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Some Selected Official NWS Definitions

Blizzard, Dew, Drizzle, Dust Devil, Dust Storm, Dust Whirl, Freezing Rain, Fog, Funnel Cloud,  

Gustnado Hail, Haze, Hurricane, Ice Crystals, Ice FogIce Pellets, Ice Storm, Mist,

Precipitation, Rain, SleetSnowSnowboard, Snow Flurries, Snow Grains,

  Snow Pellets, Snow Shower, Snow Squall, Severe Thunderstorm,

 Shelf Cloud, Tropical Cyclone, Tropical Depression, Tropical Disturbance,

Tropical Storm, Tornado, Wall Cloud, Weather Watch, Weather Warning

More NWS Official Definitions 


Blizzard: A winter storm which produces the following conditions for at least 3 hours: 1) sustained winds or frequent gusts to 35 miles per hour or greater 2) considerable falling and/or blowing snow reducing visibility to less than 1/4 mile.


Dew: Water droplets that form upon surfaces on or near the ground when air is cooled toward its dew point.


Drizzle (DZ): Fairly uniform precipitation composed exclusively of fine drops with diameters less than 0.02 inch (0.5 mm) which are very close together. Drizzle appears to float while following air currents, although unlike fog droplets, it falls to the ground. The intensity of drizzle is based solely on visibility. The following table shows drizzle intensity versus visibility:
 

Drizzle Intensity versus Visibility

Drizzle Intensity

Visibility

Light

greater than statue mile

Moderate

1/4 to statue mile

Heavy

less 1/4 statue mile


Dust Devil: A small, vigorous whirlwind, usually of short duration, rendered visible by dust, sand, and debris picked up from the ground. They range from 10 feet to greater than 100 feet in diameter, and can extend up to 1000 feet above the ground. They form are caused by intense surface heating. This heating causes the air to rapidly rise and thus, a mini low pressure system is formed. They are usually found in desert or dry climatic regions where dust and dirt can be easily lifted. Only rarely do they cause any damage. Wind speeds can reach 30 to 60 mph.


Dust Storm (DS): Severe weather conditions marked by strong winds and dust filled air over an extensive area. Visibility is reduced to mile or less.


Dust Whirl:  A rotating column of air rendered visible by dust. Similar to debris cloud; see also dust devil, gustnado, tornado.


Fog (FG): A visible aggregate of minute water particle (droplets) which are based at the Earth's surface and reduces horizontal visibility to less than 5/8 statue mile, and unlike drizzle, it does not fall to the ground. It occurs most frequently in coastal regions because of the great water vapor content of the air. However, it can occur anywhere. The rapidity with which fog can form makes it especially hazardous. It forms by any atmospheric process that does one of the following: 1) Cools the air to its dew point 2) Raises the dew point to the air temperature. Names given to fog types identify their methods of formation. The principle types are radiational fog, ice fog, advection fog, upslope fog, rain induced fog, and steam fog. These types of fog are called "dense" when the surface visibility is equal to or less than 1/4 miles. A Dense Fog Advisory will be issued when the dense fog becomes widespread.


Freezing Rain or Drizzle (ZR): This occurs when rain or drizzle freezes on surfaces (such as the ground, trees, power lines, motor vehicles, streets, highways, etc.) that have a temperature of 32o F or below. Small accumulations of ice can cause driving and walking difficulties. Meanwhile, heavy accumulations of ice can pull down trees and utility lines. In this situation, it would be called an "Ice Storm".


Funnel Cloud (FC): A condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.  


