IndianJournalofRadio& SpaceH1ysics
Vol. 25, February 1996, pp. 22-26
Wettest and driest hours of day during the southwest monsoon season oyer
Indian region
JMPathan
Indian Institute of Tropical Meteorology, Pune 411 008
Received 13 February 1995; revised received 27 September 1995; accepted'25 October 1995
The spatial distributions of wettest and driest periods of the day during the SW monsoon season
aver Indian region are presented ~ the form of charts. The hourly rainfall values for 40 self-recording
raingauge stations with data period of 5-9 SW monsoon seasons have been used for this purpose. The
chart for maximum rainfall activity showed that the wettest period occurs in the afternoon hours over
the main land region and in the night hours over the coastal region. This difference in time of maximum
is cpnsidered to be the effect of the differential response of the solar influence on precipitation formation mechanism over continental and coastal areas. The chart for minimum rainfall activity indicated
that the driest period occurs in the forenoon hours of the day over a large portion of Indian region and
there is no significant difference in the occurrence hours of the driest period over the main land region
and the coastal region of India. Results of some of the earlier relevant studies are briefly discussed.
I Introduction
The diurnal modulation of rainfall has always
been an interesting aspect of· rainfall variation and
has attracted attention of research workersI-6 from
all over the globe. The studies for different regions
of the world indicated that the nature of the diurnal variation of raipfall at a place depends upon
the location (coastal or continental), elevation or
topography of the station. In general, the inland
regions exhibit a rainfall maximum in the afternoon and maritime regions show a morning maximum. The studies7-13 for the Indian region deal
with the diurnal characteristics of rainfall at individual stations, or a group of stations. All these
studies suggest that the time of occurrence of maximum and minimum rainfall activity over the Indian region differs substantially from one area to the
other.
A total collective gross picture depicting the
distribution of the hours of maximum and minimum rainfall activity over the period of a day during the rainy season is not readily available in the
form of charts in the published literature. Such a
pictorial presentation of this valuable information
in one place may be useful for planning social and
agricultural activities, industrial and engineering
works and fDr studies in Indian geography. Therefore, the charts depicting the spatial distribution of
the wettest and the driest hours of the day during
the southwest (SW) monsoon season over the Indian region are prepared and the salient features
brought out by these charts are presented.
I
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2 Hourly rainfall data
The hourly rainfall values for each of the 24
chronological hours of the day were collected for
40 self-recording raingauge stations distributed
over India for the period of 122 days from 1 June
to 30 September of SW monsoon season. The duration of SW monsoon season is different from
one part of the country to the other. It varies from
more than 5 months over Kerala to only about
one and a half months over west Rajasthan. However, it is customary in the India Meteorological
Department to take the period of 122 days from 1
June to 30 September as the normal duration of
SW monsoon season over the entire Indian region
for all practical purposes. The present study also
confines to four months from June to September.
The period of data varied from 5 to 9 SW monsoon seasons for the different stations, depending
upon the availability of the data. The names. of
stations utilized for the study and their abbreviated symbols are listed in Table 1 together with
their geQgraphical coordinates, altitudes and normal SW monsoon rainfall amounts obtained from
the records of the India Meteorological Department. The locations of station~ with their topographical features are shown in Fig. 1, wherein only
one altitude contour line of 500 m elevation
a.m.s.l. is shown and the high altitude region above
500 m is indicated with stipples. To avoid clutter
and congestion, contour lines of high altitude steps
more than 500 m are not presented.
II III; \1 1I11~II ill
PAmAN:
0
'N
Name of Station
Colaba; Calcutta( A) = Calcutta-Alipore
WETIEST
& DRIEST
HOURS OF DAY DURING SW MONSOON
monsoon
2
01
84
217
24
27
14
42
18
85
1161036
40
92
79
1777
TRV
04
56
32
56
28
29
80
76
74
73
86
.1l1
111897
22
64
22
08
26
10
19
18
19
12
13
17
10
15
12
17
364
1841
AHM
KTD
LKN
BHP
JSD
JDP
JPR
09
26
00
07
14
58
77
79
9
257
611
3111069
02
98
2079
91
216
234
1324
1121
41
43
27
46
05
52
53
20
15
40
02
11
51
94
7
8
3
02343
13825936
921
559
129
553
06
09
88
2865
2978
297
863
1270
483
503
580
44
54
39
27
47
49
32
50
17
55
21
59
10
57
03
53
54
73
88
72
75
70
81
91
523
390
393
03
34
161238
11
26
23
23
25
22
20
09
21
22
23
18
08
21
1969
1333
863
864
1085
915
1268
1157
1124
528
·MWR
MNC
KDK
VNG.
