Where are the most significant changes in atmospheric pressure observed?

The most significant changes in atmospheric pressure are observed at lower atmospheric levels, particularly between sea level and 18,000 feet. This region is where a large percentage of the atmosphere's mass is concentrated, leading to a rapid decrease in pressure as altitude increases.

At sea level, atmospheric pressure is at its peak, around 1013.25 hPa (hectopascals). As one ascends, the pressure drops significantly nearly exponentially due to the thinning of air molecules. In fact, about half of the Earth's atmosphere is within the first 18,000 feet, making this range crucial when discussing changes in pressure.

In contrast, while atmospheric pressure continues to decrease beyond 18,000 feet, the rate of change is much slower with increasing altitude. By the time one reaches space or even the stratosphere, the pressure is substantially lower, but the most dramatic changes occur well below these heights, primarily within the lower troposphere where our weather occurs.

Therefore, the choice emphasizing the lower atmosphere between sea level and 18,000 feet as the region of significant pressure change accurately reflects the physical characteristics of atmospheric layers.