Severe weather terminology (United States)
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- Note: See the category "Weather warnings and advisories" (at the end of this article) for articles about other nations' weather terminology.
This article describes severe weather terminology used by the United States National Weather Service (NWS). The NWS defines precise meanings for nearly all its weather terms. This article describes NWS terminology and related NWS weather scales. Some terms may be specific to certain cities or regions. Information about specific weather phenomena (e.g., "tornado") can be found by following the indicated links to separate articles.
Contents |
[edit] Definitions of hazardous weather events
The NWS divides hazardous weather conditions into three types of hazardous weather/hydrologic events:
-
- Severe local storms - These are short-fused, small scale hazardous weather or hydrologic events produced by thunderstorms, including large hail, damaging winds, tornadoes, and flash floods.
- Winter storms - These are weather hazards associated with freezing or frozen precipitation (freezing rain, sleet, snow) or combined effects of winter precipitation and strong winds.
- Other hazards - Weather hazards not directly associated with thunderstorms or winter storms including extreme heat or cold, dense fog, high winds, river flooding and lakeshore flooding.
[edit] Severe local storms
Tornado Watch - Conditions are favorable for the development of severe thunderstorms and tornadoes in and close to the watch area. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for five to eight hours.
Tornado Warning - Strong rotation in a thunderstorm is indicated by Doppler radar or a tornado is sighted by skywarn spotters. These warnings are currently issued on a county by county basis, but will become more localized in 2007 with the development of the polygon warning system.
Severe Thunderstorm Watch - Conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm contains large damaging hail of 3/4 inch (20 mm) diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knots) or greater. A tornado is possible. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for five to eight hours.
Severe Thunderstorm Warning - A severe thunderstorm is indicated by Doppler radar or sighted by skywarn spotters. A severe thunderstorm contains large damaging hail of 3/4 inch (20 mm) diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knots) or greater. These warnings are currently issued on a county by county basis, but will become more localized in 2007 with the development of the polygon warning system.
(Flash) Flood Watch - Conditions are favorable for (flash) flooding in and close to the watch area. These watches are issued by the Weather Forecast Office and are usually issued six to twenty-four hours in advance of expected flood potential. In Canada, a Heavy Rainfall Warning has a similar meaning.
Flash Flood Warning - Flash flooding is occurring, imminent or highly likely. A flash flood is a flood that occurs within 6 hours of excessive rainfall and that poses a threat to life and/or property. Ice jams and dam failures can also cause flash floods. These warnings are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for up to 6 hours.
Flood Warning - General or areal flooding of streets, low-lying areas, urban storm drains, creeks and small streams is occurring, imminent, or highly likely. Flood warnings are issued for flooding that occurs more than 6 hours after the excessive rainfall. These warnings are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for 6 to 12 hours.
Special Marine Warning - A warning to mariners of hazardous thunderstorms or squalls with wind gusts of 34 knots (39 mph or 63 km/h) or more, hail 3/4 inch (2 cm) diameter or larger, or waterspouts.
[edit] Winter storms
Winter Storm Watch - Hazardous winter weather conditions including significant accumulations of snow and/or freezing rain and/or sleet are possible generally within 36 hours. These watches are issued by the Weather Service Forecast Office.
Winter Storm Warning - Hazardous winter weather conditions that pose a threat to life and/or property are occurring, imminent, or highly likely. The generic term, winter storm warning, is used for a combination of two or more of the following winter weather events; heavy snow, freezing rain, sleet and strong winds.
Blizzard Watch - Sustained winds or frequent gusts of 35 mph (56 km/h) or greater, considerable falling and/or blowing snow reducing visibility frequently to 1/4 mile (0.4 km) or less for a period of three hours or more are possible generally within the next 36 hours.
The following event-specific warnings are issued for a single weather hazard:
-
- Blizzard Warning - Sustained winds or frequent gusts of 35 mph (56 km/h) or greater, considerable falling and/or blowing snow reducing visibility frequently to 1/4 mile (0.4 km) or less for a period of three hours or more. There are no temperature criteria in the definition of a blizzard but freezing temperatures and 35 mph (56 km/h) winds will create sub-zero (below -18°C)wind chills.
