How does air pressure vary with elevation?

Air pressure is caused by the air molecules above us. These molecules are held close to Earth's surface by the planet's gravity. The farther we go from the surface, the less air molecules we find. This means that as we go higher and higher from Earth's surface, we will encounter less air molecules and hence less air pressure. In other words, air pressure decreases as we go upwards from the planet's surface. We can also say that air gets "thinner" as our elevation increases. This is the reason people experience a lack of breath as they climb mountains and many climbers have to use oxygen cylinders to compensate for the reduced air quantity. We have an air pressure of about 1,000 millibars at Earth's surface and it drops by more than 70% as we reach Mount Everest (the tallest surface feature on Earth). In fact, we lose almost 50% of the air pressure within first 5.5 km of elevation.

Hope this helps.