From sci.geo.meteorology Fri Jul 29 11:09:48 1994 Xref: news3.cis.umn.edu k12.ed.science:7248 ba.weather:36722 sci.geo.meteorology:9088 sci.edu:4610 Newsgroups: k12.ed.science,ba.weather,sci.geo.meteorology,sci.edu Path: umn.edu!zip.eecs.umich.edu!yeshua.marcam.com!usc!math.ohio-state.edu!magnus.acs.ohio-state.edu!csn!csus.edu!netcom.com!jnull From: jnull@netcom.com (Jan Null) Subject: WEATHER GUIDE Message-ID: Summary: Guide to National Weather Service Terminology Keywords: Weather Terminology Weather Guide Organization: NETCOM On-line Communication Services (408 261-4700 guest) X-Newsreader: TIN [version 1.2 PL1] Date: Thu, 28 Jul 1994 03:30:08 GMT Lines: 635 WEATHER GUIDE AND GLOSSARY (7-27-94) Note: Posting 2.0 - The original posting included only the glossary portion. The following weather guide has been put together from a variety of sources, including the National Weather Service Operations Manual and the American Meteorological Society's Glossary of Meteorology. It was originally developed as part of a media guide to be used in northern California, so there are several California specific entries. Some of these reflect local policies, which may or may not be the same in other parts of country (USA). Please let me know of any additions, clarifications or corrections that you would like to see in this document. I will try and update it regularly and post it in relevant newsgroups. Feel free to past it along to others who may find it useful. Jan Null e-mail: jnull@netcom.com Lead Forecaster National Weather Service San Francisco, CA PRECIPITATION FORECAST TERMINOLOGY Expressions of Uncertainty: No mention of Precipitation < 20% SLIGHT CHANCE 20% CHANCE 30%,40% GOOD CHANCE 50% LIKELY 60%,70% Unqualified > 70% Expressions of areal coverage: ISOLATED or FEW < 15% WIDELY SCATTERED 15% to 24% SCATTERED 25% to 54% NUMEROUS, WIDESPREAD > 54% Expressions of Intensity: Rainfall Rate: VERY LIGHT, SPRINKLES, NA DRIZZLE, FLURRIES NA LIGHT < .10 inch per hr MODERATE (or no qualifier) 10 to .30 inch per hr HEAVY > .30 inch per hr SKY CONDITION TERMINOLOGY Term: Average "Opaque" Cloud Cover: CLOUDY 9/10 to 10/10 MOSTLY CLOUDY or 7/10 to 9/10 CONSIDERABLE CLOUDINESS PARTLY CLOUDY or 3/10 to 7/10 PARTLY SUNNY MOSTLY CLEAR or 0/10 to 2/10 MOSTLY SUNNY CLEAR or SUNNY < 1/10 FAIR < 4/10, no precipitation, no extremes of visibility, winds or temperature VARIABLE CLOUDINESS: A variation in cloud amountS with respect to time and area. This is used to show an irregular sky condition, when bands of clouds may cross the region. PARTIAL CLEARING: A portion of the sky will show a clearing trend. TEMPERATURE RANGES: LOWER 50S 50, 51, 52, 53, 54 MID 50S 53, 54, 55, 56, 57 UPPER 50S 56, 57, 58, 59 50S 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 NEAR 50 49, 50, 51 WIND TERMINOLOGY: Direction: In reference to true North, to eight points of the compass. Winds are described by the direction they come from. Speed: Generally in miles per hour (mph), except Marine products which are in knots (1 knot = 1.15 mph). Terms: LIGHT < 10 mph BREEZY, BRISK, WINDY 15 to 25 mph WINDY 20 to 30 mph VERY WINDY 30 to 40 mph HIGH WINDS 40 to 60 mph DAMAGING WINDS > 60 mph GUSTY rapid fluctuations of speed of > 10 mph MARINE WARNINGS SMALL CRAFT ADVISORY Forecast winds of 18 to 33 knots; also issued for hazardous sea conditions GALE WARNING Forecast winds of 34 to 47 knots. STORM WARNING Forecast winds of 48 knots or greater. TROPICAL STORM WARNING Forecast winds of 34 to 63 knots associated with a tropical storm. HURRICANE WARNING Forecast winds of 64 knots or greater associated with a hurricane. WIND SPEED CONVERSION: MPH to Kts Knots to MPH Mph Kts Mph Kts Kts Mph Kts Mph 0 0 45 39 0 0 45 52 5 4 50 43 5 6 50 58 10 9 55 48 10 12 55 63 15 13 60 52 15 17 60 69 20 17 65 56 20 23 65 75 25 22 70 61 25 29 70 81 30 26 75 65 30 35 75 87 35 30 80 70 35 40 80 92 40 35 85 74 40 46 85 98 WIND CHILL FACTOR: The combination of wind and cold temperatures can have the same chilling effect to the skin as a colder temperature in still air. This effect is known as the wind chill factor, and is derived using the wind speed and air temperature. The index, provided below, shows the effective cooling on exposed skin. It is effectively the temperature that one feels when bare skin is exposed to a cold wind. When the wind blows across the skin it removes the insulating layers of warm molecules and replaces them with colder molecules. When all factors are the same, the faster the wind blows the greater the heat loss, and the colder we feel. EXAMPLE: Using the chart below, the equivalent temperature if the air temperature was 10 degrees above zero, and the wind speed was 25 mph the wind chill would be -29 degrees. AIR TEMPERATURE ...FAHRENHEIT... 