the dividing line being around 10 N. The division is based on the fact that the
monsoon rainfall progresses from south to north along the west coast of India and also on the known
difference in the rainfall characteristics between the southern and northern parts of the state. South Kerala
has 44 rain gauge stations and north Kerala 31, more or less uniformly distributed over the respective
regions.
Utiliing daily rainfall recorded at the rain gauge networks, daily rainfall series were constructed for south
and north Kerala for the years 1901 to 1980 by averaging the rainfall at the individual stations. Examination
of these rainfall series revealed alternating patterns of spells of light and heavy rain amounts of varying
durations. Light rain spells are a feature of the pre-monsoon months which give place to heavy rain spells
hcralding the onset of the southwest monsoon. The transition is sharp and abrupt.
January and February are the driest months in Kerala with very little rainfall. Thunderstorm activity sets
in from March and progressively increases with the advance of the season. This activity is more pronounced
over south Kerala. The long term (1901-1 980) normal daily rainfall of south Kerala for the months of March
and April is 1.9 and 4.8 mm respectively; the corresponding amounts arc lcss than 1 mm for north Kerala.
The area-averaged rainfall associated with pre-monsoon thunderstorm activity is found to be less than
10 mm per day. Averaged over individual spells whose durations are variable, the amounts are found to be
between 2 and 7 mm for south Kerala and less for north Kerala. We designate these as light rain spells.
Thc long term normal daily rainfall for May and June as found from our study is 8.9 and 19.5 mm
respectively for south Kerala and 7.3 and 24.0 mm for north Kerala. These are contributed by light rain spells
OKSF7' O F T H t SOIJTIIWEST M O N S O O K
28 5
before the monsoon onset and heavy rain spells during and after the monsoon onset. The observational data
showed that during such heavy spells the daily area-averaged rainfall exceeds 10 mm per day on most 0 1 the
days while the rainfall averaged over the spell duration varies between 15 and 50 mm per day. In view of this,
a limit of 10 mm per day was arbitrarily chosen as the demarcation between light and heavy rain spells.
Since we are dealing with the average rainfall from a large network of stations distributed over the regions,
it is obvious that the heavy spells arc associated with widespread moderate to heavy rainfall while light spells
imply scattered rainfall of small amounts with perhaps isolated heavy falls. This distinction in the nature of
the two types of rain spells is important. It implies that the physical mechanisms associated with them are
different. Heavy rain spells imply deep convection in association with synoptic scale systems indicating ITCZ
activity; light spells imply sporadic convective activity associated with prc-monsoon thunderstorms or weak
monsoon conditions after monsoon onset.
The date of monsoon onset is taken as the first day of the transition from light to heavy rain spell category
with the proviso that the average daily rainfall during the first 5 days after the transition should not be less
than 10 mm. In several years the heavy rain spell which heralds the onset of the monsoon continues for more
than 5 days with daily rainfdl in excess of 10 mm on all days. There are, however, some years in which the
daily rainfall on 1 or 2 days during the onset spell may fall below 10 mm, the average still remaining in cxccss
of 10 mm per day. This is associated with the pulsatory nature of monsoon activity.
As an illustration, the daily rainfall sequences from 1 May to 30 June for south Kerala for the years 1917,
1918, 1979 and 1980 are shown in Figure 2 in which the monsoon onset dates fixed by the above-mentioned
criteria are also indicated. The average daily rainfall for the first 5 days including the onset date in these years
is 18.1, 19.1, 27.7 and 17.3 mm respectively. I t may be noted that in the first 3 years the daily rainfall was less
than 1 0 nim on 1 day during the first 5 days following the onset.
The rainfall sequences associated with the monsoon onset over Kerala and the further northward advance
of the monsoon vary from year to year. In some years the onset spell of heavy rains is followed by a prolonged
spell of light rainfall with subsequent revival of rain activity. In such years it has been customary in the IMD
to regard the first onset as a temporary phase and consider the revival date as the onset date. We have
adopted the same convention in the present study.
Figures 3 and 4 illustrate the nature of the rainfall transition associated with the onset and advance of the
monsoon over Kerala for two years, 1936 and 1951. In these diagrams, the distribution of rainfall over the
station network is shown for 4 consecutive days starting from the date prior to the onset. The rainfall is
plotted correct to 1 mm over the station locations shown in Figure I . Stations with zero rainfall are left blank.
