Final answer:
The statement that approximately fifty percent of the atmosphere is concentrated below 5500 m is True; about half of Earth's atmospheric mass is within the first 5.5 km of altitude due to gravitational compression.
Step-by-step explanation:
The statement that approximately fifty percent of the atmosphere is compressed by gravity below an elevation of 5500 m (18,000 ft) is True. Earth's atmospheric pressure behaves such that, at lower elevations, the air is denser due to gravitational compression. In reality, about 99% of the atmospheric mass lies within 30 km of Earth's surface, but roughly half of that mass is found below the first 5.5 km.
The pressure exerted by the atmosphere at sea level is around 100 kPa (kilopascals), corresponding to the weight of a 10,000 kg column of air bearing down on every square meter of the Earth's surface. This demonstrates how atmospheric pressure decreases rapidly with altitude because of the thinning air.
Moreover, for every rise of about 8800 meters in elevation, atmospheric pressure approximately drops by a third, under simplifying assumptions of constant temperature and gravitational acceleration, which are not entirely accurate in real atmospheric conditions.