To a large extent, relative humidity, even more than temperature, determines how comfortable you are. Relative humidity is the ratio of the amount of water vapour in the air, compared to the maximum amount of vapour that the air can hold (assuming constant temperature). The higher the relative humidity, the more difficult it is for your perspiration to evaporate from your skin, thereby leaving you feeling clammy (because evaporation uses up latent heat [?] from your skin, cooling it in the process).

During the day, relative humidity is highest in December (83%), and lowest in February (76%). Within each twenty-four hour period on average, the driest period is during the early afternoon. During the pre-dawn hours, it is most humid (96% and above) in April, May and between September to December. During this latter part of the year, however, night temperatures are lower and windspeeds are higher, thereby alleviating some of the discomfort felt. This is in sharp contrast to the April-May period, when not only is relative humidity high, but so is night temperature, and the air is still.

Generally, coastal areas of Singapore receive less rain than inland areas. What is more, between December and April, there is a great variation in rainfall from place to place in Singapore, while July and August show the least variation.

Between March and May, and November-December, rainfall is likely to be most intense. However, these two sets of months differ in that during November, not only is the rain most intense, but it also falls for the longest duration (over sixty-one hours of rain in the month); whereas March has the shortest rain duration of the whole year (only twenty-eight hours of rain in the month).

Within a twenty-four hour period, most rainfall usually falls in the early to mid-afternoon. The type of rainfall that we experience in Singapore is called convectional rain. This is formed when air is heated, expands, becomes less dense and rises. As it does so, it cools, thus its relative humidity increases. If the air is cooled beyond saturation [?], it is likely to rain.

Sometimes, when rain falls through cloud, strong convection currents bring it up before it can reach the ground. Such rapidly reversing vertical movements of the water droplets within the cloud can result in the build up of static electricity [?], which in turn, results in a lightning [?] bolt. The sudden expansion of the heated air around the bolt causes a thunderclap [?] to be heard. Thunderstorms are most common in April and November (Chia, 1979/80).

Whoever said Singapore has no seasons? January has the most cloud during the day, but nights are clear and have steady, cool winds. February has the least cloud and the least humid days. March also has low cloud cover, but has intense, brief rain.

April is hot, having intense thunderstorms. Its pre-dawn hours are also the most humid, and nights are hot and windless. May has the hottest days, hot nights, cloudy afternoons, intense rain, and again windless nights with humid pre-dawn hours.

June has strong winds, warm nights, early morning showers, and is cloudy throughout the day. July to September have strong winds, early morning showers and cloudy mornings. Late August to early November tend to be more hazy than other times of the year. September also has humid pre-dawn hours, but cool, windy nights.

October too has humid pre-dawn hours and cool, windy nights. November has nights which are clear and have steady cool winds, humid pre-dawn hours, cloudy days, intense but steady rain with thunder. Finally December has cloudy, humid but cool days, and has nights which are clear and have steady cool winds, humid pre-dawn hours and intense rain.

So far, this chapter has dealt with variations in weather over time. Yet at any one time of the day, the weather varies from place to place as well. This is so even in a tiny country like Singapore. Such spatial variations in weather are known as microclimate [?].

Differences in microclimate in Singapore are due to the shape of the land (that is, the presence of hills and valleys), proximity to the sea (because the sea heats up and loses heat more slowly than the land), position with respect to the prevailing monsoons, as well as the presence of built-up areas (as opposed to open parks [!] and forest).

A phenomenon known as the 'urban heat island' is most pronounced at night. This is when temperatures over urban areas in Singapore are higher (by as much as 3.5 ^oC) than those over rural areas. This is due to heat stored in the structures of buildings during the day, which is radiated at night. However, rural areas tend to be more humid than urban areas at night, because of the transpiration from the vegetation.

The urban heat island lies along the south coast, with an extension through Toa Payoh and Ang Mo Kio, as well as another along Bukit Timah. Within the heat island, night temperatures can even exceed 30 ^oC. So much for the nightly weather forecasts then!

The climatic conditions described above for Singapore did not always exist. As the waters of the Flood receded, sea levels [?] were about two hundred metres lower than present, and Singapore was near the centre of a great landmass called Sundaland.

This meant that its climate was much more seasonal than it is today. The vegetation was thus open savanna, rather than rainforest. This is because the north-east monsoon (which was blowing even then!) no longer was moist by the time it reached present-day Singapore, simply because the South China Sea had not yet been totally filled. Therefore, the beginning and end of the year would have been much drier than they are today. It would seem that this lends a whole new meaning to the appellation 'Lion City'!

This aridity would have meant that rates of fluvial erosion of the landmass would have been much more effective than they are today, gouging out deep channels in Sundaland - a modern-day parallel would be that of the Grand Canyon [!]. These deep channels were drowned (that is to say, flooded) when sea levels [?] rose, and that is why Singapore has such deep offshore waters which are so suitable for port operations and the navigation of deep-draught vessels.