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Rainfall is one of the influencing factors for human activities which is defined as the amount of water falling in rain within a given time and area, usually expressed as a hypothetical depth of coverage. The impact of rainfall is not limited to agricultural based regions but also to industrialized urban areas. The rainfall received in any region can vary with physical factors as well as human interactions. The physical factors that can cause such variation could include micro-climatic phenomenon or global changes such as photochemical smog and global warming. The results obtained through observation by the GLOBE SOCIETY of our school clearly indicates that there are significant changes in the rainfall pattern of Colombo district which belongs to the wet zone of Sri Lanka. Annually Sri Lanka receives different rainfall types, which are Convectional rainfall, Monsoonal rainfall and Expressional rainfall. As a result of abrupt changes in nature the rainfall patterns of Colombo has been affected and this study focuses on these deviations and the changes that may occur on the crop calendar due to this. The cultivation in Sri Lanka is carried out in the two main seasons ‘Yala’ and ‘Maha’. According to the typical rainfall patterns and the crop calendar the highest rainfall in Colombo (wet zone) is expected in the months May and October. Yet our study concludes that these patterns have changed over the past decade (2005 – 2015). Through this study we are focusing on informing the general public and the cultivators about the deviation in typical rainfall pattern in Colombo district.