The Laboratory Control and Safety Solution Application Guide (1994) points out that, as an energy-efficiency measure, a conical top can be added to an exhaust stack to increase the final "velocity at the point of stack discharge." The conical top's diameter determines the actual exit velocity. Engineers should not incorporate "a weather hood or rain cap on a stack that would impede" exit velocity of the exhaust from the stack. "Rain or condensation will not run down into a stack if the exhaust air exit velocity as at least 2,500 fpm (12.7 m/s)." See Section 7, Air Distribution, Stack Discharge Fitting.