In 1936 the monsoon set in over south Kerala on 20 May. On the previous day the rainfall over south
Kerala was sporadic with the daily mean of 8.7 mm. By the next day widespread heavy rainfall with an
average amount of 41.6 mm was recorded. The monsoon advanced into north Kcrala on 21 May and by the
next day widespread heavy rainfall occurred all over the State with average amounts of 61.4 mm and
80.3 mm respectively over south and north Kcrala. In 1951 the onset over south Kerala occurred on 30 May
and over north Kerala on 1 June. The spectacular increase in rainfall over north Kerala from 31 May to 1
June may be noted.
3. ONSET DATES OVER SOUTH AND WORTH KERALA (1901 1980)
Following the criteria discussed in the previous section, the dates of monsoon onset over south and north
Kerala were fixed for the individual years 1901 1980. These dates are listed in Table I in which the monsoon
onset dates for Kerala as per IMD records are also given. The IMD records give only a single date for Kerala
as a whole. The onset dates vary between 7 May and 22 June. Comparison of the corresponding onset dates
for south and north Kerala show that, in many years, the onset over north Kerala was either simultaneous or
I to 3 days later than over south Kerala. In 1913, 1952, 1953, 1965 and 1977 the northward progress of the
heavy rain spells was slow, resulting in delayed onset of the monsoon over north Kerala by 10 days o r more
compared with south Kerala.
Table I shows that for the six years 1932, 1943. 1949, 1955, 1957 and 1959 the IMD onset dates were later
by 10 days or more than the dates we have found from the rainfall time series: in 1969 the IMD onset date
286
R A N A N T H A K R I S H N A N A N D M . K. S O M A N
mm
2;
+-MAY-
c
30
+
-,
-*loJu,:'
4ol-++-!+4---
T--- r
-
I
l
-
I
I9*O
L O
20
10
I
I
30
30
I
20-
20
I
10-
o-dll..lll'
I**Al
30 -
20-
10-
0-
20
'
I
'
I9l7
I
6
I1
k----Figure 2. Mean daily rainfall over south Kerala from 1 May to 30 Junc for selected years showing onset dates of the southwest monsoon
was 8 days earlier. Scrutiny of the rainfall data shows that the IMD onset dates have to be revised for all these
years. The daily rainfall sequence from 1 May to 30 June for some of these years is depicted in Figure 5. The
mean daily rainfall during the first 5 days including the onset dates for the ycars 1932, 1943, 1959 and 1969
are 19.5, 16.2, 14.2 and 13.4 mm respectively, satisfying the onset criteria.
Table I1 gives the number of onset years in 3-day intervals for south and north Kerala through May and
June. For the 80-year period the onset dates are equally distributed between May and June for south Kerala;
for north Kerala the corresponding numbers are in the ratio 35.
The mean, median and standard deviations of the onset dates are given in Tablc 111. The mean onset date
for south Kerala is 30 May and for north Kerala 1 June with a standard deviation of about 9 days in both
cases. The median onset dates are found to be a couple of days after the mean dates. The mean onset date for
Kerala as a whole as per IMD data in Table I is 2 June which is later by 3 days and 1 day respectively from
the mean onset dates for south and north Kerala of the present study.
4. SUPER-POGED EPOCH ANALYSIS OF THE MEAN DAILY RAINFALL SERIES IN RELATION
T O THE MONSOON ONSET
It is often stated that the pre-monsoon thunderstorm rains over Kerala progressively increase and merge
with the monsoon rains on account of which the fixation of the monsoon onset date is rendered difficult
KERALA
DAILY RAINFALL I m m l
2 2 MAY 1936
A R A B I A N
S E A
Figure 3. Rainfall sequencc associated with the onset and advance of the southwest monsoon over Kerala (1936)
KERALA
DAILY R A I N F A L L ( m m )
2 9 M A Y 1951
I0
A R A ~ I A N
S E A
IS
7-
*
KERALA
157
DAILY RAINFALL l m m )
I JUNEIS51
12.
