This summer, we’re seeing news of extreme temperatures in the western US. Oh, and airplanes were grounded in Phoenix because of temperatures as high as 119 degrees Fahrenheit! Extreme heat can cause the aerodynamics near the ground to make it impossible for some aircraft to become airborne (recent articles from the New York Times and Wired do a good job explaining how this happens). Studies are also predicting that there will be an increase in turbulent events during flight. I think it’s a good time to talk about gas. Um, I mean gases in the air and fluid dynamics.
Feeling the heat—and fluid dynamics
Studying climate change, and being a private pilot, aligns what I’ve learned about fluid dynamics across spatial and time scales. I learned to fly while I was living in Florida years ago. This was an amazing place to fly for fun—and to experience weather. The summers were hot and humid and daily thunderstorms were typical. After only an hour practicing takeoffs and landings during the summer, I would be dripping with sweat and peeling myself from the seat of the small aircraft (no air conditioning!).
Warm air would rise up from the land surface and mix with cooler air, so I had to maneuver the aircraft carefully near the runway as the invisible eddies of air swirled around and buffeted my little Cessna. This turbulence was particularly noticeable when landing on a runway that stuck out into the Atlantic Ocean or the Gulf of Mexico, because the temperature difference between the water surface and the land surface caused the air masses above them to have different temperatures. Strong winds result as air moves from high to low temperatures and pressures. When you are flying a small aircraft only a few hundred feet above the surface, you learn to respect the power of fluid dynamics!
Pushing the limits of flight
Most of us will travel by air in much larger aircraft that are powered by jets, so we are somewhat insulated by the swirling and buffeting air near the surface. Those of you who quickly locate the airsick bag after boarding a flight will likely disagree with me, but trust me that it could be worse. The engineering on these large aircraft is incredible when you understand the forces that they endure with every trip. However, climate change is causing the atmosphere to be perturbed in ways that were not always accounted for in the history of aircraft design. Engineering limits are likely to continue to be tested as the instances of extreme land and water surface temperatures increase. Thinner air and turbulence near the ground will become worse, leading to more rough landings and canceled flights. Higher up in the atmosphere, changes to larger scale weather patterns are also causing air to become more turbulent.
So what is turbulence, anyway?
Turbulent flow is fluid movement characterized by chaotic changes in velocity and pressure. It is contrasted with laminar flow, which is an organized, layered movement of the fluid in streamlined, parallel lines. In the mid-1800s, research by an Irish mathematician and physicist named George Stokes advanced the science of fluid dynamics. Experiments conducted by a fellow Irish engineer named Osborne Reynolds were used to determine the transition from laminar to turbulent flow, after which he popularized a mathematical concept introduced by Stokes. The Reynolds number, a ratio between inertial and viscous forces, is still used in many fields of science and engineering today. It describes how frictional forces within a fluid, based on the density and speed of flow, are counteracted by dynamic viscosity, or a resistance to shearing forces. For example, you may have observed slow moving water in a wide, flat creek, and it appears to flow smoothly—a laminar flow. The layers of fluid are all moving at roughly the same speed and there is little shear stress. Once the creek narrows and picks up speed, it can begin to experience more shear stress between the fluid and the creek bottom and between layers of fluid, causing different layers of the water to “trip over” each other, swirl, and create eddies as the water becomes turbulent.
It’s not usually easy to see these flow patterns in the air, but a similar phenomenon applies to that fluid. Temperature and pressure differences, along with the Earth’s rotation, dominate the flow of air in our atmosphere. On the small scale, airplane wings, known as airfoils, are designed to maximize the amount of smooth, laminar air flow over the wing while minimizing the turbulent flow created when the air meets some parts of the wing and fuselage. In this way, the aircraft optimizes the lift needed to move the plane into the air. While this is an important concept for flying, the type of turbulence that can make your flight uncomfortable or dangerous is generated at the larger scale—when air masses heat up or speed up. Near the ground, higher temperatures create temperature differences between air masses that generate winds, storms, and in some areas, more precipitation. Higher up in the atmosphere, at cruising altitudes, climate change is causing jet streams to become less stable and the increased shear can result in clear air turbulence—which is not easy to see or even detect with radar (ok, now reach for the airsick bag!).
While environmental changes due to increased concentrations of carbon dioxide (a gas!) continue and accelerate worldwide, most people in developed countries only confront the impacts of climate change occasionally. Commercial air travel has become an indispensable necessity for those lucky citizens, to the point where they relinquish many personal freedoms. However, unless many of us opt to take to the open roads or increase rail travel, this is one area where we will be forced to confront—and adapt to—climate change. Like it or not, that ship has sailed. All aboard!Katherine Moore Powell, PhD, is a climate ecologist at The Field Museum.