Air pressure drops with altitude, which is why mountain air feels noticeably thin.

The longer version

Air pressure at any level is simply the weight of all the air stacked above it. Climb a mountain and there is less atmosphere overhead, so the weight pressing down, and therefore the pressure, falls steadily with height.

Lower pressure means a given volume of air contains fewer molecules. Each breath delivers less oxygen, and liquids boil at a lower temperature because molecules escape more easily. The decrease is rapid near the surface and gentler higher up, since air is compressible and densest at sea level. Pilots and mountaineers track this closely, because it affects engines, breathing, and the boiling point of water.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.