HELP: Rain reduction factor

Hello Dears

Can any one explain to me basically what is the rain reduction factor? why do we need to calculate it.

Regards

http://descanso.jpl.nasa.gov/Propaga...ttenuation.pdf

Thanks a lot vfone for your reply. However, i know that the reduction factor use to account the inhomogeneity of rainfall BUT HOW? this is what i want to know

Please HELP friends, i have a presentation this week

Pls refer below link.

http://www.itu.int/rec/R-REC-P.838/en