Constant of Proportionality - Definition & Examples

Jane put out a cup during a rain storm to measure the rate of rainfall. During the first hour, Jane observed $7.8$ mm of rain. Later, Jane observed that there are $15.6$ mm of rain in the cup after the second hour, and $23.4$ mm after the third hour. Is the relationship between the amount of rainfall and time directly proportional? If so, what is the constant of proportionality?