Rainfall, temperature, relative humidity and evaporation these are some of the climate/atmospheric variable. There are lot other variables. Because atmospheric system is closed system, that's why each variable affecting each and every other variables. These affects on variables are consider as relation between them.
This relation is mathematically determine by regression (linear/nonlinear).
A basic and simple relation can be determined by multiple linear regression or for more accurate relation support vector regression can be applied.
If you are interested, in my thesis I described the mathematical model for that.
If you want machine learning version of regression then I will upload code for support vector regression soon...