Height of the Atmosphere

We teach that the atmosphere is 80,000 ft high and the halfway point, where half of all gases and pressure run out, is 18,000 ft. This seems wrong somehow. Please tell me what these numbers represent.
- question from Elvira Flagg
Some elements of the answer to your question can be found in previous questions we've answered on atmospheric properties. In particular, refer to questions on air density and layers of the atmosphere.

That being said, let's dig into your specific questions a little more deeply. First of all, we'll address the "height" of the atmosphere that you mention as 80,000 ft (24,405 m). To tell you the truth, I'm not exactly sure why this would be considered as the top of the atmosphere. This altitude is actually near the top of the Tropopause, a layer of the atmosphere that exists from about 6.8 mi to 15.5 mi (11 km to 25 km) above sea level as illustrated below.

Variation of properties through the layers of the atmosphere

This is the region of the atmosphere where most commercial and military aircraft operate during cruise flight. Perhaps your sources indicate that this is the "top" of the atmosphere since virtually nothing can fly higher than this altitude. In addition, many atmospheric property charts in textbooks only go up to that altitude, an example being illustrated below.

Changes in atmospheric properties with altitude

But is 80,000 ft truly the maximum height of the atmosphere? Far from it! Our very own Aerospaceweb.org Atmospheric Properties Calculator computes properties such as temperature, pressure, and density up to about 280,000 ft (86,000 m), and atmospheric tables in some textbooks go up to 350,000 ft (106,800 m). Even the above illustration of the layers of the atmosphere reaches an altitude of 120 km, or 75 mi, which is nearly 400,000 ft (122,000 m).

In addition, various aerial vehicles can fly at altitudes of 80,000 ft or more. The Mach 3 SR-71 Blackbird reconnaissance and research vehicle could reach over 85,000 ft (25,930 m) while the rocket-powered X-15 exceeded 350,000 ft (106,800 m). More recently, the solar-powered Pathfinder, Centurion, and Helios research craft have climbed to between 80,000 ft and 100,000 ft altitude. Various technology demonstrators like the X-43 and military missiles cruise at altitudes of 90,000 ft or higher.

So what is the true height of the atmosphere? To be honest, there really is no single answer. The thickness of the atmosphere can vary significantly with the behavior of the Sun and various weather phenomena. During periods of increased solar activity, for example, the eruption of solar flares sends out streams of energetic particles that collide with the magnetic field of the Earth and cause the atmosphere to expand. Other climatic and environmental changes have similar effects so that the true height of the atmosphere is constantly in a state of flux. The maximum altitude can also vary depending on where you are located above the Earth. Along the equator, the atmosphere tends to bulge outward more so than at the poles due to the centrifugal effect of the planet's rotation as well as the gravitational attraction of the Moon.

All of these influences can be so significant that oscillations in the height of the atmosphere are a significant concern for satellites and other spacecraft in low orbits. The intense solar flare activity of the late 1970s caused such a large expansion in the atmosphere that it began to affect NASA's Skylab space station. The increase in atmospheric density generated additional aerodynamic drag on Skylab great enough that it eventually pulled the station back to Earth. This unanticipated atmospheric expansion resulted in the fiery demise of Skylab over the south Pacific Ocean and Australia. Today's International Space Station is routinely boosted to higher orbit since aerodynamic drag forces affect it as well. Manufacturers of low-Earth orbit satellites must take these forces into account during the design phase to ensure that the vehicle's structure is sturdy enough to withstand the atmospheric effects.

On average, however, a good number to use for the true height of the atmosphere is about 400,000 ft (122,000 m), or 76 miles. It is at this altitude that vehicles such as the Space Shuttle are said to make "atmospheric interface" when they re-enter the atmosphere prior to landing. Another "official" value you might consider is 50 miles, or 264,000 ft (80,540 m). Anyone flying higher than this altitude is officially considered an astronaut by NASA and the US Air Force. Just for comparison, controlled airspace, which is the portion of the atmosphere regulated by government agencies like the Federal Aviation Administration (FAA), only goes up to 60,000 ft (18,305 m).

Now let's briefly address your second question about the "halfway point" of the atmosphere. The altitude you mention is 18,000 ft (5,490 m) with the reasoning that this is where the atmospheric pressure and density drop to about half of their value at sea level. While I've never heard anyone refer to this point as the halfway point of the atmosphere, it does indeed seem to be an accurate description. If you return to the second figure shown above, you'll see that it graphically portrays the ratio of atmospheric properties at altitude compared to sea level. The values of both density and pressure are indeed 0.5 of the sea level value at altitudes of about 18,000 ft (5,490 m) and 21,000 ft (6,405 m) respectively. These quantities continue to decrease rapidly at higher altitudes such that they are only about 3% of their sea level values at 80,000 ft (24,405 m) and less than 0.1% at 160,000 ft (48,820 ft)!
- answer by Jeff Scott, 18 August 2002