Of five major gaseous components of air, which is the only one to vary significantly in concentration from place to place and from day to day?

Water Vapor Let's look at the major gases in the atmosphere and see which one meets the criteria in the question. 1. Nitrogen - 78.09% and is by far the largest quantity of gas in the atmosphere. 2. Oxygen - 20.95% and is the 2nd most common gas in the atmosphere. 3. Argon - 0.93% All of the above gases are always gases under regular conditions and tend to remain fairly stable as to their percentages. 4. Carbon Dioxide - 0.04% This gas too is always a gas under normal conditions and tends to get evenly distributed through the atmosphere. You'll find higher local concentrations near burning carbon, but generally the concentrations remain fairly constant. 5. Water Vapor - Average of 0.4%, but ranges widely. Around sea level it's typically close to 1%, in deserts, it's close to 0%. So this component matches the criteria for the question.