AGT
BRD
ASL
JBP
MNG
MDS
AGD
TRP
12
71
13138017
55
45
327
04
492
52
422
545
581
1159
574
15
73
1208
m
SGRI
HYD
DNB
BNG
HZB
GYA
NGP
PNA
PBL
CPJ
41
04
35
38
86
126
25
28
31
23
752
1162
SW
AMT
ALB
502
58
08
559
38
CHN
'E'18
8
11
17
Table
Abbreviation
BMB
DBG
VVL
JOD
I-List
of stations with their coordinates and altitudes
22
Latitude
CAL
VSK
601
DUI
Altitude
Longitude
rainfall
mm
23
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INDIAN J RADIO & SPACE PHYS, FEBRUARY
N.
35
.
30
10·
70·
75·
80·
IOO·E
Fig. I-Topographical
features and locations of stations. [Only
one altitude contour line of 500 m elevation a.m.s.I. is shown
and the high altitude region above 500 m is indicated with
stipples. The stations with altitudes more than 500 m are located within the stippled region and plain stations with altitudes less than 500 m are located outside the stippled portion
of the country.]
As seen from Table 1 and Fig. 1, the heavy
rainfall station of Cherrapunjee (CPJ) on the
southern slope of the Khasi-Jaintia hills in northeast India has elevation of 1313 m (a.m.s.1.).Katmandu (KTD) in Nepal with elevation 1324 m lies
on the southern slope of the sub-montane region
of Himalayan range. The average altitude of the
Great Himalayan range which runs approximately
parallel to the northern boundary of India rises
steeply from 500 m to 5.4 km. Only the southern
periphery of the foot-hills of the Great Himalayan
range with altitude more than 500 m is seen stippled in Fig. 1. The altitude of the region north of
this stippled area increases from 0.5 km to 5.4
km. Two hill stations on the western Ghats, the
high rainfall station of Mahabaleshwar (MWR) in
the north and Kodaikanal (KDK) in the south of
peninsular India have elevations 1382 m and 2343
m, respectively. Seven stations, namely, Aurangabad (AGD), Bangalore (BNG), Bhopal (BHP), Hazaribagh (HZB), Hyderabad (HYD), Jagdalpur
(JGD) and Poana (PNA) have altitudes more.than
500 m. The locations of these stations are seen
within or near the outer fringe of the stippled region for altitudes more than 500 m in Fig. 1. All
other stations considered here are plain stations
with altitudes less than 500 m and are located
outside the stippled portion.
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11
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3 Method of analysis
The monthly totals of the hourly .:rainfall values
for individual years were added up, to obtain the
average hourly rainfall for the respective months
from June to September of SW monsoon season
at all the stations considered. Thes~ were then
combined to evaluate hourly 'rainfall for each of
the 24 chronological hours for all the 4 months
together. Thus the total rainfall for the season has
been split up into 24 hourly rainfall values which
are expressed as percentage of the total SW monsoon rainfall. In this way, the average hourly rainfall values during each chronological hour as percentage of the average daily (24-hourly) rainfall of
SW monsoon season are obtained. The average
hourly rainfall values are thus smoothed out.
Added to this, the percentage values are more
conveqient for comparison. Scanning through
these 24 hourly ''percentage rainfall values of the
individual stations, the hours of highest percentage
rainfall value (wettest period) and least percentage
rainfall value '(driest period) were picked out for
each of the stations. Thus, the hours of maxima
and minima in SW monsoon rainfall activity during the course of the day were available for all the
stations. These hours were plotted on two separate charts at each of the stations to give the wettest and driest epochs during the diurnal period
.over the Indian region. The distributions of the
hours of maximum and minimum SW monsoon
rainfall activity are shown in Figs 2 and 3, respectively.
