Rain falls straight down. You hold a large flat board out in the storm. When does the board intercept the most water per second?

D. Held horizontally, face-up to the rain

Flux is maximal when the surface normal aligns with the flow ($\cos\theta = 1$) and zero when the surface is edge-on. Every surface integral $\iint \vec{F}\cdot d\vec{S}$ is this rain-catching computation, patch by patch, over a curved surface.