123
39
I1
I0
10
A R A B I A N
S E A
09
0,
75
Figure 4. Rainfall sequence associated with the onset and advance of the southwest monsoon over Kerala (1951)
36
37
43
32
39
33
36
39
33
37
35
31
40
32
36
26
35
25
33
45
27
27
07
28
33
1901
1902
I903
1904
1905
1906
1907
1908
1909
1910
1911
i912
1913
1914
1915
1916
1917
1918
1919
1920
Year
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
IMD
38
37
43
38
41
45
39
42
33
33
37
39
33
35
46
33
31
11
34
34
NK
37
31
41
32
27
38
27
34
32
38
30
15
22
39
45
21
34
26
37
45
SK
38
31
36
31
27
36
23
34
32
36
31
14
21
37
45
20
33
32
36
43
33
31
42
33
27
37
27
34
29
39
35
33
22
39
43
19
35
26
36
45
IMD
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
,1954
1955
1956
1957
1958
1959
1960
Year
22
32
12
33
32
29
33
41
13
27
30
20
37
28
17
18
18
43
12
14
SK
15
15
44
22
41
13
34
36
34
33
40
13
27
32
33
48
32
16
20
18
NK
Year
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
IMD
23
41
29
34
36
29
34
42
23
27
31
20
38
31
29
21
32
45
31
14
SK, Onset date over south Kerala; NK, Onset date over north Kerala; IMD, Onset date for Kerala as per I M D records.
Dates are given sequentially as follows: 1 May=Ol . . . 31 May=31; I J u n e = 3 2 . . . 30 June=61.
37
41
33
37
35
36
35
35
42
28
30
08
35
34
44
NK
SK
Year
Table I. Onset dates of the southwest monsoon over Kerala (1901-1980).
10
36
36
24
31
39
38
25
25
25
53
34
23
32
30
27
27
42
31
18
SK
40
32
26
25
53
37
23
31
31
38
29
43
31
32
44
18
17
31
37
26
31
40
39
17
26
27
49
35
26
31
31
30
28
20
10
35
35
37
31
40
IMD
NK
z
3
C
I
290
R . A N A N T H A K R I S H N A N A N D M. K. S O M A N
1.
-MAY
mm
40,+
::[
6
11
I
16
21
26
1
1
I
311
I S
5
10
JUNE
I5
I
1
1
25
20
I
' I
.
Y
I
I
30
I ,
mm
-40
- 30
I I
- 20
- 10
0
0
I- 30
-
20
- 10
0
- 30
-20
b-
10
0
-30
- 20
10
0
Figure 5. Mean daily rainfall over south Kerala from 1 May to 30 June showing IMD and refixed onset dates.
(IMD, 1943).Our study shows that the types of rainfall associated with the two events are distinctly different,
as stated in Section 2. Merging occurs when the rainfall data for several years are averagcd for calendar dates
because of the dispersal in the onset dates over a period of years. Rainfall curves constructed by such
averaging give the impression of the merging of pre-monsoon and monsoon rainfall.
To highlight the abrupt nature of the rainfall transition during the onset, the daily mean rainfall amounts
for the 80-year period were composited in two ways:
(a) by averaging the rainfall amounts for calendar dates (Figures 6a, b).
(b) by averaging the daily rainfall amounts after superposing the onset dates for the individual years
(Figures 7a, b).
Figures 6a, b present the variation of the calendar date averaged daily rainfall for south and north Kerala
from the third week of March to the end of August. The mean onset dates are indicated on these diagrams. It
29 1
ONSET OF THE SOIJTHWEST MONSOON
Table 11. Number of onset years in 3-day intervals
S.K.
Interval
May 5-7
8-10
11-13
14-16
17 19
2&22
23-25
26 28
29 31
Total
40
N.K.
IMD
0
2
2
4
0
0
1
5
2
6
7
4
3
2
9
14
29
34
1
1
Interval
June 1-3
4- 6
7-9
10 12
13-15
16 18
19 21
22-25
S.K.
N.K.
1MD
15
13
15
16
8
6
4
16
8
9
7
5
1
1
5
4
2
0
0
0
1
1
40
51
46
SK, South Kerala; NK, North Kerala.
Table 111. Mean, median and standard deviation of onset dates
Parameter
Mean date
Standard deviation
Median date
Earliest onset date
Latest onset date
S. Kerala
30 May
8.8 days
1 June
7 May 1918
22June 1972
N. Kerala
1 June
9.2 days
3 June
8 May 1918
22 June 1972
IMD
2 June
7.8 days
2 June
1 1 May 1918
18 June 1972
may be noted that from about the second week of May till the onset date there is a progressive increase in the
mean daily rainfall. This arises from the superposition of the monsoon rainfall in the years of early monsoon
onset on the pre-monsoon thunderstorm rain of the remaining years. After the mean onset date, there is a
further increase in the mean daily rainfall which is more pronounced over north Kerala.