35 30 25 20 15 10 5 0 -5 -10 -15 -20 Calm 35 30 25 20 15 10 5 0 -5 -10 -15 -20 W I 5 33 27 21 16 12 7 1 -6 -11 -15 -20 -26 N D 10 21 16 9 2 -2 -9 -15 -22 -27 -31 -38 -45 S 15 16 11 1 -6 -11 -18 -25 -33 -40 -45 -51 -60 P E 20 13 3 -4 -9 -17 -24 -32 -40 -46 -52 -60 -68 D 25 7 0 -7 -15 -22 -29 -37 -45 -52 -58 -67 -75 M P 30 5 -2 -11 -18 -26 -33 -41 -49 -56 -63 -70 -78 H 35 3 -4 -13 -20 -27 -35 -43 -52 -60 -67 -72 -83 40 1 -4 -15 -22 -29 -36 -45 -54 -62 -69 -76 -87 Wind speed greater than 40 mph have little additional cooling effect. WEATHER GLOSSARY (updated 7-27-94) ACID RAIN: Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (eg. pH < 5.6) ADVISORY: Advisories are issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life-threatening situations. Advisories are issued for significant events that are occurring, are imminent, or have a very high probability of occurrence. AFOS: Automation of Field Operations and Services. Computer system linking NWS offices for the transmission of weather data. AIR MASS: A large body of air having similar horizontal temperature and moisture characteristics. AM: Area Manager. ANEMOMETER: An instrument that measures wind speed. ANTICYCLONE: A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere. ASOS: Automated Surface Observing System. AWIPS: Advanced Weather Information Processing System. New computer system integrating graphics, satellite and radar imagery. Successor to AFOS, due for installation after 1995. BAPSU: Bay Area Public Service Unit. Weather Service Office embedded in the San Francisco Weather Service Forecast Office. BAROMETER: An instrument for measuring atmospheric pressure. BLIZZARD: Snow with winds in excess of 35 mph and visibilities of 1/4 mile or less, for an extended period of time (eg. > 3 hours). BLOWING DUST: Reduction of visibility by strong winds blowing across dry ground with little or no vegetation. Visibilities of 1/8 mile or less over a widespread area are criteria for a Blowing Dust Advisory. BROKEN CLOUDS: Clouds which cover between 6/10 and 9/10 of the sky. CEILING: The height of the lowest layer of clouds, when the sky is broken or overcast. CHANCE: A 30, 40 or 50 percent chance of occurrence of measureable precipitation. CIRRUS CLOUD: High clouds, usually above 16,000 feet, composed of ice crystals. CLEAR: Sky condition of less than 1/10 cloud coverage. CLIMATE: The historical record of average daily and seasonal weather events. COASTAL FORECAST: A forecast of wind, wave and weather conditions between the coastline and 60 miles offshore. COLD FRONT: The boundary between a cold air mass that is advancing and a relatively warmer airmass. Generally characterized by steady precipitation followed by showery precipitation. CUMULONIMBUS CLOUD: A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds. CUMULUS CLOUD: A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower. CYCLONE: An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean. DENSE FOG ADVISORY: Issued when fog reduces visibility to 1/8 mile or less over a widespread area. Most common in California's central valley. See tule fog. DEW: Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature. DEWPOINT: The temperature to which the air must be cooled for water vapor to condense. DIABLO WIND: Similar to Santa Ana winds in southern California. These winds occur below canyons in the East Bay hills (Diablo range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central California coast. DMIC: Deputy Meteorologist-in-Charge. DOPPLER RADAR: A type of weather radar that determines whether the velocity of water droplets is toward or away from the radar. DOWNBURST: A severe localized downdraft from a thunderstorm. DRIZZLE: Small, slowly falling water droplets, with diameters between .2 and .5 millimeters. DUST DEVIL: A small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons. EL NINO: A major warming of the equatorial waters in the Pacific Ocean. El Nino events occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns. ENSO: El Nino-Southern Oscillation. FAIR: Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or winds. FLASH FLOOD: A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure. FLOOD: