Singapore is one giant sauna anytime of the year. November to January are the wettest months but the other months are not exactly dry either. The only real deterrent is the lack of refreshing winds during March and September. Generally, expect average temperatures to reach around 31°C (88°F) during the day and linger around 24°C (75° F) at night.
Singapore is a very safe country with low crime rates. Pickpockets are not unknown but in general crime is not a problem. This is not surprising given the harsh penalties meted out to offenders. The importation of drugs carries the death penalty, which is regularly carried out. Drugs in Singapore should be avoided at all costs.
The state of Singapore takes a tough line on antisocial issues - the most famous being its ban on chewing gum (well, it does mess up the pavement and could stop the MRT from working). While having gum for your own consumption is no big deal, bringing it in bulk into Singapore will attract a heavy fine - the standard way of stamping out un-Singaporean activities.
Smoking in all public places, littering, jay walking and eating or drinking on the MRT are all punishable with hefty fines.