4 Hours of maximum and minimum rainfall
activity
As the behaviour of the rainfall is strongly affected by the local geographical and topographical
conditions, the hours of maximum and minimum
rainfall activity given in Figs 2 and 3 have to be
considered in conjunction with the topographical
features shown in the respective figures. The stippled portion in Figs 2 and 3 indicates the region
with altitude more than 500 m ..
A critical examination of Fig. 2 shows two distinct regimes in which the epochs of maximum
rainfall could be partitioned. The first being over
the main land region and the second over the
coastal region. There is some generality, in a
broader sense, in this classification. The overall
picture that emerges from this chart is the afternoon maximum over the main land region and
night maximum over the coastal region. Broadly
speaking, the time for the maximum rainfall activity over the main land region varies from 15 to 18
hrs 1ST, i.e. immediately after or within 2-3 h of
& DRIEST
PATHAN: WETTEST
HOURS OF DAY DURING SW MONSOON
attainment of the maximum air temperature epoch
during the day time in the presence of direct solar
influence. Over the coastal region, the maximum
rainfall occurs either in the very early morning
hours or during the night hours when the direct
solar influence is absent. The night maximum over
the coastal region and the afternoon maximum
over the main land region bring out essentially the
No
35
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Fig. 2-Spatial
75°
80°
85"
90'-
95°
100"E
distribution of hours (1ST) of maximum rainfall
activity during SW monsoon season
No
35
"
30
°
20
15
"
10
5"
65°
70"
Fig. 3-Spatial
75"
80'
predominant influence of·the proximity of the sea
or the indepthness of the land ar~ and absence
or presence of direct solar influence, respectively,
in governing the precipitation formation mechanism.
A close inspection of Fig. 3 indicates that the
minimum rainfall activity over most of the land region occurs during the early part of the day (forenoon hours mostly from 09 to 12 hrs 1ST). Over
hilly region and Assam valley in NE India, the
rainfall minimum is noticed around 15 hrs 1ST
and over NW India and adjoining parts of hills
and plains of Uttar Pradesh, the minimum rainfall
activity is observed at around 20 hrs 1ST. Over
the west coast, the minimum activity occurs in the
early afternoon hours. The time of minimum.activity progressively shifts from morning to noon
hours along the east coast from north to the tip of
the peninsula. We can say _roughly that the driest
period over most parts of Indian region occurs in
the forenoon hOurs of the day and there is no
significant difference in the occurrence hours of
driest period over the main land region and over
the coastal region of India. This feature of driest
period over the Indian region is quite different
from that of the wettest period.
(I
5°
65°
25
85"
90"
95-100"[
distribution of hours (1ST) of minimum rainfall
activity during SW monsoon season
5 Discussion
The diurnal variation of rainfall over oceanic
and continental regions of the world has been
studied by a number of earlier workers. Ramagel
studied the diurnal variation of summer rainfall at
stations in East Asia during the months of MayAugust and found that many stations exhibit pronounced diurnal rainfall variation with morning
maximum between 0700 and 1000 hrs LT at tropical and sub-tropical coastal stations of southeastetn China and southern Japan and afternoon maximum at all the inland stations. While studying diurnal precipitation change over the sea, Kraus2
found that the maritime precipitation is significantly more frequent at night. In a study on the diurnal rainfall variation in northeast Brazil, Kousky3
showed that most of the coastal areas experience a
nocturnal maximum in rainfall activity, probably
due to convergence between· ·the mean onshore
flow and 'the offshore land breeze. Areas.•J.OO-300
km inland experience a daytime maximum associated with the development and inland advance
of the sea breeze. Landin and Bosart4 examined
the diurnal variability of precipitation in the populous northeastern region of the United States.on a
seasonal basis through a harmonic analysis using a
mesoscale network of stations and suggested that a
number of local and regional scale processes com-
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INDIAN J RADIO & SPACE PHYS, FEBRUARY
bine to produce the observed diurnal precipitation
cycle. Balling and Braze}5 identified the spatial and
temporal patterns in diurnal variations of Arizona's monsoon precipitation frequencies and revealed a strong maximum frequency of rain events
near midnight over the Salt and Gila riv~r basins
of Central Arizona and a more normal daytime
maximum in the monsoon precipitation over other
areas of Arizona State. Fujibe6 examined the diurnal variations of precipitation and thunderstorm
frequency in Japan during the warm season from
June· to September and sought that local precipitation and thunderstorms exhibit a pronounced afternoon maximum between 1500 and 1800 hrs LT
at stations in the inland areas. Time of maximum
differs by a few hours according to regions, local
topography and precipitation types. Maritime and
coastal stations on islands and peninsulas show a
morning maximum between 0300 and 0600 hrs
LT.
