Know The Terms

General Terminology

Advisory: A Forecast issued by the National Weather Service to highlight conditions that require caution, but are not thought to be immediately life threatening.

Acid Rain: Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic.

Air Pressure: The weight of air pressing down on the earth.  Air pressure can change from place to place, and this causes air to move, flowing from areas of high pressure toward areas of low pressure.

Air Mass: A large body of air that has similar horizontal temperature and moisture characteristics.

Atmosphere: The mass of air surrounding the earth and bound to it more or less permanently by the earth’s gravitational attraction.

Aurora Borealis: Also known as the Northern Lights- The luminous, radiant emission from the upper atmosphere over middle and high latitudes and centered around the earth’s magnetic poles.  These silent fireworks are often seen on clear winter nights in a variety of shapes and colors. 

Back Door Cold Front: A front that moves east to west in direction rather than the normal west to east movement.

Blustery: A descriptive term for gusty winds that accompany cold weather.

Box or Watch Box: A severe thunderstorm or tornado watch.

Cold Front: A narrow transition zone separating advancing colder air from retreating warmer air.  The air behind a cold front is cooler and typically drier than the air it is replacing.

Dense Fog: A fog in which the visibility is less than one-quarter mile.

Dense Fog Advisory: Issued when fog is expected to reduce visibility to ¼ mile or less over a widespread area for at least 3 hours.

Derechoe: A widespread and usually fast moving windstorm associated with convection.  Derechoes include any family of downburst clusters produced by extratropical MCS and can produce damaging straightline winds over areas hundreds of miles long and more than 100 miles across.

Dew Point: The temperature to which air must be cooled for water vapor to condense and form fog or clouds.

Disturbance: A disruption of the atmosphere that usually refers to a low pressure area, cool air, and inclement weather.

Doppler Radar: A type of weather radar that determines whether atmospheric motion is toward or away from the radar.  It determines the intensity of rainfall and uses the Doppler effect to measure the velocity of droplets in the atmosphere.

Drought: Abnormally dry weather in a region over an extended period sufficient to cause a serious hydrological (water cycle) imbalance in the affected area.  This can cause problems such as crop damage and water supply shortage.

Fog: Water that has condensed close to ground level, producing a cloud of very small that reduces visibility to less than one kilometer.

Fogbow: A rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.

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.

Frost Advisory: Issued during the growing season when widespread frost formation is expected over an extensive area.  Surface temperatures are usually in the mid 30’s.

Global Warming: A theory that increased concentrations of greenhouse gases are causing an elevation in the Earth’s surface temperature.

Greenhouse Effect: The warming of the atmosphere by the trapping of longwave radiation (heat) being radiated to space.  The gases most responsible for this effect are water vapor and carbon dioxide.

Ground Fog: Shallow fog (less than 20 feet deep) produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground.  Also known as radiation fog.

Haze: Fine dust or salt particles in the air that reduce visibility.

High Wind Warning: Issued when sustained winds from 40-73 miles per hour are expected for at least 1 hour, or when any wind gusts are expected to reach 58 miles per hour or more.

High Wind Watch: Issued when conditions are favorable for the development of high winds over all of or part of the forecast area but the occurrence is still uncertain.

Indian Summer: An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

January Thaw: A period of mild weather popularly supposed to recur each year in late January.

Lake Effect: The effect of a lake (usually a large one) in modifying the weather near the shore and down wind.  It often refers to the enhanced rain or snow that falls downwind from the lake.

NOAA: National Oceanic and Atmospheric Administration.  A branch of the U.S. department of Commerce, NOAA is a parent organization of the National Weather Service.

NOAA Weather Radio (NWR)- Continuous, 24 hour a day VHF broadcasts of weather observations and forecasts directly from the National Weather Service offices.  A special tone allows certain receivers to alarm when watches or warnings are issued.

NWS: National Weather Service

Shallow Fog: Fog in which the visibility at 6 feet above ground level is 5/8 of a mile or more.

Shear (Wind Shear): Variation in wind speed and/or direction over a short distance.  Shear usually refers to vertical wind shear.

Warning: Forecast issued when a particular weather or flood hazard is “imminent” or already occurring.  A warning is used for conditions posing a threat to life or property.

Watch: Forecast issued well in advance to alert the public of the possibility of a particular weather related hazard.  The occurrence, location, and timing may still be uncertain.

Wind: Air in motion relative to the surface of the Earth.

Wind Advisory: Issued for sustained winds 31-39 miles per hour for at least one hour or any gusts 46-57 miles per hour.  However, winds of this magnitude occurring over an area that frequently experiences such winds would not require the issuance of a wind advisory.

