How does air density change with altitude?

As altitude increases, air density decreases due to the reduction in the weight of the air above and the expansion of air, which leads to fewer air molecules being present in a given volume. At sea level, air is denser because there is more atmospheric weight pressing down, whereas at higher altitudes, there is less pressure on the air, resulting in lower density. The decrease in air density impacts various aspects of aviation, such as lift generation, engine performance, and the overall behavior of an aircraft in flight. Understanding this concept is crucial for aircrew as they need to account for changes in aircraft performance when flying at different altitudes. The other options do not accurately describe the relationship between altitude and air density, making this understanding critical for effective flight operations.