To divide one power of ten by another, simply subtract the two exponents (see Multiplication/Division). Next, convert the two numbers from scientific notation to real numbers. Now add the two numbers normally. Finally convert to scientific notation if the coefficient is less than 1 or greater than 10.
Precipitation occurs when cloud droplets combine and grow large enough to fall to the earth the rate of evaporation as a drop falls to the earth also can affect its size. from: sadia aisa.
Rainfall:  Precipitation is most often rain (hence the common term), but also includes other forms such as snow. Observations of daily rainfall are nominally made at 9 am local clock time and record the total precipitation for the preceding 24 hours. Other, more frequent, observations are also made at some sites.