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Union University Department of Physics

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Science Guys >December 2000

December 2000

Why does ice form on the wings of airplanes?

During a January, 1996 cargo flight over the mountainous West Virginia terrain, an Aerostar 601 aircraft lost power to its right engine. The pilot initially maintained altitude with the left engine alone, but the plane flew into a chilly cloud of supercooled water droplets. Immediately a layer of ice cloaked much of the plane, and the lift provided by one engine could not overcome the weight of the iced-over Aerostar. The plane crashed into Mt. Storm, West Virginia, and the pilot sustained serious injuries, but survived.

Ice formation on airplanes during flight remains a serious aviation safety threat, although measures taken by air carriers and aircraft manufacturers have partially eliminated the problem. But how does ice form on the wings of airplanes?

To answer this question adequately, you should understand several atmospheric phenomena. First, as you go up in height in the atmosphere, the air (atmospheric) pressure decreases. Since the air pressure decreases with altitude, an air mass expands as it rises in the air, and as it expands the temperature decreases. At high altitudes, air can be extremely cold, and be well below the normal freezing point of water, 32 degrees Fahrenheit. Air normally contains a certain amount of water vapor. As the temperature decreases the amount of water vapor increases to the point of saturation. The water vapor in the air can now be many degrees below freezing. Under the right conditions, when condensation nuclei are present, tiny water droplets slowly form from the water vapor which is several degrees below the freezing point. Now we have a cloud of extremely cold water droplets. These initial droplets are very small, about one-tenth the diameter of a human hair. However, they coalesce into larger drops before falling as rain.

Thus, cloud droplets do not necessarily freeze at 32 degrees; pure water suspended in air may not freeze until nearly 40 degrees below zero. Water in liquid form below 32 degrees is called supercooled. In a cloud of supercooled water droplets, any agitation of the cloud or contact of the droplets with solid particles can trigger rapid freezing. When a plane flies through a supercooled cloud the disturbance causes droplets to collect on the airplane, and since the water droplets are already supercooled they solidify into ice almost instantly. While flying through a large cloud of supercooled water droplets, an airplane can experience considerable ice buildup.

Consider what is commonly called freezing rain, rain that freezes upon contact with objects. The raindrops have come from a supercooled cloud thousands of feet above the Earth’s surface and may be several degrees below freezing as they fall through the atmosphere. At the surface of the Earth people are surprised when they find ice forming as it rains with the temperature above freezing, maybe 35 degrees, yet as the rain strikes objects on the surface it "instantly" freezes. This is one method whereby an ice storm can occur.

Believe it or not, the formation of frost on vegetation is the result of bacteria on leaf surfaces. The bacteria actually serve as condensation nuclei for the frost! Scientists hope to genetically engineer these bacteria so that they no longer help to trigger ice formation. Without the aid of the bacteria, water vapor in the air would not form frost on tender plants until a temperature of 18 to 21 degrees Fahrenheit.

So we see that the formation of ice on surfaces is really a complex process. It’s really not enough to just get the air to 32 degrees Fahrenheit.