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.

Evaporation

Among all the factors affecting the rate of evaporation, cloudiness and temperature are two of the most important ones in this country. These two factors are however inter-related. 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 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 4to 5 mm per day, Cameron Highlands has a rate of only about 2.5 mm per day.