Weather: Types of Precipitation
Types of Precipitation
Precipitation can take on many different forms. The temperature plays a major role in determining the various types of precipitation. Also important are the motions within the atmosphere. Raindrops aren't the only elements that could be falling upon our heads.
"I love snow, and all forms
Of the radiant frost."
—Percy Bysshe Shelley
Precipitation can take many forms. Ice crystals can link to each other and form snowflakes. John Burroughs once said, "I was born with a chronic anxiety about the weather." Most people share that anxiety, and—once you're out of school and the possibility of "snow days"—few elements of weather bring on as much anxiety as snow. Although snow is beautiful, it is disruptive to traffic and communities. My own students who work part-time at grocery stores say they can tell how much snow I'm predicting based on the length of the check-out lines. The perceived need for loaves of bread and quarts of milk is directly proportional to the amount of snow in the forecast.
Can it ever be too cold for snow? Almost. If the air is very cold, the water-vapor content is quite low: Cold air delivers less precipitation because it's dry. Still, it can never really be too cold to snow. The coldest place in the world, the South Pole, gets about 10 to 20 inches of snow each year.
Snow has been a focus of study for centuries. In the sixteenth century, Swedish historian Olaus Magnus was the first to study individual snow crystals. Later French philosopher/mathematician René Descartes and even German astronomer Johannes Kepler studied the shapes and sizes of snowflakes. But not until the turn of the twentieth century did William Bentley put together a detailed catalogue of snowflake forms. Bentley was an American farmer who was completely fascinated by snowflakes. From age 20, he dedicated 40 years of his life to building a collection of 6,000 photographs of snow crystals—all different. An infinite variety of crystal shapes exists. Two snowflakes can look alike, but there is no shortage of different forms. Temperature and humidity determine the shape of snowflakes, along with the nature of the nuclei around which the crystals form.
Sleet and Hail
Sleet is frozen rain and occurs during the winter. As rain falls into a cold layer near the ground, it sometimes will refreeze into ice pellets.
These two elements are confused more than any other precipitation types. Both are pieces of ice, but there are major differences between the two. If rain freezes into pellets while falling to the ground, those pellets are called sleet. Sleet occurs in the winter when warm air streams over a cold layer. Rain falls from the warm layer, then the drops pass through a shallow freezing layer that isn't exceptionally cold, so snowflakes are unable to form—just those pellets. During many winter storms, the precipitation will begin as snow, but as the warm air reaches overhead, the snow changes to sleet. As the warmer air becomes even more dominant, the sleet changes to rain. Just before the change from snow to sleet, the snowflakes become thick and heavy.
Sleet is quite different from hail, which is a warm-weather phenomenon. Hail forms when ice slowly descends through a cumulonimbus cloud. Strong updrafts allow layer after layer of ice to build up until the hail becomes so heavy that it overcomes the updrafts and falls to the ground. The hail can be tossed around the clouds as well, between melting and freezing layers, so layers of clear and opaque ice form—something like an onion skin.
When rain freezes on contact with surfaces that are below freezing, it's called "freezing rain." The air may be warm aloft, too warm for snow or sleet. But because of stubborn cold air at ground level, the rain forms a sheet of ice on the ground. A glaze of ice coats roads and walkways. When the ice build-up becomes extensive, widespread power outages occur. The weight of the ice can cause tree limbs to break as well as power lines.
Braving the Elements
In an attempt to control hail, in 1896 Albert Stiger, Austrian wine grower, invented a hail cannon, which consisted of a cast-iron mortar. A metal smokestack was attached to the muzzle to act as a sound box that would increase the noise even more. Stiger's hail cannon became popular throughout the world because of the apparent diminishing of hailstorms during the first season of operations in Austria. By 1900, 7,000 of these devices were in use in northern Italy alone. During that year, 11 deaths and 60 serious injuries were attributed to the hail explosions. By 1902, the concept was discredited after two years of intensive barrages by the Austrian and Italian governments. Both areas experienced disastrous storms during those years of explosive experimentation.
Braving the Elements
During the winter of 1997 and 1998, historic ice storms struck northern New England and Canada. Millions were without power for weeks when a stream of tropical air spread northward while a very shallow pool of cold air hung tough at the ground. Another record-setting ice storm occurred in the Northeast in December 1973. During that storm, the temperature ranged from 20 degrees at ground level to 50 degrees at 5,000 feet. That storm provided the setting for a 1997 film named, you guessed it, The Ice Storm. Kevin Kline and Sigourney Weaver starred in the film.
The single costliest hailstorm in U.S. history occurred in 1990 in Denver, Colorado, where damage came to $600 to $700 million. In 1984 in Munich, Germany, a hailstorm caused $1 billion worth of damage. The largest documented hailstone occurred in Aurora, Nebraska, on June 22, 2003. The hail weighed 1.67 pounds and measured 18.75 in.
Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.