The z score shows the number of standard deviations by which the raw score is above or below the mean. If the raw score is above the mean then the z score is positive but if the raw score is less than the mean then the raw score is negative. The z score is given by:
μ = 20.66°C, σ = 2°C
For x < Singapore temperature, i.e. x < 26
From the normal distribution table P(x < 26) = P(z < 2.67) = 0.9962