Cold boiling liquid escapes radially from a point source and flows along the ground or sea until it flashes into cool vapor.  Until it has mixed with sufficient atmospheric oxygen, its is not an explosive mixture.  If you light a match, the match is extinguished.  As the cool vapor expands on heating from surface contact and atmospheric mixing, it continues moving outward from the source until it is eventually heated to ambient air temp.  At that point it becomes lighter than air and starts rising.  As the gas  mixes with the oxygen in the air, it eventually reaches explosive limits and starts looking for an ignition source.  <At this point it is very important to extinguish all smoking materials>  If no ignition source is encountered, the gas rises into the air and <"harmlessly"> contributes to the greenhouse effect.  If ignition occurs, rapid combustion follows and heated expanded vapors quickly cause an updraft usually sufficient to pull back any remaining gas mixture in the surroundings near the ground and all is sucked into the updraft and rising fireball.  Consequently, surrounding gas now well mixed with atm O2 is rapidly displaced upward into contact with fireball causing even bigger and more spactular fireball that rises several hundred meters into the air, as radiated heat converts all nearby objects <if there are still any nearby> into melted crispy critters.  Just digging into a large pipeline with a backhoe can cause fireballs and flames that can easily reach 200 meters high and melt the backhoe and unfortunate operator in a few ms.  Happening on the scale of an LNG tanker would be a hell of a photo opportunity.