The suddenness of rainfall (13)
I believe the answer is:
precipitation
I believe this is a double definition.
'the suddenness' is the first definition.
(thesaurus)
'rainfall' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for precipitation that I've seen before include "Snow, rain, etc - impetuosity" , "Undue rushing" , "Rain, hail, snow etc" , "Rain, snow, sleet or hail" , "Eg, rain, snow or sleet" .)