When planning out a pyrotechnic display, the pyrotechnist (is that a word? dunno) takes into account the expanding cloud of smoke when deciding where the next one is going to go off, and at what time. It all depends on weather this is a sky bursting shell, or lance work. Even so, the hope is for a night with a mild upper level wind, but sometimes, you don't get that. Or the fireworks end up composed slightly wonky, and fail to make the perfect display. If you need a scholarly work on the topic, you should look up something by Lancaster. He's an Anglican minister from England, commonly known as Lancaster, Master Blaster Pastor.