Normalized spectral weight at each of the final (nonoverlapping) 3672 frequency channels displayed in the corresponding velocity units for the entire set of ALFALFA grids (upper) and for two strips of grids across the northern Galactic hemisphere (lower) at decl. = +07° (blue line) and +33° (dashed red line). The main cause of missing data (reduced spectral weight) is RFI, most notably the 1350 MHz (cz ∼ 15,600 km s⁻¹) FAA radar at the San Juan airport. Narrower features at 8800, 3300, and 2200 km s⁻¹ are modulations of the FAA radar occurring within the WAPP spectrometer. Comparison of the panels illustrates the varying nature of the RFI during different observing periods and the serious contamination caused by the 1345–1350 MHz radar system at velocities above 15,000 ﹩\mathrm{km}\,{{\rm{s}}}^{-1}﹩.