Weather Definitions

Black Ice- thin, new ice that forms on fresh water or dew covered surfaces; it is common on roadways during the fall and early winter and appears “black” because of its transparency.

Blizzard- Includes winter storm conditions of sustained winds or frequent gusts of 35 mph or more that cause major blowing and drifting of snow, reducing visibility to less than one-quarter mile for 3 or more hours. Extremely cold temperatures often are associated with dangerous blizzard conditions.

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.

Brisk- Wind in the range of 15 to 25 mph when the temperature is cold.

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 Drizzle- Drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less, possibly producing a thin coating of ice. Even in small amounts, freezing drizzle may cause traveling problems.

Freezing Rain- Rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact Frost- The formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the exposed surface falls below 32 degrees Fahrenheit and water vapor is deposited as a solid. 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- Liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

Nor’easter- A low-pressure disturbance forming along the South Atlantic coast and moving northeast along the Middle Atlantic and New England coasts to the Atlantic Provinces of Canada. It usually causes strong northeast winds with rain or snow. Also called a Northeaster or Coastal Storm.

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. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub-freezing air near the surface and refreezes.

Snow- Frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice.

Snow Flurries- Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation.

Snow Shower- 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.

Snowburst- Very intense shower of snow, often of short duration, that greatly restricts visibility and produces periods of rapid snow accumulation.

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 additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. The (equivalent) wind chill temperature is the temperature the body “feels” for a certain combination of wind and air temperature.

Wind Chill Factor- The apparent temperature which describes the cooling effect on exposed skin by the combination of temperature and wind, expressed as the loss of body heat. Increased wind speed will accelerate the loss of body heat. The formula to calculate wind chill is: WC=.0817(3.71 V^.5 + 5.81 – .25 v)(T-91.4)+91.4 where V=wind speed in MPH and T=temperature F. See our Wind Chill page

Winter Storm- A heavy snow event. A snow accumulation of more than 6 inches in 12 hrs or more than 12 inches in 24 hrs.

Source: National Weather Service Forecast Office, Taunton, MA, Web Page

See our Winter Storm Awareness Information Page for definitions of winter weather advisories, watches and warnings.

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