On 08 April 2005, a strong gravity wave activity (of more than 3 hours) was observed in São João do Cariri (7.4o S, 36.5o W). These waves propagated to the southeast and presented different spectral characteristics (wavelength, period and phase speed). Using the hydroxyl (OH) airglow images, the characteristics of the observed gravity waves were calculated; the wavelengths ranged between 90 and 150 km, the periods from ~26 to 67 min and the phase speeds ranged from 32 to 71 m/s. A reverse ray-tracing analysis was performed to search for the possible sources of these detected waves. The ray-tracing database was composed of temperature profiles from NRLMSISE-00 model and SABER measurements and wind profiles from HWM model and meteor radar data. According to the ray path, the likely source of these observed gravity waves was the Inter Tropical Convergence Zone with intense convective processes taking place in the northern part of the observatory. Also, the observed preferential propagation direction of the waves to the southeast could be explained using blocking diagrams, i.e. due to the wind filtering process.