Although snow versus sleet seems a straightforward topic at first, it becomes interestingly detailed upon examination. It’s useful to begin with a couple of basic similarities, followed by key differences:
Both snow and sleet are types of precipitation; products of the condensation of atmospheric water vapor. Snow and sleet are forms of water ice, and are produced by winter (cold) weather conditions.
Some rudimentary differences:
Falling sleet is generally translucent; a small, frosty “ball” of ice. Falling snow appears white in color. (“…. (D)ue to diffuse reflection of the whole spectrum of light by the small ice particles.” – Wikipedia)
Sleet is defined as rounded pellets of ice by the U.S. National Weather Service (NWS), which often bounce as they strike hard surfaces. By this definition sleet is not the same as rain and snow mixed, a type of precipitation composed of rain and partially melted snow. Rain and snow mixed is a “softer” form of precipitation, unlike ice pellets. In a few areas outside of the United States, this translucent, “slushy”, mixed precipitation is sometimes described as sleet.
The NWS notes that sleet can accumulate on the ground to a half-inch or more; though a somewhat rare occurrence. (The eastern U.S. is one exception, where collection of an inch or two (2-5 centimeters) is not uncommon.) Pure sleet often has the consistency of sand, and does not adhere to trees and wires. In equal volume, larger, more solid ice pellets will be heavier in weight than snow.
Sleet is also not the same as snow grains or snow pellets (a/k/a/ graupel). The former are smaller, white, ice particles usually less than one millimeter in diameter. They resemble “grains”, and do not bounce upon impact. Snow pellets, which may fall along with sleet, are usually fragile, round- or conical-shaped particles, about two to five millimeters in diameter. They are white in color, and may be described as ice-accreted snow crystals.
Sleet is formed differently in the atmosphere than snowflakes. Ice pellets occur when raindrops or melted snowflakes freeze as they pass through a significant layer of below-freezing air at the earth’s surface. This indicates the presence of a warmer, above-freezing layer in a separate region (higher) above the surface. (Usually between 5,000-10,000 feet.) As the now-melted/liquid droplets descend through the sub-freezing air near the ground, they become icy spheres.
Snow is formed when water vapor in clouds changes directly to ice. (Note: Unless very cold cloud temperatures exist, an aerosol particle or an ice nucleus must exist within, or be in contact with an individual water droplet, to form a nucleus.) As tiny ice crystals, snow rises within a cloud before it falls. With sub-freezing temperatures at all layers, each crystal grows on the rising air in the cloud, to the point where the flakes become heavy enough to fall all the way to the ground. No melting occurs on this journey. (Per above: In the formation of snow pellets, still-liquid droplets (in the cloud) freeze (accrete) upon the surface of the snowflake.)
Icy, bouncy pellets of sleet are in fact quite different from white, crystalline flakes of snow, as we’ve seen. At times stinging sleet can almost feel like “debris from the sky”. But even this might be preferable to another wintry circumstance: Precipitation plus a shallower sub-freezing layer of air at the ground. Many would take battering sleet over the freezing rain that glazes every surface in that situation.
Reference material (linked):
http://en.wikipedia.org/wiki/Ice_pellets
http://www.weather.gov/glossary/
http://en.wikipedia.org/wiki/Snow
http://en.wikipedia.org/wiki/Rain_and_snow_mixed
http://forums.accuweather.com/index.php?showtopic=4710
http://www.isws.illinois.edu/atmos/statecli/Winter/glossary.htm
