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Blizzard

A snowstorm with winds of at least 35 mph, temperatures 20 degrees F or lower over the period of the storm is a plain "blizzard."

A severe blizzard has 45 mph or greater winds, blowing snow and temperatures at 10 degrees F or below.

Derecho

A widespread and long-lived, violent convectively induced windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo. Derechos are usually not associated with a cold front, but a stationary front. A warm weather phenomenon, derechos occur mostly in late spring to summer, especially July, but can occur at any time of the year and occur as frequently at night as in the daylight hours

A severe localized downdraft from a thunderstorm or shower.  This outward burst of cool or colder air creates damaging winds at or near the surface.  Sometimes the damage resembles tornado damage.

Downdraft

A sudden descent of cold or cool air to the ground, usually with precipitation, and associated with a thunderstorm or shower.   Contrast with an updraft.

Flash Flood

A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area.  Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.

Flood

High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.

Flood Plain

Level land that may be submerged by flood waters.

Flood Stage

The level of a river or stream where overflow onto surrounding areas can occur.

Fog

A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles.  It is created when the temperature and the dew point of the air have become the same, nearly the same, and sufficient condensation nuclei are present.

Hurricane / Typhoon / Cyclone / Tropical Storm

The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).

Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called "tropical depressions". (This is not to be confused with the condition mid-latitude people get during a long, cold and gray winter wishing they could be closer to the equator ;-)) Once the tropical cyclone reaches winds of at least 17 m/s they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called: a "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E); a "typhoon" (the Northwest Pacific Ocean west of the dateline); a "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a "severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993).

Note that just the definition of "maximum sustained surface winds" depends upon who is taking the measurements. The World Meteorology Organization guidelines suggest utilizing a 10 min average to get a sustained measurement. Most countries utilize this as the standard. However the National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC) of the USA use a 1 min averaging period to get sustained winds. This difference may provide complications in comparing the statistics from one basin to another as using a smaller averaging period may slightly raise the number of occurrences (Neumann 1993).

Microburst

A severe localized wind blasting down from a thunderstorm.  It covers an area less than 2.5 miles (4 kilometers) in diameter and is of short duration, usually less than 5 minutes.  See downburst.

Thunderstorm

Produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulance, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes.

Tornado

A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth.   It is the most destructive of all storm-scale atmospheric phenomena.  They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachains in the east.

Turbulence

The irregular and instantaneous motions of air that is made up of a number of small eddies that travel in the great air current.  Atmospheric turbulence is caused by random fluctuations in the wind flow.   It can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure.

Twister

A slang term used in the United States for a tornado.

Warning

A forcast issued when sever weather has developed, is already occuring and reported, or is detected on radar.  Warnings state a particular hazard of imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.

Watch

A forcast issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.

Waterspout

A small, weak tornado that is not formed by a storm-scale rotation.  It is generally weaker than a super-cell tornado and is not associated with a wall cloud or mesocyclone.  It may be observed beneath cumulonimbus or towering cumulus clouds and is the water equivalent of a landspout.

Wind

Air that flows in relation to the earth's surface; generally horizontally.  There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts.  Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons or aircraft reports.