The superposed epoch diagrams, Figures 7a, b, bring out a sharp and spectacular increase in rainfall
accompanying the monsoon onset, compared to the pre-onset phase. For south Kerala the level of daily
rainfall prior to the onset is around 5-6mm which is due to pre-monsoon thunderstorm activity. It is
important to note that the pre-monsoon thunderstorm rain does not progressively increase but levels off by
the middle of April. On the onset date the rainfall abruptly jumps up to the level of 17 mm and continues to
increase reaching a peak value of about 28 m m pcr day during the next 5 or 6 days. This is followed by a
gradual oscillatory decline. The mean pre-monsoon rainfall level for north Kerala is about 4 to 5 mm per day
which shoots up to a little over 20 mm per day during the onset with further increase during the next 5 to 6
days.
It should be emphasized that the mean rainfall variation depicted by Figures 6a, b is not a correct
representation of the actual rainfall variation in individual years. This is more correctly represented by
Figures 7a, b. Thus, the present study categorically shows that the prevailing notion that the pre-monsoon
thunderstorm rainfall over Kerala progressively increases and merges into the monsoon rain resulting in a
gradual transition is not in conformity with facts. The onset of the monsoon is heralded by a sudden and
spectacular increase in the rainfall from the pre-monsoon level.
5. CHARACTERISTICS O F THE ONSET SPELL
The onset spell has been defined as a heavy spell of rain with a duration of 5 days or more and mean rainfall
in excess of 10 mm. We have examined the duration and average daily rain amount associated with the onset
mm
mm
135
3 51
D A I L Y M E A N RAINFALL OF SOUTH K E R A L A : 1901-1980
CALENOAR DATES : M E A N O N S E T D A T E 3 0 M A Y
-
-30
25-
- 25
30
m m
mm
35
DAILY M E A N R A I N F A L L OF N O R T H K E R A L A : 1901- 1980
C A L E N D A R D A T E S : M E A N O N S E T DATE I J U N E
3c
25
20
M E A N ONSET
I5
I
5
0
Figure 6. (a) Mean daily rainfall over south Kerala averaged for calendar dates for the years 1901 -1980 showing mean onset date of the
southwest monsoon. (b) Mean daily rainfdl over north Kerala averaged for calendar dates for the years 1901-1980showing mean onset
date of the southwest monsoon
35
mm
mm
DAILY MEAN RAINFALL OF SOUTH KERALA: 1901-1980
SUPERPOSED EPOCH : ONSET DAY 0
--I
DAILY MEAN RAINFALL OF NORTH KERALA
( 1901-l980)
b-
DAYS
D E F O R E ONSET +DAYS
AFTER Ol(fET
Figure 7. (a) Mean daily rainfall over south Kerala after superposing onset dates of the southwest monsoon for the period 1901 1980.(b)
Mean daily rainfall over north Kerala after superposing onset dates of the southwest monsoon for the period 190-1980
~
294
R. ANANTHAKRISHNAN A N D M. K. S O M A N
spell for the individual years of the 80-year period for both south and north Kerala. The relevant data for
south Kerala are shown graphically in Figure 8. Here the vertical pillars indicate the spell duration in days
while the thick dots (joined by the broken curve) denote the mean daily rainfall for the respective spells. Note
that both the duration as well as the rainfall associated with the onset spells show wide dispersion.
Statistical data relating to the analysis of the onset spells are given in Table IV (a),(b). The spell duration in
individual years varied from 5 days (1976) to 50 days (1922) with a mean duration of 15.4 days. The onset
spells were generally of longer duration during the first-half of the period compared with the second half. The
mean daily rainfall associated with the onset spell is 26 mm. This quantity showed a wide range from
1 0 3 mm in 1976 to 495 mm in 1933. It may be of interest to note that 1976 was the year of lowest rainfall for
Kerala during the 80-year period covered by the present study. The data in Table IV provide justification for
our criteria to fix the onset date of thc southwest monsoon on the basis of the mean daily rainfall over the rain
gauge network over south Kerala. The data for north Kerala show similar behaviour.
