If descending, partially melted snowflakes or raindrops fall through a colder layer near the ground, they can refreeze into sleet, which is tiny clear or translucent ice pellets that sound like falling rice when they hit your window.
When the layer of colder surface air is shallow, raindrops falling through it won’t have time to freeze and will hit the surface as freezing rain, which spreads out into a thin film of ice as soon as it hits any cold surface. While sleet is relatively harmless, ice storms caused by freezing rain can be killers, as roads become slick with ice, causing auto accidents and bringing even foot traffic to a standstill. Freezing rain can create winter wonderlands by coating trees with a twinkling, crystalline glaze, but it can also bring down telephone and power lines, cutting off communications and creating severe electrocution hazards.
Sleet can form only when the weather is very cold, while hail is a warm-weather phenomenon based on heat convection. Hail forms while bouncing around in a thunderstorm, while sleet is created when a snowflake or raindrop refreezes during a winter storm.
Aircraft are especially vulnerable to ice, which in a freezing rain can build up very quickly and is very difficult to remove. A coating of ice on a plane’s wings increases its weight, which makes it more difficult to gain altitude at takeoff. Moreover, the ice disturbs the airflow over the wings and fuselage, which makes it more difficult for the plane to stay airborne. Airports in ice-prone areas maintain de-icing crews, who spray aircraft with an antifreeze mixture designed to melt ice before it can accumulate to dangerous levels.
The National Center for Atmospheric Research (NCAR) has found that the most dangerous icing forms when planes fly through supercooled drizzle in clouds. Although the drops are small, they freeze quickly and form a rough ice layer called rime that decreases lift and increases drag much more than a layer of smooth ice would. The National Weather Service’s Aviation Weather Center in Kansas City, Missouri, is using supercomputers to develop forecast maps that will enable pilots to steer clear of icy drizzle while aloft.
Down at ground level, however, even a good forecast isn’t always enough to protect people and property from the dangers of an ice storm. In January of 1998 a severe ice storm hit the northeastern United States and Canada, causing forty-four deaths. In some places more than 3 inches of freezing rain fell, coating trees, buildings, and cars with ice that was more than an inch thick. In the aftermath 500,000 people were without power in the United States, including more than 80 percent of the population of Maine. Things were even worse in Canada, where more than three million people lost electricity. Damage estimates for both countries totaled $4.5 billion.