At what altitude is air density typically lower?

Air density decreases with increasing altitude, primarily due to the fact that atmospheric pressure diminishes at higher elevations. At sea level, air is compressed by the weight of the atmosphere above it, resulting in a higher density. As you ascend into high altitude regions, there is less air above pressing down, leading to a decrease in air pressure and a corresponding reduction in the number of air molecules in a given volume. This phenomenon is why aviation, for example, recognizes that aircraft performance can be affected by altitude, as they encounter thinner air at higher elevations. Therefore, high altitude regions are characterized by significantly lower air density compared to sea level and low altitude areas.