Micro-Tutorial #4 – Light as Air? That’s heavy.

The Smithsonian’s National Air and Space Museum is one of my favorite museums in Washington, D.C. The museum has two prominent locations: one in downtown D.C. and one in Chantilly, VA. Once the pandemic is over, I recommend visiting! Until then, you can visit their website to view illustrations and read descriptions from previous eras when balloons were used for entertainment, science, adventure, exploration, and war! Amazingly, people continue to use balloons for many purposes.

Partial screenshot of the Smithsonian’s National Air and Space Museum website. Click on the image to explore the website. [Website: https://airandspace.si.edu/exhibitions/clouds-bag]

Balloon technology works at lifting people and cargo into the air through controlled application of the buoyancy force. According to the American Meteorological Society’s (AMS) Glossary of meteorology, buoyancy is “That property of an object that enables it to float on the surface of a liquid, or ascend through and remain freely suspended in a compressible fluid such as the atmosphere.”

Both the density of the air in the balloon and the air outside of the balloon affect how fast the balloon can rise or descend. Air density is also directly related air temperature.

Air Has Mass & Weight

Atmospheric gases each have their own mass (the amount of matter in an object) and their own weight. For the most part, atmospheric gases are well-mixed and equally distributed in the troposphere (i.e., the lower portion of the atmosphere where most weather occurs).

Weight is the force of gravity on a particular mass. Some gases weigh more than others and have more mass. Aeronauts (ballooners) take advantage of the different characteristics of gases to operate balloons.

Hot mixed air, hydrogen, and helium are three gases aeronauts have used over the years to operate balloons. Due to its extreme flammability, aeronauts no longer use hydrogen. Helium is a really light gas and is used in some balloons because it weighs less than the surrounding air. This increases the upward buoyancy force on the balloon causing the balloon to accelerate upward relative to Earth. Similarly, hot air is less dense relative to cooler surrounding air and will also rise, as soon with hot air balloons.

That air has weight and mass means that it applies a pressure to anything it touches. Typically, anything on the surface of the Earth experiences an air pressure of 1013 millibars (a millibar is a unit of air pressure). The AMS Glossary definition of air pressure is: “The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the “column” of air lying directly above the point in question.”

A Quick Note on the Ideal Gas Law

Through many experiments, scientists demonstrated that pressure, volume, and temperature relate to one another with the equation of state (also known as the ideal gas law). The ideal gas law applies to all gases and mixture of gases, such as the atmosphere. We will cover more about the ideal gas law in a later tutorial.

Air Pressure & Density Varies with Altitude

We are starting to tie things together! Density is the ratio of mass of matter to the volume it occupies. It’s basically how much stuff is crammed into a space. Recall, we talked about the concept of air occupying a space and having a volume in Micro-Tutorial #3.

In the real world, most things are complex (as they say, “nothing is ever easy”). This is also true in how atmospheric density varies with altitude. Specifically, atmospheric density decreases with height, but so does pressure. This lesson’s programming exercise will walk you through the atmospheric equations that define how air pressure and density vary with height (a.k.a altitude).

Aeronauts boldly explored the skies during the 1800s. Some adventurers even died! Our knowledge of how atmospheric pressure, temperature, and density decrease with height stems from their early experiments and adventures. During one flight in 1862, two scientists by the names of James Glashier and Henry Coxwell, flew as high as 37,000 feet. They experienced hypoxia (oxygen deprivation) and exposure to very cold temperatures.

Travels in the Air – Falling stars as observed from the balloon. Source: (Public domain / Wikimedia Commons)

Programming Exercise

Difficulty Level: Easy to Moderate


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