Sentence:  On May 15 , a very large increase occurred with * * f of mass between * * f and * * f ; for the next two days , the impact <WSD>rate</WSD> was * * f ; and for the next nine days , the impact rate was less than * * f ( Nazarova , 1960 ) . 
Meaning of the word marked with <WSD> tags:  a magnitude or frequency relative to a time unit