During a severe storm in Palm Beach, Fla., on January 2, 1999, 31 inches of rain fell in a period of nine hours. Assuming that the raindrops hit the ground with a speed of 14 m/s, estimate the average upward force exerted by one square meter of ground to stop the falling raindrops during the storm. (Note: One cubic meter of water has a mass of 1000 kg.)

So far I have tried using F_av = (P_f - P_i)/(t_f - t_i) but I haven't been able to even figure out exactly what the mass is of the rain...how does 31 inches of rain relate to 1 cubic meter of rain? And also, am I using the correct equation?