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RAINFALL
There are four rainfall seasons during the
year. These are :-
1. The South - west monsoon period (May to September)
2. The Intermonsoon period following the South-west monsoon (October
to November)
3. The North - east monsoon period (December to February)
4. The
Intermonsoon period following the North- east monsoon (March to
April
)
Rainfall is of three types- monsoonal, convectional and depressional.
Monsoon rain occurs during the two monsoon periods, namely, the South -
west and North - east, and is responsible for nearly 55% of the annual
precipitation. Convectional rain occurs during the intermonsoon periods,
mainly in the afternoon or evening and is likely to be experienced
anywhere over the Island. Depressional rain also occurs during the
intermonsoonal periods, particularly during the second intermonsoon
(October to November).
The annual average rainfall
varies from below 1000 mm (39’’) over a small region in the arid parts
of the North - west and South - east of the Island to over 5000 mm
(197”) at a few places in the Kegalle and Nuwara Eliya districts (on
the South - western slopes of the central hills).
Rainfall during the South -
west monsoon is mostly over the South - western parts of the Island. At
the beginning it occurs in the South - western low country. As winds
strengthen, it spreads gradually to the interior, with considerable
heavy rain in the hill country from June to August. South - west monsoon
rainfall exceeds 3000 mm (118”) at a few places in the Kegalle and
Nuwara Eliya districts.
During the North - east
monsoon, the eastern half of the Island receives from about 200 mm (8”)
to over 1200 mm (47’’) of rain. The higher rainfalls are experienced
over the Rangala range of hills.
During the intermonsoon
periods, winds are generally light except for the sea breeze which
develops from about noon and is also responsible for the thundershowers
that occur in the afternoon or evening. These showers may occur anywhere
over the Island. Another source of rain during these periods is
depressional activity.
Depressions are responsible
for a good part of the precipitation during the intermonsoon period,
October to November. Rainfall during this period is therefore widespread
and exceeds 500 mm (20’’) at many places. Taking the Island as a whole,
this is the rainiest period of the year.
Conditions are similar during the other intermonsoonal period, from
March to April, but the rainfall is less, mainly because of less
depressional activity
TEMPERATURE
The mean annual temperature over Sri Lanka, which lies within latitudes
6 deg N and of approximately 10 deg N and longitudes of approximately 80
deg E and 82 deg E, is about 27.5 deg C over the lowlands. The oceanic
influence (the maximum width of the Island being only 225 kilometers)
helps to reduce the temperature in lowlands by sea breeze. The highlands
in the central region enjoy a cooler climate with a mean temperature of
18 deg C.
Temperature decreases at a
steady rate of about 6.5 deg C for each 1,000 metre rise. Thus, at Kandy,
which is 488 metres above mean sea level, the mean annual temperature is
about 24.5 deg C; at Diyatalawa, (1,250 metres above mean sea level) the
mean annual temperature is about 20.2 deg C, and at Nuwara-Eliya, where
the elevation is 1895 metres, the mean annual temperature is about 15.8
deg C.
A noteworthy feature in many
parts of Sri Lanka is the small variation in the mean monthly
temperatures throughout the year. On average, the mean temperature of
25.0 deg C during the coolest months, November to February, is only 2.4
deg C lower than that during the warmest months April and May.
Higher temperatures are
experienced generally in the Northern, North - central and Eastern
regions of the Island and range between 33.3 deg C and 34.7 deg C, on
average.
Lower temperatures are
experienced during the early hours of the day, a little before dawn.
Along the coast, these temperatures occur during December and January
and range between 21.0 deg C and 24.2 deg C, on average. At Nuwara Eliya
the average maximum temperature is 22.8 deg C in April (highest) and the
average minimum temperature is 9.4 deg C in January (lowest). The
minimum temperature at Nuwara Eliya falls below the freezing point (0
deg C) only very occasionally.
Diurnal variation of
temperature, i.e., the rise to a maximum early in the afternoon and the
fall to a minimum shortly before dawn, is well marked. Its magnitude
depends on the season. There is a gradual increase in the range with
altitude as well as with distance from the sea.
RELATIVE HUMIDITY
Relative humidity varies generally from about 70 percent during the day
to about 90 to 95 percent at night. In the dry zone, however, these
values are lower by about 5 per
cent, while in the driest areas in the North - west and South - east
relative humidity drops to about 60 per cent.
HAIL
During intense thunderstorms, hail is experienced occasionally. It
occurs mainly in the hill - country but there have been reports of the
occurrence of hail in low - country stations too.
GROUND FROST
Ground frost occurs in Nuwara - Eliya on a few days of the year during
the months of January and February.
CLIMATE CHANGE
Statistical analysis of air temperature and
rainfall data collected by the Department of Meteorology over a period
of more than 100 years have shown an increasing trend in the annual mean
air temperature over the entire Island, particularly during the more
recent period, 1961 - 1990. This increase was found to be approximately
0.16 deg C per decade. Rainfall trends were found to be some what
complex; there were decreasing trends over most of the island except for
some isolated areas in the north-western province, where an increasing
trend was indicated. Thunder activity, in addition to showing an
increasing trend, was found to be positively correlated with air
temperature.
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