We consider the polar corrdinates in which (by mapping theorem), raindrops are a point process with $\lambda r$ $drops/cm$ ~~ $$ \begin{align} P[\Lambda^1([0,r] \times [0, 1] \times R^2)=0] &=exp(-\int_{0}^r \int_{0}^{1} \int_{R^2}\lambda p(x,y)dxdydtdr)\\ &= exp(-\lambda t 2\pi \int_0^r \rho e^{-\rho}d\rho)\\ &= exp(-\lambda t 2\pi(1-e^{-r}(r+1)))\\ \end{align} $$ ~~