Definitions of precipitation and Winter Weather

Rain: Falling drops of water larger than 0.02 inch in diameter. In forecasts, "rain" usually implies that the rain will fall steadily over a period of time. (See "showers" below).

Light rain: Falls at the rate of 0.10 inch or less an hour.

Moderate rain: Falls at the rate of 0.11 to 0.30 inch an hour.

Heavy rain: Falls at the rate of 0.30 inch an hour or more.

Drizzle: Falling drops of water smaller than 0.02 inch in diameter. They appear to float in air currents, but unlike fog, do fall to the ground.

Light drizzle: Drizzle with visibility of more than 5/8 of a mile.

Moderate drizzle: Drizzle with visibility from 5/16 to 5/8 of a mile.

Heavy drizzle: Drizzle with visibility of less than 5/16 of a mile.

Showers: Rain that falls intermittently over a small area. The rain from an individual shower can be heavy or light, but doesn't cover a large area or last more than an hour or so.

Snow: Falling ice composed of crystals in complex hexagonal forms. Snow forms mainly when water vapor turns directly to ice without going through the liquid stage, a process called sublimation.

Snowflakes: Aggregations of snow crystals.

Snow flurries: Light showers of snow that do not cover large areas and do not fall steadily for long periods of time.

Snow grains: Very small snow crystals. The ice equivalent of drizzle.

Snow pellets: White, opaque ice particles that form as ice crystals fall through cloud droplets that are below freezing but still liquid (supercooled). The cloud droplets freeze to the crystals forming a lumpy mass. Scientists call snow pellets "graupel." Such pellets falling from thunderstorms are often called "soft hail."

Sleet: Drops of rain or drizzle that freeze into ice as they fall. They are usually smaller than 0.30 inch in diameter. Official weather observations list sleet as "ice pellets." In some parts of the country "sleet" refers to a mixture of ice pellets and freezing rain.

Freezing rain or drizzle: Falling rain or drizzle that cools below 32F, but does not turn to ice in the air. The water is "supercooled." When the drops hit anything they instantly turn into ice.

Ice storm: A storm with large amounts of freezing rain that coats trees, power lines and roadways with ice. Often the ice is heavy enough to pull down trees and power lines.

Hail: Falling ice in roughly round shapes at least 0.20 inch in diameter. Hail comes from thunderstorms and is larger than sleet. Hailstones form when upward moving air -- updrafts -- in a thunderstorm keep pieces of graupel from falling. Drops of supercooled water hit and freeze to the graupel, causing it to grow. When the balls of ice become too heavy for the updrafts to continue supporting them, they fall as hailstones. Sleet, in contrast, consists of raindrops that freeze on the way down.

Thunderstorm: A rain or snow shower in which there is lightning. Thunder is always caused by lightning. In general, the upward and downward winds, updrafts and downdrafts, in thunderstorms are more violent than those in ordinary showers.

Thundersnow: A thunderstorm with snow instead of rain falling on the ground.

Severe thunderstorm: A thunderstorm with winds of 57 mph or faster or hail more than 3/4 inch in diameter reaching the ground. Severe thunderstorms can also produce tornadoes.

