What if you climbed Mount Everest?

Every time you breathe in, you take in air. Air has oxygen, and your body needs it.

What if you climbed to the top of the highest mountain? Would the air be the same up there?

Every breath you take brings oxygen into your lungs, and your body needs that oxygen to keep going. About 21 percent of the air around us is oxygen.

Mount Everest is the highest mountain in the world. Its summit is 8,849 meters above sea level. What would the air be like if you stood up there? In this lab, Olly's model takes you up through the atmosphere, kilometer by kilometer.

Air is about 78% nitrogen and 21% oxygen (UCAR). Our bodies depend on drawing enough oxygen from each breath, so what matters at altitude is how much air, and so how much oxygen, a breath holds.

Mount Everest's summit stands at 8,848.86 m, the height jointly declared by China and Nepal in 2020 (Britannica). This lab uses the International Standard Atmosphere, an agreed average used by pilots and engineers, to compare the air at sea level with the air at 8.8 km, the model's Everest setting.

At the beach, there is lots of air above us. It presses down.

The air is thick, and each breath is full of oxygen.

At sea level, the whole thick blanket of air above you presses down. In the model, that air pressure is 101.3 kilopascals, the temperature is 15 °C, and water boils at 100 °C.

NASA explains why pressure is highest at the bottom: the pressure of the air comes from the weight of all the air above it. Climb higher and there is less air above you, so the pressure falls, and the air gets thinner.

At sea level the ISA gives 101.3 kPa, 15.0 °C and an air density of 1.225 kg/m³; water boils at 100.0 °C. These are average conditions, not any particular day.

NASA Glenn's atmosphere model explains that air pressure reflects the weight of the air above a location; with altitude there is always less air above, so pressure falls. Density depends on both pressure and temperature through the equation of state, ρ = p ÷ (R·T), and also falls with height.

Now Olly zooms up to the top of Everest. The air is much thinner!

It is also very, very cold. Brrr!

Now set the height to 8.8 kilometers, the model's Everest setting. The air pressure drops to 31.7 kilopascals, only about 31 percent of what it is at sea level. That's close to what Encyclopaedia Britannica says: at the top of Everest there is about one third as much breathable oxygen as at sea level.

The model's temperature falls to −42.2 °C, and water would boil at only 69.6 °C, so a pot of boiling water there would be much cooler than at home.

At 8.8 km the ISA gives 31.7 kPa (31% of sea level), −42.2 °C and 0.478 kg/m³. Britannica states that the breathable oxygen at Everest's summit is one-third of the sea-level amount, consistent with the pressure ratio.

The boiling point falls to 69.6 °C; the model's documentation notes that steam tables give about 70.4 °C at this pressure, so its formula is about 1 °C low here. A stretchy balloon launched at sea level would be 2.6 times its starting size.

Up high, there is less air pressing down. So the air spreads out.

Each breath has less air in it, and less oxygen.

The model imagines each breath taking in the same amount of space, wherever you are. High up, the air is spread out more thinly, so that space holds less air and less oxygen.

The model's 'air in each breath' readout says 39 percent at 8.8 kilometers, a bit more than the pressure's 31 percent. That's because the model compares air at the outside temperature, and very cold air is packed more tightly. If the summit air were warmed to the same temperature as sea-level air, the amount in each breath would follow the pressure: about a third.

For a fixed breath volume, the mass of air inhaled scales with density, ρ = p ÷ (R·T). The model's 'air in each breath' compares outside densities: (31.7 ÷ 101.3) × (288.15 ÷ 231.0) ≈ 0.39, so the cold summit air raises the figure. Compared at the same temperature, the amount per breath follows pressure alone, about 31%, matching Britannica's one-third.

The model's limits: the ISA is a mid-latitude average with no weather or humidity, valid to 20 km; real pressure and temperature can differ by several kilopascals and 20 °C. It says nothing about how a human body responds to thin air.

The real top of Everest is very cold and windy. No plants or animals live there.

People first climbed to the top in 1953.

The model's −42.2 °C is an average for that height, not a forecast for Everest. Britannica reports that the warmest average daytime temperature at the real summit, in July, is only about −19 °C; in January it averages −36 °C and can drop as low as −60 °C. With so little oxygen, powerful winds and extreme cold, no plants or animals can live there.

Edmund Hillary and Tenzing Norgay made the first confirmed climb to the summit on May 29, 1953. Reaching it without bottled oxygen took much longer: Reinhold Messner and Peter Habeler first did it in 1978.

Britannica gives summit averages of about −19 °C in July and −36 °C in January, with lows as cold as −60 °C. The ISA's −42 °C is colder than either monthly average: the ISA is a mid-latitude average for a given height, not a forecast for a Himalayan summit. It notes that lack of oxygen, wind and cold preclude plant or animal life at the summit.

The first confirmed ascent was by Edmund Hillary and Tenzing Norgay on May 29, 1953. In 1978 Reinhold Messner and Peter Habeler became the first to climb Everest without contained oxygen for breathing, and in 1980 Messner made the first solo ascent, also without it.