GustnadoSlang for a gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud


Hail (GR): Precipitation in the form of balls or lumps usually consisting of concentric layers of ice. A thunderstorm is classified as severe when it produces hail 3/4 of an inch or larger in diameter. The following table shows hail size estimates:
 

Hail Size Estimates

Description

Size

Description

Size

Pea

0.25 inch

Golfball

1.75 inches

Dime

0.50 inch

Hen Egg

2.00 inches

Penny

0.75 inch

Tennis Ball

2.50 inches

Nickel 

0.88 inch

Baseball

2.75 inches

Quarter

1.00 inch

Tea Cup

3.00 inches

Half Dollar

1.25 inches

Grapefruit

4.00 inches

Ping Pong Ball

1.50 inches

Softball

4.50 inches


Haze (HZ): A concentration of salt particles or other dry particles not readily classified as dust or other phenomenon. Occurs in stable air usually only a few thousand feet thick, but may extend as high as 15,000 feet. Haze layers often have definite tops above which the visibilities are good. However, the visibility in the haze layer can be very poor.


Hurricane A warm-core tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 kph) or more. The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian.  It has a diameter of 250 to 500 miles and a cyclonic circulation typically extending to near 50,000 feet. It is called a Typhoon in the western Pacific north of the Equator and west of the International Dateline, a Cyclone in the Indian Ocean, and Baguio in the Philippines area.

Saffir-Simpson Hurricane Intensity Scale

Scale  Number (Category)

Central Pressure

Winds (MPH)

Storm Surge (Feet)

  Damage

Millibars

Inches

1

greater than 980

greater than 28.94

74 - 95

4 - 5

Minimal

2

965 - 979

28.50 - 28.91 

96 - 110

6 - 8

Moderate

3

945 - 964

27.91 - 28.47

111 - 130

9 - 12

Extensive

4

920 - 944

27.17 -27.88

131 - 155

13 - 18

Extreme

5

less than 920

less than 27.17

greater than 155

greater than 18

Catastrophic

 

Damage Associated with the Saffir-Simpson Hurricane Intensity Scale
Category Damage Description
1 Damage mainly to trees, shrubbery, and unanchored mobile homes. There is no substantial damage to other structures. Some damage occurs to poorly constructed signs. Low lying coastal roads are inundated. There is some minor damage to piers. Some small craft in exposed anchorages are torn from their moorings.
2 There is considerable damage to shrubbery and to tree foliage, with some trees blown down. Major damage occurs to exposed mobile homes. There is extensive damage to poorly constructed signs and some damage to roofing materials of buildings, windows, and doors. No major destruction occurs to buildings. Coastal roads and low-lying escape routes inland are cutoff by rising water about 2 to 4 hours before the arrival of the hurricane center. There is considerable damage done to piers, and marinas are flooded. Small craft in unprotected anchorages are torn from their moorings. Evacuation of some shoreline residences and of low-lying areas is required.
3 Foliage removed from trees and large trees blown down. Nearly all poorly constructed signs are blown down. There is some damage to roofing materials of buildings, windows, and doors. Some structural damage occurs to small buildings. Mobile homes are destroyed. Serious flooding occurs at the coast and many smaller structures near the coast are damaged by battering waves and floating debris. Low-lying escape routes inland are cut by rising water about 3 to 5 hours before the hurricane center arrives. Flat terrain 5 feet or less above sea-level is flooded up to 8 or more miles inland. Evacuations of low-lying residences within several blocks of the shoreline may be required.
4 Shrubs, trees, and all signs are blown down. There is extensive damage to roofing materials, windows, and doors, with complete failure of roofs on many small residences. Mobile homes are demolished. Flat terrain which 10 feet or less above sea-level is flooded inland as far as 6 miles. The flooding and the battering by waves and floating debris cause major damage to the lower floors of structures near the shore. Low-lying escape routes inland are cut off by rising water about 3 to 5 hours before the arrival of the hurricane center. There is major erosion of beaches. Massive evacuation of all residences within 500 yards of the shore may be required, as well as of single-story residences on low ground within 2 miles of shore.
5 Trees, shrubs, and all signs are blown down. There is considerable damage to roofs of buildings, with very severe and extensive damage to windows and doors. Indeed, complete failure of roofs occur on many residences and industrial buildings. There is extensive shattering of glass in windows and doors. Some buildings are destroyed completely. Small buildings are overturned or blown away, and mobile homes are demolished. There is major damage to lower floors of all structures which are less than 15 feet above sea-level within 1,500 feet of the shore. Low-lying escape routes inland are cut off by rising water about 3 to 5 hours before the arrival of the hurricane center. Massive evacuations of residential areas on low ground within 5 to 10 miles of the shore may be required.

