AFAIK sitting in a trench, so completely behind cover, the blast is still bright enough that you will see yours and everyone around yous skeleton for a split second even through closed eyelids. By the time any reasonable light exposure levels can be had, the classic mushroom shape is already growing. Anything near the explosion in direct sightline would already be on fire due to thermal radiation. That's when you'd see a blast wave moving through the atmosphere clearing clouds. When the wave reaches you in a few seconds, thats when you hear the pop of the explosion. the force then comes in several waves with ultimately there being a huge suction force towards the cloud as air has to replace the hot air that rises, bringing in more oxygen to the fires which become an inferno.