lucynorton63; yes, at equal temperatures methane is lighter than air and will rise in an open environment. When an LNG tank is burst the LNG flows close to the ground at a density 1.6 times that of air and "boils" rapidly producing methane vapor. That area will be momentarily very cold unless there is a strong breeze and this will have a negative convection effect. i refer you to google "Basic Properties of LNG-GiiGNL" (pdf/adobe acrobat). Figure 2. in that report is an excellent photo of an LNG vapor cloud running along the ground at a Texas A+M LNG Fire Training session. Please also see my reply to your post in the thread "CNG In A Box".
wskip99: there is a world of difference between a fire and an explosion. in the rare ( i don' t know of more than one fire In the 37,000+ Chevrolet Volts in use worldwide) event of a plug-in hybrid fire in your garage a smoke detector, especially one connected to a monitoring company, can probably keep you from harm and possibly keep your house usable. A gas vapor explosion is instantaneous, can severely burn you or worse, level your house and possibly that of your neighbors. Yes , both types of events are rare, but far different. A relative was severely burned and their house was totalled in a propane explosion.
the other day you said "For safety it is best that all those vehicles that use any of the above including cars always be parked outdoors ........."
what I was trying to convey in my reply to you is that people need to use reasonable precautions in regard to NGVs as with just about anything.
saying that you should always park your NGV outdoors seems kind of like saying that if you cross the street you could get hit by a bus, so don't cross the street.
wouldn't it be more reasonable to look both ways first?