The acoustic environment is known to influence the vertical directionality of the ambient noise. For example, in shallow water with a downward refractive (summer) sound-speed profile, the noise vertical directionality is expected to show a deep (>5 dB) null in the horizontal direction for wind generated noise below 1 kHz. This ``horizontal null'' is a consequence of the fact that low-order modes are weakly excited by the noise sources which are located near the ocean surface. This phenomenon has been experimentally observed in limited cases. This paper concentrates on a quantitative analysis of when the vertical directionality will exhibit a notch in the horizontal direction. The answer will depend on the acoustic frequency, the sound-speed profile, the water depth and bathymetry, the bottom properties, the vertical array aperture, and finally the noise source distribution. Currently no simple way exists to model the noise notch except to run the numerical noise model repeatedly. A modal analysis is presented of the ambient noise directionality with a closed-form expression that allows one to predict and model the noise notch under a typical condition.