Freezing rain and sleet
For the most part, winter ice doesn't do much more than inconvenience people for a few hours. Worldwide, it's far less common than snowfall, making it an unusual enough phenomenon to grab people's attention. The shimmering beauty of an ice storm, for example, offers a gratifying encounter with the work of Mother Nature while packing a truly destructive force. The worst ice storms can uproot trees and power lines, bringing civilization to a standstill for days. In some regions, ice-coated pavements are the leading cause of broken bones and other winter-weather injuries. Vehicles, too, can slip and slide just as easily as people do, resulting in many deaths on the road. Ice build-ups on aircraft wings have led to many an aviation disaster.
Pelted by pellets
Winter's subdued answer to summertime hail is ice pellets, also known as soft hail in the UK and sleet in the US (in the UK, sleet refers to a mix of rain and snow). A hailstone forms within a strong thunderstorm updraught and fells into ground-level temperatures that may be quite hot. Ice pellets, on the other hand, form in flatter, stratiform clouds. Since the upward motions are much weaker, ice pellets never attains the gargantuan sizes of the biggest hailstones.
Unlike hail, ice pellets verge on becoming rain and then pull back. The key is a layer of above-freezing air sandwiched within the heart of a winter storm. An ice pellet begins as a snowflake, then falls into the warmer middle layer and melts either partially or completely. Before reaching the surface, though, it hits a ground-hugging layer of air where the temperature is again below 0°C/32°F. This freezes the melted snowflake into a rounded, solid mass.
Because ice pellets are sometimes associated with big winter storms, they tend to occur in the parts of the world where intense cyclones have access to frigid surface air as well as warm maritime air flowing on top of it. Ice pellets typically occur in eastern North America, western and central Europe, and Japan – some of the same areas prone to the most serious bouts of freezing rain. Even in the most favoured locales, you're unlikely to experience ice pellets for more than a few days in a given winter.
Unlike all other forms of winter precipitation, ice pellets are most likely to occur around midday. One theory exists that the relative warmth of the daytime helps to nourish the warm, middle layer of a storm, thereby giving snowflakes a chance to melt and refreeze with ease.
Freezing rain vs freezing drizzle
Only a slight tweak to the conditions for ice pellets can yield one of winter's most treacherous visitors, freezing rain. If the sandwiched-in warm layer is sufficiently thick, a snowflake may completely melt yet not have time to refreeze as an ice pellet before it strikes the ground. In this case, it refreezes on whatever surface it hits – but only if the surface is adequately chilled.
Ideally, the air at ground level has been well below freezing for at least a few hours, if not for several days. Sometimes only the most exposed surfaces, such as highway bridges and overpasses, are cold enough for rain to freeze on them That's why a motorist driving on wet roads at temperatures near freezing can run into an unexpected ice patch across a bridge.
When a cloud lacks the nuclei to create large snowflakes that melt and form full-sized raindrops, drizzle may result. Freezing drizzle begins as supercooled water droplets that remain liquid while in air that's below freezing. Like freezing rain, these droplets may pass through a layer above 0°C/32°F and then turn to ice on contact once they hit the cold ground. But since freezing drizzle starts out in liquid form, there's nothing to melt, so the warm layer isn't really necessary. Some recent studies have found that a large amount of freezing drizzle – at least in the United States, and perhaps elsewhere – forms without the presence of any above-freezing layer at all.
Freezing rain and freezing drizzle have their favourite haunts, and those don't always coincide. The most likely place for freezing drizzle in North America is a swath through the centre of the continent, from the high plains of Texas up to Hudson's Bay. Another familiar zone is along the northern and eastern fringes of the continent, in the cold maritime air often prevalent from Alaska across Canada and south to New England. This air tends to be relatively clean, with few ice nuclei – a good environment for freezing drizzle to form.
The peak zones see as many as 30 or 40 hours of freezing drizzle a year, spread out over a few days: St Johns, Newfoundland, gets over 100 hours per year. Most areas experience only 10 or 20 hours a year, if that much, although by and large freezing drizzle tends to last longer than freezing rain.
The numbers are roughly similar across western and central Europe: here, Atlantic winds can easily surmount Arctic air near the ground. The prime spots are slightly inland: they include central England, as well as the prairies of the Ukraine and a belt just northeast of the Alps from southern Germany to Romania. The mountains and adjacent lowlands of central China are another designated region at risk, as moist air from the Indian Ocean overrides persistent Siberian air at lower levels. Some Chinese mountains have reported weeks of continuous glazing, with astounding ice build-ups of more than 30cm/10in.
Ice storms – the result of heavy freezing rain – are most common in the vicinity of big winter storms with large temperature contrasts. Some of the most famous icings in American history have occurred from the Deep South northeast to New England. The states of Oklahoma, Arkansas and Texas were hit by an ice storm on Christmas 2000 that left millions of trees damaged or destroyed and some half a million people without electricity for days. Thousands of people across rural Arkansas were without power, water, heat and telephone for as long as a week. A similar disaster struck the more urban setting of Montreal in January 1998.
Ice storms do their dirty work by coating everything in sight – tree limbs, power lines, highways – with a glaze whose thickness can exceed 5cm/2in. Even a thinner coating of ice can be surprisingly heavy, since it carries roughly ten times the weight of a similar depth of wet snow. Although normally flexible in high winds, power lines and transmission towers (not to mention trees) were not designed to bear huge amounts of weight. The cold northwest winds that often follow an ice storm make matters worse as they add to the stress on ice-coated trees and power lines.
Both freezing rain and freezing drizzle can seriously threaten aircraft. Radar and satellite displays can pinpoint the whereabouts of cloud or rain, but neither offers a foolproof method of locating exactly where the air is below freezing (though new types of radar may soon help). The deicing devices attached to most planes are designed to prevent cloud-sized water droplets from coating the aircraft. They're not meant to protect against the larger droplets that can freeze onto vulnerable parts of the plane, with adverse effect. Deicing fluid, applied before take-off to prevent ice build-up, helps, but it only works for a limited time after application. Each year an average of about 25 plane crashes in the US are blamed on in-flight icing. Most of these, however, involve aircraft smaller than the typical passenger plane.