Winter Terminology

Alberta Clipper: A fast moving low-pressure system that forms in western Canada and travels southeastward into the United States.  These storms, which generally bring little precipitation, generally precede an Arctic Air Mass.

Arctic Air: A mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska.

Black Ice: Black ice is a deadly driving hazard defined as patchy ice on roadways or other transportation surfaces that cannot easily be seen. It is often clear (not white) with the black road surface visible underneath. It is most prevalent during the early morning hours, especially after snow melt on the roadways has a chance to refreeze over night when the temperature drops below freezing. Black ice can also form when roadways are slick from rain and temperatures drop below freezing overnight. 

Blizzard: Winds of 35 mph or more with snow and blowing snow reducing visibility to less than 1/4 mile for 3 hours or more.

Blizzard Warning: A dangerous storm with winds that are 35 mph or greater in combination with falling and/or blowing snow that reduces visibility to 1/4 mile or less for a duration of at least 3 hours. Canceling travel plans is advised.

Blowing Snow: Wind driven snow that reduces visibility causing significant drifting.  Blowing snow may be snow that is falling and/or loose snow on the ground that is picked up by the wind.

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 of a significant period of time.

Freeze Warning: Issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless if frost develops or not.

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 Fog: A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0 degrees Celsius, based at the Earth’s surface, which reduces horizontal visibility; also called ice fog.

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: Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects fall below 32 degrees F. As with the term "freeze," this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a "killing frost". Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a thermometer level temperature in the mid-30's.

Hard Freeze: A freeze where vegetation is killed and ground surface is frozen solid.

Heavy Snow: Depending on the region in the U.S., this generally means that 4 or more inches of snow is accumulated in 12 hours or 6 or more inches in 24 hours.

Ice Jam: Long cold spells can cause rivers and lakes to freeze. A rise in the water level or a thaw breaks the ice into large chunks which become jammed at manmade and natural obstructions. Ice jams can act as a dam, resulting in severe flooding. 

Ice Storm: Liquid rain falling and freezing on contact with cold objects creating ice buildups of ¼ inch or more that can cause severe damage.

Polar Air: A mass of very cold, very dry air that forms in polar regions.

Polar Vortex: A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.

Sleet: Raindrops 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 intermittent nature and short duration with no measurable accumulation.

Snow Showers: Snow falling at varying intensities for brief periods of time. Some accumulation is possible. 

Snow Squalls: Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant.

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.

Wind Chill Advisory: Issued for bitter cold wind chills of 20 to 34 below zero (25 to 34 below zero in the northwest portion of the state)

Wind Chill Warning: Issued with wind chills of 35F below zero (40F below zero for far NW portion of Wisconsin) Frostbite is possible when outside for 10 minutes or less.

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

Winter Storm Watch: Winter storm conditions including freezing rain, sleet, and heavy snow are possible within the next 36-48 hours. Continue monitoring the weather forecast.

Winter Storm or Ice Warning: A significant winter storm is occurring or will begin in the next 24 hours. Snowfall rates could be in the range of 6 inches in 12 hours or 8 inches in 24 hours. The combination of snow, sleet, freezing rain and moderate winds will impact travel and outdoor activities. An Ice Storm Warning is issued when mostly freezing rain is expected with ice accumulations of 1/4 inch or more within a 12-hour period. Take necessary precautions - consider canceling travels plans. 

Winter Weather Advisory: There is a high confidence that a hazardous winter event will happen with 3 to 5 inches of snow over a 12 hour period, but the event likely will not meet warning criteria.

Frostbite: Damage to body tissue caused by extreme cold. A wind chill around -20 degrees F could cause frostbite in just 15 minutes or less. Frostbite causes a loss of feeling and a white or pale appearance in extremities such as fingers, toes, ear tips or the tip of the nose. If symptoms are detected, seek medical care immediately!

Hypothermia: A condition that develops when the body temperature drops below 95 degrees F. It is very deadly. Warning signs include uncontrollable shivering, disorientation, slurred speech and drowsiness. Seek medical care immediately!

Overexertion: is dangerous. Cold weather puts an added strain on the heart. Unaccustomed exercise such as shoveling snow or pushing a car can bring on a heart attack or make an existing medical condition worse. 

Pets: They also need extra care when the temperatures fall. They should be brought inside when the temperature reaches 30 degrees F with wind chill. Dogs and cats can get frost bitten ears, nose and feet if left outside during bitter cold weather. Chemicals used to melt snow and ice can also irritate pets' paws - be sure to keep anti-freeze, salt and other poisons away from pets.