Ever wonder why the cabin windows in most airplanes don’t fog up? Most automobiles experience fogged-up windows from time to time, so conventional wisdom may lead you to believe that airplanes experience them as well. After all, the high altitudes at which airplanes fly will expose them to cold air. Airplanes, however, are designed so that their windows stay clear even at cruising altitude.
The Science Behind Fogged-Up Windows
Fogged-up windows are the result of condensation. As moisture vapor settles on the cold surface of a window, it will condense. This condensation will manifest in the form of fog or ice.
Airplanes, of course, fly at high altitudes. Most commercial airliners fly at roughly 30,000 feet where the air temperature is between minus 40 and minus 70 Fahrenheit. The cabin temperature, on the other hand, will remain around 70 degrees Fahrenheit. The 140-degree difference between the outdoor air and the cabin air creates the perfect conditions for window fogging.
Defogging Solutions
There are different ways for airplanes to prevent fogged-up windows, one of which is the application of anti-fog coatings. Anti-fog coatings are exactly what they sound like: window coatings that are designed to prevent fogging. They are typically applied to the interior, cabin-facing side of the windows.
Consisting of oleophobic substances, anti-fog coatings work by reducing the surface tension of moisture. Moisture vapor will still try to collect on the windows. With an anti-fog coating, though, it won’t be able to condense. Instead, the moisture vapor will scatter into a thin layer while preserving the clarity and visibility of the windows.
One of the most common defogging solutions is heating. In the past, airplane windows were heated using wires similar to those found in automobile windows. While some airplanes still use wire-based defogging solutions such as this, though, most modern-day airplanes have since switched to conductive heating.
The innermost layer of a typical airplane’s windows is made of an electrically conductive material, such as indium tin oxide (ITO). ITO is prized for its ability to conduct electricity. To prevent fogging, airplanes will apply a voltage to this conductive layer. As electricity flows through the layer of ITO, the window will heat up. This heat will prevent condensation from forming.
In Conclusion
Airplanes use different techniques to maintain clear, fog-free windows. Some of them use anti-fog coatings that disperse moisture vapor so that it can’t condense. Others use heat via a thin layer of ITO or a similar electrically conductive material.
