You are not likely to stroll down a street and encounter a suddenly high concentration of oxygen or a pocket of unmixed argon gas. The constant motion of the weather due to fronts, pressure changes, varying temperatures, storms, and so on, is like putting the atmosphere in a food processor and hitting the “blend” button, and never turning it off. The percentage of each gas, therefore, will be constant anywhere below an elevation of 50 or 60 miles (80 to 95 kilometers).