Wind flow in Malaysia
Though the wind over the country is generally light and variable, there are, however, some uniform periodic changes in the wind flow patterns. Based on these changes, four seasons can be distinguished, namely, the southwest monsoon, northeast monsoon and two shorter inter monsoon seasons.
The southwest monsoon is usually established in the later half of May or early June and ends in September. The prevailing wind flow is generally south westerly and light, below 15 knots.
The northeast monsoon usually commences in early November and ends in March. During this season, steady easterly or north-easterly winds of 10 to 20 knots prevail. The more severely affected areas are the east coast states of Peninsular Malaysia where the wind may reach 30 knots or more during periods
of intense surges of cold air from the north (cold surges).
The winds during the two inter monsoon seasons are generally light and variable. During these seasons, the equatorial trough lies over Malaysia.
It is worth mentioning that during the months of April to November, when typhoons frequently develop over the west Pacific and move westwards across the Philippines, south-westerly winds over the northwest coast of Sabah and Sarawak region may strengthen reaching 20 knots or more.
As Malaysia is mainly a maritime country, the effect of land and sea breezes on the general wind flow pattern is very marked especially over days with clear skies. On bright sunny afternoons, sea breezes of 10 to 15 knots very often develop and reach up to several tens of kilometer inland. On clear nights, the reverse process takes place and land breezes of weaker strength can also develop over the coastal areas.
The seasonal wind flow patterns coupled with the local topographic features determine the rainfall distribution patterns over the country. During the northeast monsoon season, the exposed areas like the east coast of Peninsular Malaysia, Western Sarawak and the northeast coast of Sabah experiences heavy rain spells. On the other hand, inland areas or areas which are sheltered by mountain ranges are relatively free from its influence. It is best to describe the rainfall distribution of the country according to seasons.
Seasonal Rainfall Variation in Peninsular Malaysia
The seasonal variation of rainfall in Peninsular Malaysia is of three main types:
(a) Over the east coast districts, November, December and January are the months with maximum rainfall, while June and July are the driest months in most districts.
(b) Over the rest of the Peninsula with the exception of the southwest coastal area, the monthly rainfall pattern shows two periods of maximum rainfall separated by two periods of minimum rainfall. The primary maximum generally occurs in October - November while the secondary maximum generally occurs in April - May. Over the northwestern region, the primary minimum occurs in January - February with the secondary minimum in June - July while elsewhere the primary minimum occurs in June - July with the secondary minimum in February.
(c) The rainfall pattern over the southwest coastal area is much affected by early morning "Sumatras" from May to August with the result that the double maxima and minima pattern is no longer discernible. October and November are the months with maximum rainfall and February the month with minimum rainfall. The March - April - May maximum and the June -July minimum are absent or indistinct.
Seasonal Rainfall Variation in Sabah and Sarawak
The seasonal variation of rainfall in Sabah and Sarawak can be divided into
five main types:
(a) The coastal areas of Sarawak and northeast Sabah experience a rainfall regime of one maximum and one minimum. While the maximum occurs during January in both areas, the occurence of the minimum differs. In the coastal areas of Sarawak, the minimum occurs in June or July while in the northeast coastal areas of Sabah, it occurs in April. Under this regime, much of the rainfall is received during the northeast monsoon months of December to March. In fact, it accounts for more than half of the annual rainfall received on the western part of Sarawak.
(b) Inland areas of Sarawak generally experience quite evenly distributed annual rainfall. Nevertheless, slightly less rainfall is received during the period June to August which corresponds to the occurence of prevailing southwesterly winds. It must be pointed out that the highest annual rainfall area in Malaysia may well be found in the hill slopes of inland Sarawak areas. Long Akah, by virtue of its location, receives a mean annual rainfall of more than 5000 mm.
(c) The northwest coast of Sabah experiences a rainfall regime of which two maxima and two minima can be distinctly identified. The primary maximum occurs in October and the secondary one in June. The primary minimum occurs in February and the secondary one in August. While the difference in the rainfall amounts received during the two months corresponding to the two maxima is small, the amount received during the month of the primary minimum is substantially less than that received during the month of the secondary minimum. In some areas, the difference is as much as four times.