-
- Heavy Snow Warning - Heavy snowfall amounts are imminent and the criteria for amounts varies significantly over different county warning areas.
-
- Lake Effect Snow Warning - Heavy lake-effect snowfall amounts of generally 6 inches (15 cm) in 12 hours or less or 8 inches (20 cm) in 24 hours or less are imminent or highly likely. Lake-effect snow squalls can significantly reduce visibilities with little notice.
-
- Ice Storm Warning - Heavy ice accumulations are imminent and the criteria for amounts varies over different county warning areas. Accumulations range from 1/4 to 1/2 inch (6 to 12 mm) or more of freezing rain. In Canada, these are known as Freezing Rain Warnings.
-
- Sleet Warning - Heavy sleet accumulations of 2 inches (5 cm) or more in 12 hours or less are imminent.
-
- Wind Chill Warning - Extreme wind chills making it feel very cold, criteria varies significantly over different county warning areas.
Winter Weather Advisory - Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, and may result in a potential threat to life and/or property. The generic term, winter weather advisory, is used for a combination of two or more of the following events; snow, freezing rain or freezing drizzle, sleet, blowing snow.
The following event-specific advisories are issued for these single weather hazards:
-
- Snow Advisory - Moderate snowfall amounts are imminent and the criteria for amounts varies significantly over different county warning areas. They can range from as little as one inch in Florida to up to half a foot out West.
-
- Freezing Rain Advisory - A trace to 1/4 inch (1 - 6 mm) of freezing rain is expected is needed in any county warning area to prompt a freezing rain advisory.
-
- Freezing Drizzle Advisory - A trace to 1/4 inch (1 - 6 mm) of freezing drizzle is expected is needed in any county warning area to prompt a freezing rain advisory.
-
- Snow and Blowing Snow Advisory - Sustained winds or frequent gusts of 25 to 35 mph (40 to 56 km/h) accompanied by falling and blowing snow, occasionally reducing visibility to 1/4 mile (0.4 km) or less.
-
- Blowing Snow Advisory - Widespread blowing snow with winds 25 to 35 mph (40 to 56 km/h), occasionally reducing visibility to 1/4 mile (400 m) or less.
-
- Wind Chill Advisory - Dangerous wind chills making it feel cold, criteria varies significantly over different county warning areas.
[edit] Other hazards
Excessive Heat Warning - Extreme heat index making it feel very hot, typically above 110°F (43°C) for 3 hours or more during the day and at or above 80°F (27°C) at night. Specific criteria varies over different county warning areas.
Heat Advisory - Extreme heat index making it feel hot, typically between 105°F to 110°F (40°C to 43°C) for 3 hours or more during the day and at or above 75°F (24°C) at night. Specific criteria varies over different county warning areas.
High Wind Warning - Sustained winds of 40 mph (65 km/h) or greater for a duration of one hour or longer or frequent gusts to 58 mph (93 km/h) or greater.
Wind Advisory - Sustained winds of 30 mph (48 km/h) or greater or gusts to 45 mph (72 km/h) or greater for a duration of one hour or longer.
Dense Fog Advisory - Widespread dense fog reducing visibility to less than 1/4 mile (0.4 km).
Flood Warning (river flood)- A warning for specific communities or areas along a river where flooding is imminent or occurring. Flood warnings normally give specific crest forecasts.
Freeze Warning - Widespread temperatures at or below 32°F (0°C) during the growing season. A freeze may occur with or without frost. A hard freeze occurs with temperatures below 28°F (-3°C).
Frost Advisory - Widespread frost during the growing season. Frost generally occurs with fair skies and light winds.
Lakeshore Flood Warning - Lakeshore flooding that is occurring or is imminent in the next 12 hours, which poses a serious threat to life and/or property. A Seiche Warning is issued for rapid and large fluctuations in water level in Lake Michigan usually caused by a strong line of thunderstorms moving rapidly southeast across the lake (similar to the sloshing in a bath tub).
Red Flag Warning -- Highly favorable conditions for wildfires, typically for areas under drought conditions with low humildity and high winds.
