Specific Weight is defined as weight per unit volume. Weight is a force. The Specific Gravity - SG - is a dimensionless unit defined as the ratio of density of the material to the density of water at a specified temperature. The specific gravity of some common gases can be found in the table below: As a reference, normal air has a specific gravity of 1. If the specific gravity is greater than 1 the gas is heavier than air and the vapor will be found low; the larger the number the heavier the gas is and the lower vapors will be found. If the specific gravity is less than 1 the gas is lighter than air and the vapor will be found high; the smaller the number the lighter the gas is and the higher vapors will be found. Various vapors can be "layered" when multiple gasses are present. Gas Specific Gravity - SG - Acetylene (ethyne) - C2H2 0.907 Air 1.000 Ammonia - NH3 0.596 Argon - Ar 1.379 Arsine 2.69 Benzene - C6H6 2.6961 Blast Furnace gas 1.02 Butadiene 1.869 Butane - C4H10 2.0061 Carbon dioxide - CO2 1.5189 Carbon monoxide - CO 0.9667 Carbureted Water Gas 0.63 Chlorine - Cl2 2.486 Coke Oven Gas 0.44 Cyclobutane 1.938 Cyclopentane 2.422 Cyclopropane 1.451 Decane 4.915 Digestive Gas (Sewage or Biogas) 0.8 Ethane - C2H6 1.0378 Ethylene (Ethene) - C2H4 0.9683 Fluorine 1.31 Helium - He 0.138 Heptanes 3.459 Hexane 2.973 Hydrogen 0.0696 Hydrogen chloride - HCl 1.268 Hydrogen sulfide - H2S 1.1763 Isobutane 2.01 Isopentane 2.48 Krypton 2.89 Methane - CH4 0.5537 Methyl Chloride 1.74 Natural Gas (typical) 0.60 - 0.70 Neon 0.696 Nitric oxide - NO 1.037 Nitrogen - N2 (pure) 0.9669 Nitrogen - N2 (atmospheric) 0.9723 Nitrous oxide - N2O 1.530 Nonane 4.428 Octane 3.944 Oxygen - O2 1.1044 Ozone 1.660 Pentane 2.487 Phosgene 1.39 Propane - C3H8 1.5219 Propene (Propylene) - C3H6 1.4523 Sasol 0.42 Silane 1.11 Sulfur Dioxide - SO2 2.264 Toluene-Methylbenzene 3.1082 Water Gas (bituminous) 0.71 Water Vapor 0.6218 Xenon 4.53 Resource: http://www.engineeringtoolbox.com/