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 northeast monsoon (Figure 1), southwest monsoon (Figure 2), and two shorter periods inter-monsoon seasons (Figure 3).
Usually commences in early November and ends in March.
Steady easterly or northeasterly winds of 10 to 30 knots prevail.
Wet season (approximately 4-5 monsoon surges that can cause flooding).
Usually established in the later half of May or early June and ends in September.
The prevailing wind flow is generally southwesterly and light, below 15 knots.
Relatively dry weather (dry season), except for Sabah.
Inter - Monsoon
Starting in late March to early May and October to mid-November.
Winds are generally light and variable.
The equatorial trough lies over Malaysia.
Frequent thunderstorms in the afternoon.
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, southwesterly winds over the northwest coast of Sabah and Sarawak region may strengthen to reach 20 knots or more.
As Malaysia is mainly a maritime country, the effect of land and sea breezes on wind flow patterns is large 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 kilometres 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 experience 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 states, 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 and the secondary minimum occurs in June - July while elsewhere the primary minimum occurs in June - July with the secondary minimum occurs 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 distinguishable. October and November are the months with maximum rainfall and February the month with the minimum rainfall. The March - April - May maximum and the June - July minimum rainfall are absent or indistinct
(d) The average monthly rainfall (annual) for Peninsular Malaysia is shown in Figure 4.
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 rainfall occurs during January in both areas, the occurrence of the minimum rainfall differs. In the coastal areas of Sarawak, the minimum rainfall 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 occurrence 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 coastal areas of Sabah experience a rainfall regime of which two maxima and two minima can be distinctly identified. The primary maximum occurs in October and the secondary maximum occurs in June. The primary minimum occurs in February and the secondary minimum occurs 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 have evenly distributed rainfall. The annual rainfall total received is comparable over the central part of Sabah. The period February to April is, however slightly drier than the rest of the year.
(f) The mean monthly rainfall (annual) for Peninsular Malaysia is shown in the following.
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. Annual average of sunshine duration for Peninsular Malaysia and Sabah Sarawak are shown in Figure 6 and 7 respectively.
Among all the factors affecting the rate of evaporation, cloudiness and temperature are two of the most important factors in this country. These two factors are however interrelated. A cloudy day will mean less sunshine and thus less solar radiation resulting in a lower temperature.
An examination of the evaporation data shows that the cloudy or rainy months are the months with lower evaporation rate while the dry months are the months with higher evaporation rate. It is noted that Senai has an average evaporation rate of 2.6mm/day in the month of November, the lowest for lowland stations. On the other end of the scale, Kota Kinabalu has the highest average evaporation rate of 6.0 mm/day in the month of April. For highland areas such as Cameron Highlands where the air temperature is substantially lower, the evaporation rate is proportionally lower too. While lowland areas have an annual average evaporation rate of 4 to 5 mm/day, Cameron Highlands has a rate of only about 2.5 mm/day.
As a country located at the equator, Malaysia has uniform temperature throughout the year. The annual variation is less than 2°C except for the east coast of Peninsular Malaysia which are often affected by cold surges from Siberia during the northeast monsoon. However, the annual variation is less than 3°C.
The daily temperature range is large; from 5°C to 10°C for stations near the coast and between 8°C to 12°C for stations at the interior but the daily high temperatures such as those found in the tropical continents have never been experienced. Although the days are often hot, but the nights are reasonably cool.
Although the seasonal and spatial changes in temperature variations are relatively small, but in some ways it can be determined. For Peninsular Malaysia, there is an obvious change in temperature during monsoon in the east coast of peninsula. April and May are the months in which the temperature is highest monthly average temperature while December and January are the months with the lowest monthly average temperature.
Mean daily temperatures in most areas of east of the mountains is low compared to areas in the west. These changes are due to the low daily temperatures experienced in the eastern region during the northeast monsoon and the result of extensive cloud cover. At Kuala Terengganu, for example, during the northeast monsoon the daytime temperature rarely reaches 32°C and often do not reach 27°C. Some situations have been recorded where the temperature does not exceed 24°C i.e. the lowest temperature reached during the night in most places. The temperatures at night is usually between 21°C to 24°C. But the temperatures in almost all stations can be reduced significantly lower than these temperatures ranges; cool nights are usually followed by a hot afternoon.
The following is mean annual temperature for Peninsular Malaysia and Sabah Sarawak.
As mentioned earlier, Malaysia has high humidity. The average monthly relative humidity falls within 10% to 90%, varies from one place to another and months. For any specific area, the range of the average relative humidity varies from a minimum of 3% (Bintulu) to a maximum of approximately 5% (Alor Star). In Peninsular Malaysia, the minimum range of mean relative humidity varies from a low of 80% in February to a high of only 88% in February. The maximum range is available in the area northwest of the Peninsula (Alor Star) where the mean relative humidity varies from 72% to 87% in February.
It is noted that in Peninsular Malaysia, a minimum relative humidity occurs in January and February except for the east coast states of Kelantan and Terengganu, where the minimum relative humidity usually occurs in March. The maximum relative humidity is usually occurs in November
As in the case of temperature, the daily changes of relative humidity is greater than the annual changes. Average daily minimum can be as low as 42% during the dry months and as high as 70% during wet months. However, the maximum daily average was little changed from place to place, which is over 94% .It may reach as high as 100%. States in the northwest of Kedah and Perlis have the largest diurnal variation of relative humidity.