[edit] Hazardous weather risks
The various weather conditions described above have different levels of risk. The NWS uses a multi-tier system of weather statements to notify the public of threatening weather conditions. These statements are used in conjunction with specific weather phenomenea to convey different levels of risk. In order of increasing risk, these statements are:
Outlook - A hazardous weather outlook is issued daily to indicate that a hazardous weather or hydrologic event may occur in the next several days. The outlook will include information about potential severe thunderstorms , heavy rain or flooding, winter weather, extremes of heat or cold, etc., that may develop over the next 7 days with an emphasis on the first 24 hours of the forecast. It is intended to provide information to those who need considerable lead time to prepare for the event.
Advisory - An advisory is issued when a hazardous weather or hydrologic event is occurring, imminent or likely. Advisories are for less serious conditions than warnings, that cause significant inconvenience and if caution is not exercised, could lead to situations that may threaten life or property. NWS may activate weather spotters in areas effected by advisories to help them better track and analyze the event.
Watch - A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location or timing is still uncertain. It is intended to provide enough lead time so those who need to set their plans in motion can do so. A watch means that hazardous weather is possible. People should have a plan of action in case a storm threatens and they should listen for later information and possible warnings especially when planning travel or outdoor activities. NWS may activate weather spotters in areas affected by watches to help them better track and analyze the event.
Warning - A warning is issued when a hazardous weather or hydrologic event is occurring, imminent or likely. A warning means weather conditions pose a threat to life or property. People in the path of the storm need to take protective action. NWS may activate weather spotters in areas effected by warnings to help them better track and analyze the event.
[edit] Hazardous weather statements
Hazardous weather forecasts are provided to the public using the NOAA Weather Radio All Hazards system and through news media such as television and radio. Some of the most common NWS hazardous weather statements are described in the following table:
| Tornado Watch | Conditions are favorable for the development of severe thunderstorms producing tornadoes in and close to the watch area. Watches are usually in effect for several hours, with 6 hours being the most common. (Also automatically indicates a Severe Thunderstorm Watch) |
| Tornado Warning (TOR) | Tornado is indicated by radar or sighted by storm spotters. The warning will include where the tornado is and what towns will be in its path. (Also automatically indicates a Severe Thunderstorm Warning) |
| Severe Thunderstorm Watch | Conditions are favorable for the development of severe thunderstorms in and close to the watch area. Watches are usually in effect for several hours, with 6 hours being the most common. |
| Severe Thunderstorm Warning (SVR) | Issued when a thunderstorm produces hail 3/4 inch (20 mm) or larger in diameter and/or winds which equal or exceed 58 mph (93 km/h). Severe thunderstorms can result in the loss of life and/or property. Information in this warning includes: where the storm is, what towns will be affected, and the primary threat associated with the storm. Tornadoes can also and do develop in severe thunderstorms without the issuance of a tornado warning. |
| Severe Weather Statement | Issued when the forecaster wants to follow up a warning with important information on the progress of severe weather elements. |
| Flash Flood Watch | Indicates that flash flooding is possible in and close to the watch area. Those in the affected area are urged to be ready to take quick action if a flash flood warning is issued or flooding is observed. |
| Flash Flood Warning | Signifies a dangerous situation where rapid flooding of small rivers, streams, creaks, or urban areas are imminent or already occurring. Very heavy rain that falls in a short time period can lead to flash flooding, depending on local terrain, ground cover, degree of urbanization, degree of man-made changes to river banks, and initial ground or river conditions. |
| Tropical Storm Watch | An announcement for specific coastal areas that tropical storm conditions are possible within 36 hours. |
| Tropical Storm Warning | A warning that sustained winds within the range of 34 to 63 kt (39 to 73 mph or 63 to 117 km/h) associated with a tropical cyclone are expected in a specified coastal area within 24 hours or less. |
| Hurricane Watch | An announcement for specific coastal areas that hurricane conditions are possible within 36 hours. |
| Hurricane Warning | A warning that sustained winds 64 kt (74 mph or 118 km/h) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force. |
[edit] Related weather scales as defined by the NWS
The NWS uses several scales in describing weather events or conditions. Several common scales are described below.
