You may not find what your looking for in the available scientific literature with respect to empirical data. If you apply fundamentals, you will find the answer to your question.

The criteria that is relevant for ignition of gasoline vapors by the gas-fired water heater is a flammable concentration of gasoline vapors and air at the location of the source of ignition (inside the water heater enclosure). The criteria that the presence of any gasoline vapor at a vertical height of 18 inches is a necessary, but insufficient condition for the ignition of the gasoline vapors.

The "weight" of the gasoline vapors is not the relevant variable for consideration. It is the density of the gasoline vapors relative to the density of air that is relevant to whether gasoline vapors will be buoyant in air and will rise and opposed to falling. This ratio is refereed to as the Specific Gravity.

While humid air is less dense than dry air and will change the specific gravity, the effect of humidity is much less than other modes of mass transfer, such as diffusion and temperature effects. The forces associated with diffusion are strong enough to overcome the forces associated with gravity (buoyancy). The math suggests that the effect of humidity on the density of air is not significant at "room temperatures." This effect starts to change the specific gravity at temperatures well above "room temperature."

Gasoline is a multi-component mixture. All of the significant molecules of the individual components are "heavier" than air and will "fall" as opposed to rise. The way to change the specific gravity is to heat the gasoline vapors and reduce their density and specific gravity.

Since the characteristic time of diffusion is orders of magnitude greater than the characteristic time of forced convection, air movement is much more of a likely contributor for the flammable mixture at the source of ignition than the humidity in the air in most practical time frames. Given that the gas-fired water heater creates a buoyant flow internally and the combustion system requires air flow from outside, this is probably the determining factor for ignition. Again, the presence of a flammable concentration at the high of 18 inches outside the water heater is a necessary, but insufficient condition for ignition of gasoline vapors.

In summary, there is a change of density effect for changes in humidity of air, so one can formulate a hypothesis that this can cause a change in the vertical concentration gradient of gasoline vapors.However, if one tests this hypothesis using fundamentals of gas dynamics, basic chemistry, and thermodynamics, it would disprove this hypothesis (too small of an effect to be a driving force), but allow evidence to formulate other hypotheses.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com