Ice Crystals (IC): A fall of unbranched (snow crystals are branched) ice crystals in the form of needles, columns, or plates. They are also referred to as Diamond Dust.


Ice Fog: Occurs when the temperature is much below freezing and water vapor condenses directly as ice crystals (sublimation). It is a radiational fog and the conditions for its formation are the same as for radiational fog except that the temperature must be cold. It occurs mostly in Arctic regions, but it is not unknown in middle latitudes during the cold season.


Ice Pellets (PL): Precipitation of transparent and translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (5 mm), or less. Ice Pellets bounce when they make contact with the ground. It is sometimes called "Sleet". There are two main types:

          1) Hard grains of ice consisting of frozen raindrops, or largely melted and refrozen snowflakes.

          2) Pellets of snow encased in a thin layer of ice which have formed from the freezing, either of droplets intercepted by the pellets, or of water resulting from the partial melting of the pellets.

The following table shows how ice pellet intensity is determined:
 

Ice Pellet Intensity

Ice Pellet Intensity

Rate-of-fall  in 6-minutes

Rate-of-fall  in one hour

Visual Estimation

Light

less than 0.01 inch

Up to 0.10 inch 

Scattered pellets that do not completely cover an exposed surface regardless of duration. Visibility is not affected.

Moderate

0.01 to 0.03 inches

0.11 to 0.30 inches

Slow accumulation on the ground. Visibility reduced by ice pellets to less than 7 statue miles.

Heavy

more than 0.03 inches

more than 0.30 inch

Rapid accumulation on ground. Visibility reduced by ice pellets to less than 3 statue miles.

Ice Storm: It is usually used to describe occasions when damaging accumulations of ice are expected during freezing rain situations. Significant accumulations of ice pull down trees and utility lines resulting in the loss of power and communications. These accumulations of ice make walking and driving extremely dangerous. Significant ice accumulation are accumulations of 1/4 inch or greater.


Mist (BR): A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than 7 statue miles, but greater than or equal to 5/8 statue miles.


Precipitation: 1) The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc. 2) As used in hydrology, precipitation is the discharge of water, in a liquid or solid state, out of the atmosphere, generally onto a land or water surface. It is the common process by which atmospheric water becomes surface, or subsurface water. The term "precipitation" is also commonly used to designate the quantity of water that is precipitated. Precipitation includes rainfall, snow, hail, and sleet, and is therefore a more general term than rainfall.


Rain (RA): Precipitation, either in the form of drops larger than 0.02 inch (0.5 mm), or smaller drops, which in contrast to drizzle, are widely separated. The following table shows how rainfall intensity is determined:
 

Rainfall Intensity

Rainfall Intensity

Rate-of-fall in 6-minutes

Rate-of-fall in one hour

Visual Estimation

Light

less than 0.01 inch

Up to 0.10 inch 

From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.

Moderate

0.01 to 0.03 inches

0.11 to 0.30 inches

From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.

Heavy

more than  0.03 inches

more than 0.30 inch

Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to the height of several inches is observed over hard surfaces.

