Which atmospheric gas has the greatest effect on the weather conditions associated with an air mass?

Water vapor is the atmospheric gas that has the most impact on an air mass and the weather associated with the interactions of different air masses.  

An air mass is a large body of air that has similar temperature and moisture throughout.  The air mass gets its moisture and temperature characteristics from the area over which the air mass forms.  That area is called a "source region."  In general, the source regions are combinations of hot, dry, cold, and wet.  For example, an air mass that forms over the Gulf of Mexico is likely to be warm and wet.  Compare that with an air mass that forms over the southwestern United States.  That air mass is going to be warm and dry.  An air mass that forms over Canada would be cold and dry, and an air mass off the coast of New England would be cold and wet.  The temperature and humidity are both of equal importance for the associated weather conditions of a specific air mass, but water vapor is the only atmospheric gas of the two.  

Generally speaking, the weather conditions associated with an air mass are stable.  Air masses do not extend forever though.  Eventually they "bump" into another air mass that has distinctly different weather conditions.  The two air masses do not usually mix, so when they meet, a boundary forms between them.  This boundary is called a "front."  Weather conditions along a front are usually cloudy and stormy.  There are four different types of fronts.  They are warm fronts, cold fronts, occluded fronts, and stationary fronts. Fronts are identified and named based on their motion.  For example, a cold front occurs when a colder air mass replaces a warmer air mass.