[edit] Hail Diameter Sizes and Updraft Speed needed to create them
| Hailstone size | Measurement in. | Measurement cm. | Updraft Speed mph | Updraft Speed m/s |
| BB | < 1/4 | <0.64 | <24 | <11 |
| pea | 1/4 | 0.64 | 24 | 11 |
| plain M&M | 1/2 | 1.3 | 35 | 16 |
| dime | 7/10 | 1.8 | 38 | 17 |
| penny | 3/4 | 1.9 | 40 | 18 |
| nickel | 7/8 | 2.2 | 46 | 21 |
| quarter | 1.00 | 2.5 | 49 | 22 |
| half dollar | 1 1/4 | 3.2 | 54 | 24 |
| walnut | 1 1/2 | 3.8 | 60 | 27 |
| golf ball | 1 3/4 | 4.4 | 64 | 29 |
| hen egg | 2.00 | 5.1 | 69 | 31 |
| tennis ball | 2 1/2 | 6.4 | 77 | 34 |
| baseball | 2 3/4 | 7.0 | 81 | 36 |
| tea cup | 3 | 7.6 | 84 | 38 |
| grapefruit | 4 | 10.1 | 98 | 44 |
| softball | 4 1/2 | 11.4 | 103 | 46 |
[edit] Beaufort wind scale and other wind terms
Beaufort wind scale
| Beaufort number | Wind speed | Conditions |
| 6 | 25 to 31 mph (40 to 50 km/h) | Large branches in motion; whistling in telephone wires. |
| 7 | 32 to 38 mph (51 to 62 km/h) | Whole trees in motion; inconvenience felt walking against wind. |
| 8 - 9 | 39 to 54 mph (63 to 88 km/h) | Twigs break off trees; wind generally impedes progress. |
| 10 - 11 | 55 to 73 mph (89 to 117 km/h) | Damage to chimneys and TV antennas; pushes over shallow-rooted trees. |
| 12 - 13† | 74 to 112 mph (118 to 181 km/h) | Peels surfaces off roofs; windows broken; mobile homes overturned; moving cars pushed off road. |
| 14 - 15† | 113 to 157 mph (182 to 252 km/h) | Roofs torn off houses; cars lifted off ground. |
†:Beaufort levels above 12 are non-standard in the United States. Instead, the Saffir-Simpson Hurricane Scale (Category 1, Category 2, etc.) is used.
Maritime advisory terms
| Wind speed | Description | Flags | Beaufort number |
| 25 to 38 mph (22 to 33 knots) | Small craft advisory |  |
6 and 7 |
| 39 to 54 mph (37 to 47 knots) | Gale warning |  |
8 and 9 |
| 55 to 73 mph (48 to 63 knots) | Storm warning |  |
10 and 11 |
| over 73 mph (over 64 knots) | Hurricane warning |  |
12 |
Other wind terms
| Sustained Wind Speed | Descriptive Term |
| 0 to 5 mph (0 to 8 km/h) | light or light and variable |
| 5 to 15 mph (8 to 24 km/h) | none used |
| 10 to 20 mph (16 to 32 km/h) | none used |
| 15 to 25 mph (24 to 40 km/h) | breezy (mild weather) or brisk,
blustery (cold weather) |
| 20 to 30 mph (32 to 48 km/h) or 25 to 35 mph (40 to 56 km/h) | windy |
| 30 to 40 mph (48 to 64 km/h) or 35 to 45 mph (56 to 72 km/h) | very windy |
| 40 to 73 mph (64 to 117 km/h) | high, strong, damaging or
dangerous winds |
[edit] Fujita tornado intensity scale
The Fujita scale, developed by Dr. Theodore Fujita, assigns a numerical rating from F0 to F5 to rate the damage intensity of tornadoes. F0 and F1 tornadoes are considered "weak" tornadoes, F2 and F3 are classified as "strong" tornadoes, where F4 and F5 are categorized as "violent" tornadoes. The F scale is based on tornado damage (primarily to buildings), so there is some ambiguity in the scale, but nonetheless, it provides a good baseline for classifying tornadoes according to their intensity.
