Like water in the ocean, air in the atmosphere exerts pressure that increases with depth. Most gas molecules in the atmosphere are pulled close to Earth’s surface by gravity. As a result, air pressure decreases quickly at lower altitudes and then more slowly at higher altitudes. This is illustrated in Figure below . Air pressure is greatest at sea level, where the depth of the atmosphere is greatest. At higher altitudes, the pressure is less because the depth of the atmosphere is less. For example, on top of Mount Everest, the tallest mountain on Earth, air pressure is only about one-third of the pressure at sea level. At such high altitudes, low air pressure makes it hard to breathe and is dangerous to human health.
Mark had looked forward to climbing Mt. Denalit for years. Today he would complete the last leg and be on top op the mountain more than 20,000 miles above sea level. As the day went on Mark often had to slow down as he was getting light headed and tired.
Was the air pressure increasing or decreasing as Mark climbed??