DAYS
I
Imm
r
n
YEAll--r
Figure 8. Duration and averagc rainfall of onset spells over south Kerala (1901 -1980)
Table IV. Statistical data pertaining to onset spells
(a) Spell duration
Duration
(days)
5-9
10-14
15-19
20-24
25-29
3 0 34
235
Total
Frequency
24
23
14
7
6
3
3
80
Mean spell duration
(days)
Standard deviation
1901 -1940
1941 1980
1901-1980
17.9
10.9
12.9
7.0
15.4
9.5
(b) Mean daily rainfall
Frequency
2
Mean daily spell
rainfall (mrn)
Standard deviation
(mm)
18
19
18
13
5
2
3
1901--I 940
1941- 1980
1901-1 980
27.6
24.4
26.0
8.0
7.2
7.8
80
295
ONSET OF THE SOUTHWEST MONSOON
6. DISCUSSION
In an earlier study by Ramdas et al. (1954) the mean daily rainfall data over south Kerala (referred to as
Travancore Cochin in their paper) for May-June have been made use of to fix the date of establishment of
the southwest monsoon for the period 1891-1950. Visual examination of the rainfall curve depicting the
rainfall sequence was the basis on which the dates were arrived at. The date of ‘commencement of persistent
heavy rainfall’ was taken by them as the date of establishment of the monsoon. Neither the threshold value of
the rainfall nor the minimum duration of persistence has been specified. We have found it possible to fix such
limits in the present study. The onset dates for south and north Kerala in Table I furnish a set of dates based
on uniform criteria with the least measure of subjectivity. The delay in the mean onset date by 2 days between
north and south Kerala seen in Table 111 is in accordance with the average rate of northward advance of the
southwest monsoon along the west coast of India.
Examination of the dates of monsoon onset for the first and second halves of the 80-year period shows
some interesting features, which are shown in Table V. During the period 1901-1940 the monsoon onset was
more often in June than in May while the reverse feature occurred during 1941-1980. This feature is reflected
in the mean onset dates for the two periods which differ by 4 days.
A significant feature brought out by the present study is the clear demarcation between the rainfall
associated with the pre-monsoon thunderstorm activity over Kerala and the rainfall that heralds the onset of
the monsoon. The super-posed epoch rainfall diagrams (Figures 7a, b) show that the average daily rainfall
from pre-monsoon thunderstorm is of the order of 5 mm. This limit is reached by about the middle of April
and there is little increase thereafter. Following the monsoon onset the mean daily rainfall steeply increases to
about 30 mm. The limit of 10 mm chosen for demarcating the light and heavy rain spells roughly corresponds
to twice the average pre-monsoon rainfall level.
In two recent papers, Subbaramayya et al. (Subbaramayya and Bhanukumar, 1978; Subbaramayya et
ul. 1984) have discussed various features associated with the onset and advance of the southwest monsoon
over India. For the 25-year period 1956- 1980 these authors have, in one of their diagrams, indicated a mean
onset date of 20 May over Kerala. This is totally at variance with the mean onset dates for the same period as
per IMD records (30 May) and as per the present study (29 May for south Kerala and 30 May for north
Kerala). It is obvious that these authors distinguished not between the pre-monsoon thunderstorm rains and
the monsoon rainfall over Kerala.
7. CONCLUSIONS
The main points brought out by the present study are:
(i) The mean daily rainfall series for south and north Kerala enable formulation of objective criteria for
fixing monsoon onset dates in individual years. From the dates thus arrived at, the mean onset dates for
south and north Kerala are found to be 30 May and 1 June respectively for the period 1901-1980 with a
standard deviation of about 9 days.
Table V. Comparison of monsoon onset over Kerala during 1901- 1940
and 1941 -1980.
~
1901 1940
Item
Mean date
Std. Dev.
(days)
May onset
(No. of years)
June onset
(No. of years)
S.K
N.K
1 June
7.5
3 June
7.9
15
11
25
29
1941-1980
IMD
S.K
N.K
IMD
4 June 28 May 30 May 31 May
7.0
9.6
10.1
7.9
9
31
25
18
25
15
22
15
296
R. A N A N T H A K R I S H N A N AND M. K. SOMAN
(ii) The mean onset date for 1901-1940 is later than for 1941-1980 by 4 days.
(iii) Super-posed epoch analysis of the mean daily rainfall reveals a sharp and spectacular increase heralding
the monsoon onset.
(iv) The statement often made that the pre-monsoon thunderstorm rain over Kerala progressively increases
and merges with the monsoon rains resulting in a gradual transition is not valid.
(v) The spell that heralds the onset of the monsoon has a mean duration of about 15 days and the associated
daily mean rainfall is 26 mm. These have large variations in individual years.
ACKNOWLEDGEMENTS
We wish to thank the Director, Indian Institute of Tropical Meteorology for his interest in this study and for
providing the necessary facilities. We also wish to express our thanks to Shri R. M. Soni for preparation of
fair copies of the diagrams, Shri R. P. Mali for photographic assistance and Miss S. R. Kamble for typing the
manuscript.
The data utilised in this study were kindly made available by the National Data Centre of IMD. We arc
grateful to the India Meteorological Department for a research grant.
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