The National Weather Service adopted a new Fujita Scale (The Enhanced Fujita Scale) and went into effect on February 1, 2007.
| F number | Wind speed | Damage |
| 0 | < 73 mph (<118 km/h) | Light damage. Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards. |
| 1 | 73 to 112 (118 to 181 km/h) | Moderate damage. The lower limit is the beginning of hurricane-force wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off roads. |
| 2 | 113 to 157 (182 to 252 km/h) | Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated; Cars lifted off ground. |
| 3 | 158 to 206 (253 to 331 km/h) | Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown. |
| 4 | 207 to 260 (332 to 418 km/h) | Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated. |
| 5 | 261 to 318 (419 to 510 km/h) | Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile-sized missiles fly through the air in excess of 100 meters (109 yd); trees debarked; incredible phenomena will occur. |
| 6 | 319 to 379 (511 to 609 km/h) | Inconceivable damage. It is believed that tornadoes of this intensity are physically impossible. It is said that the damage of an F6 tornado would not be recognizable as caused by an F6 wind because of the outer F4 and F5 winds that would surround the tornado. Therefore, in theory, there is no such thing as an F6 tornado, which is why tornadoes are operationally assessed only on the F0 to F5 scale However it is said that the Oklahoma City Tornadoes of 1999 reached winds over 320 mph making some meteorologists believe that this tornado was stronger then what the F-5 is however still a F-6 is impossible. |
[edit] TORRO tornado intensity scale
The TORRO tornado intensity scale (or T-Scale) is a scale measuring tornado intensity between T0 and T10. It was developed by Terence Meaden of the Tornado and Storm Research Organization (TORRO), a meteorological organization in the United Kingdom, as an extension of the Beaufort scale.
| TORRO Intensity | Wind Speeds | Tornado Description | Damage |
| FC | none | Funnel Cloud | No damage except to tops of extremely tall towers , balloons, and aircraft. Generally no damage on land, except to the tops of very large trees. |
| T0 | 39 - 54 mph (63 - 87 km/h) | Light | Loose, light weight object spiraled in air from the ground; twigs snapped and visible path through crops. |
| T1 | 55 - 72 mph (88 - 116 km/h) | Mild | Things like light chairs and potted plants and other slightly heavier objects become airborne. Dislodging of tiles on roofs becomes more evident. Minor damage to bushes and trees; wooden fences can be flattened. |
| T2 | 73 - 92 mph (117 - 148 km/h) | Moderate | Heavy mobile homes displaced, light caravans blown over, garden sheds destroyed, garage roofs torn away, much damage to tiled roofs and chimney stacks. General damage to trees, some big branches twisted or snapped off, small trees uprooted. |
| T3 | 93 - 114 mph (150 - 183 km/h) | Strong | Mobile homes heavily damage or possibly over turned. Light caravans may be destroyed; Garages and weaker outbuildings are destroyed as well; House roof timbers are thoroughly exposed; Bigger trees uprooted or destroyed |
| T4 | 115 - 136 mph (185 - 219 km/h) | Severe | Cars lifted from ground; Mobile homes become airborne and are destroyed; roofs lifted from some houses; roof timbers of stronger brick and stone houses are completely exposed; many trees uprooted or snapped in half. |
| T5 | 137 - 160 mph (220 - 257 km/h) | Intense | Heavy cars lifted into the air; more serious house damage than that of a T4, but walls usually remain standing; some of the oldest and weakest building may collapse completely. |
| T6 | 161 - 186 mph (259 - 299 km/h) | Moderately Devastating | Well built houses lose roofs completely and possibly one or two walls; many slightly weaker built building collapse completely. |
| T7 | 187 - 212 mph (301 - 341 km/h) | Strongly Devastating | Wooden framed houses completely destroyed; some walls built of stone or brick beaten down or collapsed; steel framed warehouse-type constructions may buckle slightly; cars thrown through the air; noticeable debarking of trees caused by flying debris. |
| T8 | 213 - 240 mph (343 - 386 km/h) | Severely Devastating | Cars hurled great distances; Wood framed houses and object inside are spread out across long distances; irreplaceable damage to stone or brick houses; steel framed building buckled. |
| T9 | 241 - 269 mph (388 - 433 km/h) | Intensely Devastating | Numerous steel framed buildings are damaged moderately; cars and trains hurled long distances; complete and thorough de-barking of any standing tree trunks. |
| T10 | 270 - 299 mph (434 - 481 km/h) | Super | Entire framed houses and other similar building lifted bodily from the foundation and hurled great distances; steel-reinforced buildings and such are severely damaged. |
[edit] Saffir-Simpson hurricane category scale
Category |
Sustained winds | 33–42 m/s | 74–95 mph | 64–82 kt | 119–153 km/h |
|---|---|---|---|---|---|
| Storm surge | 4–5 ft | 1.2–1.5 m | |||
| Central pressure | 28.94 inHg | 980 mbar | |||
| Potential damage | No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal flooding and minor pier damage.[1] | ||||
| Example storms | Bess (1974) – Jerry (1989) – Ismael (1995) – Danny (1997) – Gaston (2004) | ||||
Category |
Sustained winds | 43–49 m/s | 96–110 mph | 83–95 kt | 154–177 km/h |
| Storm surge | 6–8 ft | 1.8–2.4 m | |||
| Central Pressure | 28.50–28.91 inHg | 965–979 mbar | |||
| Potential damage | Some roofing material, door, and window damage. Considerable damage to vegetation, mobile homes, etc. Flooding damages piers and small craft in unprotected anchorages may break their moorings.[1] | ||||
| Example storms | Carol (1954) – Diana (1990) – Erin (1995) – Marty (2003) – Juan (2003) | ||||
Category |
Sustained winds | 50–58 m/s | 111–130 mph | 96–113 kt | 178–209 km/h |
| Storm surge | 9–12 ft | 2.7–3.7 m | |||
| Central pressure | 27.91–28.47 inHg | 945–964 mbar | |||
| Potential damage | Some structural damage to small residences and utility buildings, with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland.[1] | ||||
| Example storms | Alma (1966) – Alicia (1983) – Roxanne (1995) – Fran (1996) – Isidore (2002) | ||||
Category |
Sustained winds | 59–69 m/s | 131–155 mph | 114–135 kt | 210–249 km/h |
| Storm surge | 13–18 ft | 4.0–5.5 m | |||
| Central pressure | 27.17–27.88 inHg | 920–944 mbar | |||
| Potential damage | More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Terrain may be flooded well inland.[1] | ||||
| Example storms | "Galveston" (1900) – Hazel (1954) – Iniki (1992) – Iris (2001) – Charley (2004) | ||||
Category |
Sustained winds | ≥70 m/s | ≥156 mph | ≥136 kt | ≥250 km/h |
| Storm surge | ≥19 ft | ≥5.5 m | |||
| Central pressure | <27.17 inHg | <920 mbar | |||
| Potential damage | Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required.[1] | ||||
| Example storms | "Labor Day" (1935) – "Mexico" (1959) – Camille (1969) – Gilbert (1988) – Andrew (1992) | ||||
[edit] Precipitation probabilities
Technically, the probability of precipitation (PoP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount (.01 inch/0.2 mm or more) of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Forecasts are normally issued for 12 hour time periods. Descriptive terms for uncertainty and areal coverage are used as follows:
| PoP | Expressions of Uncertainty | Areal Coverage |
| 0% | none used | none used |
| 10% | none used | isolated |
| 20% | slight chance | isolated |
| 30 to 50% | chance | scattered |
| 60 to 70% | likely | numerous |
| 80 to 100% | definite | none used |
- The following terms of duration imply a high probability (80 to 100%) of precipitation occurrence:
-
- brief
- periods of
- occasional
- intermittent
- frequent.
-
[edit] Sky condition
| Term | Predominant or Average Sky Condition |
| Cloudy | 95 to 100% opaque cloud cover |
| Mostly Cloudy or Considerable Cloudiness | 65 to 95% opaque cloud cover |
| Partly Sunny | 50 to 65% opaque cloud cover |
| Partly Cloudy | 25 to 50% opaque cloud cover |
| Mostly Clear or Mostly Sunny | 9 to 25% opaque cloud cover |
| Clear or Sunny | 3 to 9% opaque cloud cover |
| Abundant Sunshine | 0% opaque cloud cover (No clouds visible) |
| Fair | Less than 40% opaque cloud cover, no precipitation and
no extremes of temperature, visibility, or wind. |
[edit] General weather terms as defined by the NWS
Air stagnation - A condition in which an air mass remains in one place for a long period of time, allowing pollutants to accumulate in the air.
