But when these so-called supercooled water droplets hit an object, they freeze onto it. That's why trees at mountaintops in the winter are often covered in rimethe white ice formed in a dense, chilly fog.
The same thing can happen to airplanes flying through cold, wet clouds.
"When an airplane flies through a cloud of precipitation containing supercooled water droplets, the drops stick to the airplane, changing the shape of the airflow, and that makes the plane not fly so well," Politovich said.
Gene Addy is an icing researcher at the NASA Glenn Research Center in Cleveland, Ohio. Generally speaking, he said, the more water, the thicker the cloud, and the more dangerous the icing conditions.
"Ice builds up more quickly," he noted. "Conversely, where the temperature is quite a bit below freezing, the drops freeze on impact, and it can be more of a challenge to get the ice removed, which protects the airplane from accretion."
The study of accretion, or how ice builds up, on aircraft wings is a major focus of icing research at the NASA research center, Addy said.
(See a National Geographic magazine feature: "The Future of Flying.")
NASA-built computer models of the accretion process are widely used throughout the aviation industry in the design of ice-protection systems, such as the placement of heaters on wings.
"We also get involved with getting a handle on how icing affects the airplanes what happens to the airplane when it's trying to fly with ice accretion on it," he said.
To study the dangers of icing, the NASA researchers fly themselves into icing conditions on a DH-6 Twin Otter aircraft to collect data on cloud conditions, ice formation, and plane performance, he explained.
NCAR's Politovich called the data collected by the NASA team "critical" in the development of CIP. The data verifies the accuracy of their weather models.
With confidence in the CIP data, pilots of properly equipped airplanes can fly through areas of light icing knowing that the conditions will not be severe enough to cause a problem, she said, instead of taking extended, fuel-costly detours.
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES