Vapor Density

Each fuel vapor's density will cause it to rise, sink or remain neutral in comparison to air.

Once ignited,
Fuel vapor will
travel upward in the convection column

This should not be confused with
Specific Gravity (Next section) which pertains to a substance (Liquids/Solids) floating, sinking or remaining neutrally buoyant in water!

Vapors from gasoline are heavier than air, will sink, continuing to seek lower ground.

Technical note:  We are most familiar with vapor density in comparison to air. Vapor density is also compared to Hydrogen. When researching, assure that your reference tables are referring to a comparison to air which is what we will encounter on incidents.

You should be concerned for ignition sources downhill!