Cold air funnel - A funnel cloud or (rarely) a small relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold. They are much less violent than other types of tornadoes.
Crest - The highest level of a flood wave as it passes a point.
Degree Day - Gauges the amount of heating or cooling needed for a building using 65 °F as a baseline. To compute degree days, the average temperature for a day is taken and referenced to 65 °F. An average temperature of 50 °F yields 15 heating degree days, while an average temperature of 75 °F would yield 10 cooling degree days.
Dew Point - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming pressure and moisture content are constant).
Doppler Radar - Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar).
Downburst - A strong downdraft from a thunderstorm resulting in an outward burst of damaging winds on or near the ground. Downburst winds are often 50 to 100 mph (80 to 160 km/h) and in a few cases, 100 to 150 mph (160 to 240 km/h). They can do as much damage as a small tornado.
Microburst - a small downburst affecting an area less than 2 1/2 miles (4 km) in diameter with peak winds lasting generally less than five minutes.
Macroburst - a large downburst affecting an area greater than 2 1/2 miles (4 km) in diameter with peak winds generally lasting five minutes or longer.
Flood Stage - The level or stage at which a stream overflows its banks or the stage at which the overflow of a stream begins to cause damage.
Freezing Rain/Freezing Drizzle - Rain or drizzle which falls in liquid form and freezes on impact with cold surfaces to form a glaze on the ground and exposed objects.
Funnel Cloud - A rotating column of water vapor or air, extending from a towering cumulus or cumulonimbus cloud, that is not in contact with the ground.
Gustnado - A gust front tornado. A small, weak, short-lived tornado that occurs along a gust front - the leading edge of a thunderstorm, caused by rain-cooled air flowing out from the storm's downdraft. A gustnado is often visible as a debris cloud or dust whirl.
Hail - Precipitation in the form of lumps of ice that form during some thunderstorms.
Heat Index - The apparent temperature that describes the combined effect of high temperatures and high levels of humidity, which reduces the body's ability to cool itself.
Lake Breeze - A local wind blowing from the cooler surface of a large lake to warmer inland areas, usually during the afternoon in spring and summer.
Lake Effect Snow - Snow that occurs to the lee of the Great Lakes when cold air moves across relatively warm waters of the lakes.
Rain/Showers - Rain is a nearly steady and uniform fall of precipitation over an area. Showers are intermittent and/or scattered convective rainfall of varying intensity.
Seiche - An oscillation of the surface of one of the Great Lakes usually caused by a squall line moving rapidly south-southeast down the lake. Rapid changes in the lake level pose a serious threat to marinas and people on piers and breakwaters. Similar to a storm surge.
Relative Humidity - The ratio of the amount of water vapor actually present in the air to the greatest amount possible at the same temperature.
Sleet - Sleet or ice pellets are solid grains of ice formed from the freezing of rain or the refreezing of melted snow, which bounce off the ground and other objects.
Squall line - A line of thunderstorms or squalls which may extend over several hundred miles.
Tornado - A violently rotating column of air in contact with both a thunder cloud and the ground.
Virga- Precipitation that drys up in the atomosphere and therefore does not reach the ground.
Waterspout - In general, a tornado occurring over water. Specifically, it refers to a small, relatively weak rotating column of air over water beneath a towering cumulus or cumulonimbus cloud. A waterspout may rotate counter-clockwise or clockwise.
Wind Chill - An apparent temperature that describes the combined effect of wind and low temperature on exposed skin.
Snow shower - A snow shower is a short duration of moderate to possibly heavy snowfall. Some accumulation is possible. Snow showers are usually localized.
[edit] See also
- NOAA Weather Radio All Hazards
- Emergency Alert System
- National Weather Service
- Severe weather terminology (Canada)
