A torrential downpour drove sheets of water against the corrugated tin overhead, setting the entire market eaves into deafening vibration. I stepped deeper into the rafter niche above the fruit stall, keeping well clear of the cascading overflow. Wedged against a rusted crossbeam beam sat an overturned metal sardine tin, washed clean by previous seasons. Using the brass rim of my folding magnifier, I braced the empty tin against a heavy iron bolt. Each lightning strike and subsequent water surge resonated through the tin's thin wall with a sharp ping, giving me a reliable gauge of the storm's intensity without exposing my position to the elements.