Apply a logarithm or square root to the data

For some variables, for instance precipitation, the distribution is
closer to a normal distribution after taking te logarithm or square
root of the data. When the operation assumes a normally distrbuted
variable (e.g., correlation, regression) the results may be better
after such a transformation. For plots the variables are transformed
back to the original scale, so that a straight-line regression becomes
an exponential or parabolic line.
Negative data are replaced by undefined. In the case of a logarithmic transformation, zeros are arbitrarily replaced by 1/5 of the lowest positive value encountered.