(d) In the central parts of Sabah where the land is hilly and sheltered by mountain ranges, the rainfall received is relatively lower than other regions and is evenly distributed. However, two maxima and two minima can be noticed, though somewhat less distinct. In general, the two minima occur in February and August while the two maxima occur in May and October.
(e) Southern Sabah has evenly distributed rainfall. The annual rainfall total received is comparable to the central part of Sabah. The period February to April is, however slightly drier than the rest of the year.
Being an equatorial country, Malaysia has uniform temperature throughout the year. The annual variation is less than 2�C except for the east coast areas of Peninsular Malaysia which are often affected by cold surges originating
from Siberia during the northeast monsoon.
Even there, the annual variation is below 3�C.
The daily range of temperature is large, being from 5�C to 10�C at the coastal stations and from 8�C to 12�C at the inland stations but the excessive day temperatures which are found in continental tropical areas are never experienced. It may be noted that air temperature of 38�C has very rarely been recorded in Malaysia. Although the days are frequently hot, the nights are reasonably cool everywhere.
Although the seasonal and spatial temperature variations are relatively small, they are nevertheless fairly definite in some respects and are worthy of mention. Over the whole Peninsula, there is a definite variation of temperature with the monsoons and this is accentuated in the east coast districts. April and May are the months with the highest average monthly temperature in most places and December and January are the months with the lowest average monthly temperature. The average daily temperature in most districts to the east of the Main Range is lower than that of the corresponding districts west of the Main Range. The differences in the average values in the east and the west are due almost entirely to the low day temperatures experienced in the eastern districts during the northeast monsoon as a result of rain and greater cloud cover. At Kuala Terengganu, for example, the day temperature rarely reaches 32�C during the northeast monsoon and often fails to reach 27�C. A number of occasions have been recorded on which the temperature did not rise above 24�C which is quite frequently the lowest temperature reached during the night in most districts. Night temperatures do not vary to the same extent, the average usually being between21�C to 24�C. Individual values can fall much below this at nearly all stations, the coolest nights commonly follow some of the hottest days.
As mentioned earlier, Malaysia has high humidity. The mean monthly relative humidity falls within 70to 90%, varying from place to place and from month to month. For any specific area, the range of the mean monthly relative humidity varies from a minimum of 3% to a maximum of about 15%. In Peninsular Malaysia, the minimum range of mean relative humidity varies from a low 84% in February to a high of only 88% in November. The maximum range is found in the northwest area of the Peninsula (Alor Setar) where the mean relative humidity varies from a low of 72% in February to a high of 87%. It is observed that in Peninsular Malaysia, the minimum relative humidity is normally found in the months of January and February except for the east coast states of Kelantan and Terengganu which have the minimum in March. The maximum is however generally found in the month of November.
As in the case of temperature, the diurnal variation of relative humidity is much greater as compared to the annual variation. The mean daily minimum can be as low as 42% during the dry months and reaches as high as 70% during the wet months. The mean daily maximum, however, does not vary much from place to place and is at no place falls below 94%. It may reach as high as nearly 100%. Again, the northwest states of Kedah and Perlis have the largest diurnal variation of relative humidity.
Sunshine and Solar Radiation
Being a maritime country close to the equator, Malaysia naturally has abundant sunshine and thus solar radiation. However, it is extremely rare to have a full day with completely clear sky even in periods of severe drought. The cloud cover cuts off a substantial amount of sunshine and thus solar radiation. On the average, Malaysia receives about 6 hours of sunshine per day. There are, however, seasonal and spatial variations in the amount of sunshine received. Alor Setar and Kota Bharu receive about 7 hours per day of sunshine while Kuching receives only 5 hours on the average. On the extreme, Kuching receives only an average of 3.7 hours per day in the month of January. On the other end of the scale, Alor Setar receives a maximum of 8.7 hours per day on the average in the same month.
Solar radiation is closely related to the sunshine duration. Its seasonal and spatial variations are thus very much the same as in the case of sunshine.
Source - Malaysia Meteorological Service