Severe Thunderstorm: A thunderstorm that produces either of the following: winds of 58 miles an hour or greater (these speeds can result in structural or tree damage), hail 3/4 of an inch in diameter or larger, or a tornado. Lightning frequency is not a warning criteria for issuing a severe thunderstorm warning. Severe thunderstorms can result in the loss of life and property. They can also produce a tornado with little or no advanced warning. A table of hail sizes can be found in this glossary, under the definition of hail. The following table shows an estimate of winds that meet Severe Thunderstorm criteria:
 

Estimated Winds that Meet Severe Thunderstorm Criteria

Wind Speed Damage
58-72 mph Damage to chimneys and TV antennae; uproots shallow rooted trees and blows down limbs or branches.
73-112 mph Peels surface off roofs; windows broken; mobile homes moved or overturned; moving automobiles pushed off roads
113-157 mph Roofs torn off houses; weak buildings and mobile homes destroyed; large trees snapped or uprooted
158+ mph Severe damage; vehicles lifted off ground

Shelf Cloud:  A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn. It is accompanied by gusty, straight-line winds and is followed by precipitation.


Sleet (PL): Describes solid grains of ice formed by the freezing of raindrops or the refreezing of largely melted snowflakes. These grains usually bounce upon impact with the ground or pavement. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of 1/2 inch or more. See Ice Pellets.


Snow (SN): Precipitation of snow crystals, mostly branched in the form of six-pointed stars. It usually falls steadily for several hours or more. Qualifiers, such as occasional or intermittent, are used when a steady, prolonged (for several hours or more) fall is not expected. Like drizzle, its intensity is based on visibility. The following table shows snow intensity versus visibility:
 

Snow Intensity versus Visibility

Snow Intensity

Visibility

Light

greater than statue mile

Moderate

1/4 to statue mile

Heavy

less 1/4 statue mile


Snowboard A flat, solid, white material, such as painted plywood, approximately two feet square, which is laid on the ground, or snow surface by weather observers to obtain more accurate measurements of snowfall and water content.


Snow Flurries: They are intermittent light snowfalls of short duration (generally light snow showers) with no measurable accumulation.


Snow Grains (SG): Precipitation of very small, white, and opaque grains of ice. They can be distinguished from ice pellets, because ice pellets bounce and snow grains do not bounce at all.


Snow Pellets (GS): Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm).


Snow Shower (SHSN): It is a moderate snowfall of short duration. Some accumulation is possible.


Snow Squalls (SQSN): They are intense, but limited duration, periods of moderate to heavy snowfall. They are accompanied by strong, gusty surface winds, and possibly lightning (generally moderate to heavy snow showers). Snow accumulations may be significant.


Tornado (+FC):  A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel. It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. Tornadoes are classified by the amount of damage that they cause. See Fujita Scale.


Tropical Cyclone: It is a warm-core low pressure system which is non-frontal. It originates over tropical and subtropical waters and a has a organized cyclonic (counter-clockwise) surface wind circulation.


Tropical Depression: Cyclones that have maximum sustained winds of surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 kph) or less. They are either located in the tropics or subtropics. They characteristically have one or more closed isobars. They usually intensify slowly and may dissipate before reaching Tropical Storm intensity.


Tropical Disturbance:  A discrete tropical weather system of apparently organized convection--generally 100 to 300 nautical miles in diameter---originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. In successive stages of intensification, it may be subsequently classified as a tropical wave, tropical depression, tropical storm, or hurricane.


Tropical Storm: It is a warm-core tropical cyclone that has maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 kt (39 mph or 63 kph) to 63 kt (73 mph or 118 kph).  


Wall Cloud: It is formed in a supercell thunderstorm.  A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm.  Even though this cloud is lowering, it remains attached to the rain free cloud base of the thunderstorm. It is usually located south or southwest of the visible precipitation area, and marks the strongest updraft in the thunderstorm. The wall cloud develops as the strong updraft draws in surface moisture from several miles away. Eventually, this updraft will pull air from the rain cooled area of the thunderstorm. Since the rain cooled air is very humid, it will quickly condense in the updraft at a lower altitude than the rain free cloud base. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. "Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is "eyewall".


Warning:  A product issued by NWS local offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning).


WatchAn NWS product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).


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