Considerable research efforts are focused to forecast consequences of aerial overpressure and impulse due to fuel-air explosions. Statistics indicate that some two-thirds of the financial loss is attributable to explosions. Previous work of the authors [1] was done on propane-oxygen stoichiometric clouds ignited by SEMTEX™ high explosives. In this second study, stoichiometric mixture of methane and oxygen filled a polyethylene envelope with a total volume of the mixture of 5.5 m3. Different image processing was performed to visualize simultaneously the propagation of combustion flame in the envelope and the overpressure in ambient air, outside the envelope. To represent realistic conditions, the shape of the gas envelope was elongated like the dispersion shape from an accidental leak with wind. An electric spark ignited the mixture gas volume on one side. Both flame propagation velocity and overpressure in open-air were recorded with direct visualization. Near field scene (10 m x 6.9 m i.e. about 1.7 times the envelope length and 6.2 times the envelope diameter) was recorded with direct illumination from the flame propagation through the use of a 20 kfps (fps, frame per second) camera. Far field scene (32.4 m x 17.3 m i.e. about 5 times the envelope length and about 16.8 times the envelope diameter) was recorded with direct lighting of the scene through the use of a 9.4 kfps camera. The mixture envelope and the camera were positioned in order to use far-field vegetation as contrasted natural background. In order to visualize the overpressure wave, natural Background Oriented Schlieren (BOS) was applied as image processing [2,3]. Because of the light fluctuation due to the explosion, the reference image for a frame corresponds to the previous frame. Direct visualization shows the combustion of the mixture with increasing velocity. A transition from deflagration to detonation is identified while the combustion front reaches the middle of the envelope. Analysis of the BOS images shows the presence of two overpressure waves of decreasing velocity outside the envelope. Evolution of the global dynamics of the phenomenon is achieved through the use of synchronization of near and far field in a composite image where RGB image layers are respectively set with the BOS image, the global view and the flame view scaled to fit the far field view.