Similarly, a large number of studies have been
made on the diurnal characteristics of rainfall over
the Indian region. Prasad7 studied 'diurnal variation of rainfall' at a number of Indian,stations and
pointed out' that the nature of the variation depends on the location (continental or maritime),
elevation of the station and, to some extent, on the
season. In another study Prasad8 found a wellmarked diurnal variation in the rainfall with a
maximum in the early morning hours and minimum in the afternoon hours at 4 inland stations
distributed throughout the Brahmaputra valley in
the northeast India. This, he attributed to the effect of mountain winds. The low level convergence
and upper level divergence associated with katabatic flow appears to be a possible mechanism for
the enhanced rainfall activity from midnight to
early morning hours at these stations8,9.' Diurnal
variation of southwest ·monsoon rainfall with enhanced rainfall activity from midnight to early
morning hours and suppressed activity in the afternoon ~e the striking features at the stations on
the west coast of India10-13• As stated by Ananthakrishnan10, land and sea breeze effects playa
dominant role in increasing or decreasing low convergence at coastal stations. The 4tcreased rainfall
activity at night and early morning hours is attributed to greater low level convergence resulting
from the interaction of land breeze with the prevailing wind when they are from opposite direc-
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tions. At inland stations convective activity resulting from heating of the ground and adjoining layers of the atmosphere tends to be maximum towards afternoon/evening and least in the forenoon
hours .. Afternoon rainfall maximum at inland
stations is, generally, attributed to this increased
convective activity. The discussion made so far
may be useful in understanding the information
contained in the charts given in the present study.
.Despite the fact that the diurnal variation. of
rainfall. over different parts of the globe is a well
known feature, the physical mechanisms responsible for the same are not well-understood. The observed features seem to be the result of a complex
interaction of several mechanisms operating simultaneously.
Acknowledgements
Sincere thanks are due to the Director of the
Institute and Dr A S R Murty, Deputy Director,
for the support and facilities. Thanks are due to
Mr M I R Tinmaker for assistance in the preparation of the manuscript. The author received valuable suggestions during the course of the preparation of the paper from Dr G K Manohar of P M
& A Division of the Institute. The hourly rainfall
data utilized in this study were made available
through the Additional Director General of Meteorology (Research), India Meteorological Department, Pune. The critical suggestions offered by the
anonymous referee for the improvement of the paper are thankfully acknowledged.
References
1
2
3
4
5
6
7
8
9
10
11
12
13
Ramage C S,J Meteorol( USA), 9 (1952) 83.
Kraus E B, J Atmos Sci( USA), 20 (1963) 551.
Kousky V E, Mon Weather Rev ( USA), 108 (1980) 488.
\.-andin M G & Bosart L F, Mon Weather Rev (USA), 113
(1985) 989.
Balling R C & Brazel S W, Mon Weather Rev (USA), 115
(1987) 342.
FujibeF, Pap Meterorol &Geophys(Japan), 39 (1988) 79.
Prasad B,InditmJ Meteorol &Geophys, 21 (1970) 443.
Prasad B, IndianJ Meteorol &Geophys, 25 (1974) 245.
Bhattacharya P K & Bhattacharyya S G, Mausam (IndiD),
31 (1980) 51.
Ananthakrishnan
R, Pure & Appl Geophys (Swilzerland),
115 (1977) 1209.
Sarwade G S, Vayu Mandal(IndiD), 8 (1978) 105.
Ananthakrishnan R, Aralikatti S S & Pathan J M, Proc IndiDnAcadSciA,88
(1979) 177.
PathanJM,AdvAtmosSci(China),
11 (1994) 111.
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