 Source: The USA TODAY Weather Book

Winter weather glossary

National Weather Service Watches, warnings and advisories

  • Blizzard warning: Sustained winds or frequent gusts of 35 mph occurring in combination with considerable falling and/or blowing snow for a period of at least three hours. Visibilities will frequently be reduced to less than one-quarter mile and temperatures will often remain extremely cold in a blizzard.
  • Heavy snow warning: Snow accumulations are expected to approach or exceed six inches in 12 hours but will not be accompanied by significant wind. A heavy snow warning could also be issued if eight inches or more of accumulations are expected in a 24 hour period. In addition, during a heavy snow warning, freezing rain and sleet are not expected.
  • Ice storm warning: A significant coating of ice, one-quarter inch or more, is expected.
  • Wind chill warning: Life-threatening wind chills reach minus 50 or lower. Criteria varies across the nation.
  • Winter storm watch: A significant winter storm may affect your area, but its occurrence, location and timing are still uncertain. A winter storm watch is issued to provide 12 to 36 hours notice of the possibility of severe winter weather. A watch will often be issued when neither the path of a developing winter storm nor the consequences of the weather event are as yet well defined. Ideally, the winter storm watch will eventually be upgraded to a warning when the nature and location of the developing weather event becomes more apparent. A winter storm watch is intended to provide enough lead time so those who need to set plans in motion can do so.
  • Winter storm warning: Issued when hazardous winter weather is occurring, imminent or likely. A warning is used for winter weather conditions posing a threat to life and property. A winter storm warning are usually issued for heavy snow approaching or exceeding six inches, ice accumulations, dangerous wind chills, or a combination of the three. Warnings can be issued for lesser amounts of snow, say 3 to 6 inches, if the snow occurs with strong winds in excess of 20 miles an hour and/or significant sleet or heavy ice accumulations from freezing rain. Expected snow accumulation during a winter storm warning is four inches or more in 12 hours or six inches or more in 24 hours at low, flat areas such as the Plains or South. For mountainous areas less than or equal to 7,000 feet, a snowfall of six inches or more in 12 hours or 10 inches or more in 24 hours would prompt a warning. For elevations greater than 7,000 feet, snowfall of eight inches or more in 12 hours or 12 inches or more in 24 hours would qualify for a warning.

Winter weather definitions

  • Blizzard: Winds of 35 mph or more along with considerable falling and/or blowing snow reducing visibility to less than one-quarter mile for three or more hours. Extremely cold temperatures often are associated with dangerous blizzard conditions, but are not a formal part of the definition. The hazard created by the combination of snow, wind and low visibility significantly increases, however, with temperatures below 20 degrees.
  • Blowing snow: Wind driven snow that reduces visibility to six miles or less causing significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind.
  • Drifting snow: Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility.
  • Flurries: Light snow falling for short durations. No accumulation or just a light dusting is all that is expected.
  • Freeze: Occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time.
  • Freezing rain or drizzle: Occurs when rain or drizzle freezes on surfaces such as trees, cars and roads, forming a coating or glaze of ice. Temperatures above the ground are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact. Even small accumulations of ice can be a significant hazard.
  • Frost: Describes the formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the earth's surface falls below 32 degrees Fahrenheit, but because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with air temperatures in the middle 30s.
  • Graupel: Small pellets of ice created when supercooled water droplets coat, or rime, a snowflake. The pellets are cloudy or white, not clear like sleet, and often are mistaken for hail.
  • Heavy snow: Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours.
  • Ice storm: An ice storm is used to describe occasions when damaging accumulation of ice are expected during a freezing rain situation. Significant accumulations of ice are defined as one-quarter inch or greater. This can cause trees, utility and power lines to fall down causing the loss of power and communication.
  • Sleet:Rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. However, it can accumulate like snow and cause a hazard to motorists. Heavy sleet occurs when a half of an inch of sleet accumulates.
  • Snow showers: Snow falling at varying intensities for brief periods of time. Some accumulation is possible.
  • Snow squalls: Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning.
  • Watch: A watch is used when the risk of a hazardous weather event has increased significantly, but the occurrence, location and timing are still uncertain.
  • Warning/Advisory: These products are issued when a hazardous weather event is occurring, is imminent or has a very high probability of occurrence. A warning is used for conditions posing a threat to life or property. Advisories are for less serious conditions that cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life and property.
  • Whiteout: A condition caused by falling and/or blowing snow that reduces visibility to nothing or zero miles; typically only a few feet. Whiteouts can occur rapidly often blinding motorists and creating chain-reaction crashes involving multiple vehicles. Whiteouts are most frequent during blizzards.
  • Wind chill: The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. As the wind increases, heat is carried away from the body at an accelerated rate, driving down the body temperature. This temperature is the reading the body "feels" given the combination of wind and air temperature. At wind speeds of four mph or less, the wind chill temperature is the same as the actual air temperature.

    (Related graphic: Wind chill chart)