High flow, overflow or inundation of a normally dry area which causes or threatens damage. FLOOD STAGE: The level of a river or stream at which considerable inundation of surrounding areas will occur. FOG: A cloud whose base is at the earth's surface. FRONT: The transition zone between two distinct airmasses. FROST: The covering of ice that is formed on exposed surfaces whose temperature falls below freezing. FUNNEL CLOUD: A rotating, cone-shaped column of air extending downward from the base of a thunderstorm. When it reaches the ground it is called a tornado. GALE: Wind speeds from 39 to 54 mph (34 to 47 knots). GEOSTATIONARY SATELLITE: A satellite that rotates at the same rate as the earth, remaining over the same spot above the equator. GOES: Geostationary Operational Environmental Satellite GREENHOUSE EFFECT: The warming of the atmosphere by the trapping of longwave radiation being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide. GROUND FOG: Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog, and in California as tule fog. GUST: A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph. HAIL: Precipitation in the form of balls or irregular lumps of ice. HALOS: Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds. HAZE: Fine dry or wet dust or salt particles in the air that reduce visibility. HEAT INDEX: An index that combines air temperature and humidity to give an apparent temperature (eg. how hot it "feels"). HIC: Hydrologist In Charge. HIGH WIND: Sustained winds greater than or equal to 40 mph or gust greater than or equal to 58 mph. HIGH: An anticyclone. HUMIDITY: The amount of water vapor in the atmosphere. (See relative humidity). HURRICANE: A severe tropical cyclone with wind speeds in excess of 74 mph (64 knots). INDIAN SUMMER: An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather. INVERSION: An increase in temperature with height. ISOBAR: A line of equal barometric pressure on a weather map. JET STREAM: Strong winds concentrated within a narrow band in the atmosphere. KNOT: One nautical mile per hour (1.15 mph). LAND BREEZE: A wind that blows from the land towards a body of water. Also known as an offshore breeze. LA NINA: A cooling of the equatorial waters in the Pacific Ocean. LIGHTNING: An electrical discharge from a thunderstorm. LIKELY: In probability of precipitation statements, the equivalent of a 60 or 70 percent chance. LOW: A cyclone. METEOROLOGY: The study of the atmosphere and atmospheric phenomena. MIC: Meteorologist In Charge. MICROBURST: A strong localized downdraft from a thunderstorm. MILLIBAR: A unit of atmospheric pressure. Normal surface pressure is approximately 1013 millibars (29.92 inches of mercury). MONSOON: A persistent seasonal wind, often responsible for seasonal precipitation regime. NCDC: National Climatic Data Center. Located in Asheville, North Carolina, the agency that archives climatic and forecast data from the National Weather Service. NEXRAD: NEXt Generation RADar. A system of Doppler radars planned for nationwide installation between 1992 and 1996. NHC: National Hurricane Center. The office of the National Weather Service in Miami that is responsible for tracking and forecasting tropical cyclones. NMC: National Meteorological Center. Central computer and communications facility of the National Weather Service; located in Washington, DC. NOAA: National Oceanic and Atmospheric Administration. A branch of the US Department of Commerce, NOAA is the parent organization of the National Weather Service. NOAA WEATHER WIRE (NWWS): Mass dissemination via satellite of National Weather Service products to the media and public. NOAA WEATHER RADIO (NWR): Continuous, 24 hour a day VHF broadcasts of weather observations and forecasts directly from National Weather Service offices. A special tone allows certain receivers to alarm when watches or warnings are issued. (San Francisco Bay Area 162.40 MHz, Santa Clara Valley/Monterey Bay 162.55 MHz.) NSSFC: National Severe Storms Forecast Center. Located in Kansas City, this office is responsible for monitoring and forecasting severe convective weather in the continental U.S. This includes the issuance of Tornado and Severe Thunderstorm Watches. NUMERICAL WEATHER PREDICTION (NWP): Forecasting weather by the use of numerical models, run on high speed computers. Most of the NWP for the National Weather Service is done at the National Meteorological Center. NWS: National Weather Service. OCCLUDED FRONT: A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion. OFFSHORE BREEZE: A wind that blows from the land towards a body of water. Also known as a land breeze. OFFSHORE FORECAST: A marine weather forecast for the water between 60 and 250 miles off the coast. OIC: Official In Charge. ONSHORE BREEZE: A wind that blows from a body of water towards the land. Also known as a seabreeze. OROGRAPHIC UPLIFT: The vertical forcing of air by terrain features such a hills or mountains. This can create orographic clouds and/or precipitation. OVERCAST: Sky condition when greater than 9/10 of the sky is covered. OZONE: A form of oxygen containing 3 molecules, usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation. PACIFIC HIGH: A semipermanent anticyclone located in the Eastern North Pacific. PARTLY CLOUDY: Sky condition when between 3/10 and 7/10 of the sky is covered. PRECIPITATION: Liquid or solid water molecules that fall from the atmosphere and reach the ground. PRESSURE: The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure. RADAR: An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. RADIATION FOG: See ground fog. RADIOSONDE: An instrument attached to a weather balloon that transmits pressure, humidity, temperature and winds as it ascends. RAIN: Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle. RELATIVE HUMIDITY: The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage). RFC: River Forecast Center. The California/Nevada River Forecast Center is located in Sacramento. RIDGE: An elongated area of high pressure in the atmosphere. SANTA ANA WINDS: Relatively warm, dry winds that blow into Southern California coastal areas from an anticyclone located over the high deserts of California or Nevada. The warmth and dryness are due to compressional heating. See also Diablo winds. SCATTERED CLOUDS: Sky condition when between 1/10 and 5/10 are covered. SEA BREEZE: A wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. SEVERE THUNDERSTORM: A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4" or more. SHOWER: Precipitation that is intermittent, both in time and space. SLEET: A type of frozen precipitation, consisting of small transparent pellets. SLIGHT CHANCE: In probability of precipitation statements, equivalent to a 20 percent chance. SMALL CRAFT ADVISORY: Winds in excess of 20 knots (23 mph), and less than 34 knots (39 mph), that may cause hazardous conditions for operators of small vessels. SMOG: Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health. SNOW: Frozen precipitation composed of ice particles in complex hexagonal patterns. SNOW ADVISORY: An advisory issued when snow is expected to create hazardous or restricted travel conditions, but not as severe as expected with a winter storm. SNOW FLURRIES: Light snow showers, usually of an intermittent nature. SOUTHERN OSCILLATION: A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events. SQUALL LINE: A non-frontal band, or line, of thunderstorms. STATIONARY FRONT: A transition zone between airmasses, with neither advancing upon the other. STORM: In marine usage, winds 48 knots (55 mph) or greater. STORM SURGE: A rise of the sea, preceding a storm (usually a hurricane) due to the winds of the storm and low atmospheric pressure. SWELLS: Ocean waves of regular and longer duration than wind waves. THERMAL: Small rising columns of air due to surface heating. THUNDER: The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand. THUNDERSTORM: A storm with lightning and thunder, produced by a cumulonimbus cloud. TORNADO: A violenting rotating column of air, in contact with the ground, pendant from a cumulonimbus cloud. A tornado does not require the visible presence of a funnel cloud. TRADE WINDS: Persistent tropical winds that blow from the subtropicial high pressure centers towards the equatorial low. TROPICAL DEPRESSION: Tropical mass of thunderstorms with a cyclonic wind circulation and winds between 20 and 34 knots. TROPICAL DISTURBANCE: An organized mass of tropical thunderstorms, with a slight cyclonic circulation, and winds less than 20 knots. TROPICAL STORM: An organized cyclone in the tropics with wind speed between 35 and 64 knots. TROUGH: An elongated area of low pressure at the surface or aloft. TSUNAMI: An ocean wave generated by a submarine earthquake, volcano or landslide. (Also known as a seismic seawave, and incorrectly as a tidal wave). TULE FOG: Radiation fog in the central valley. It forms during night and morning hours in late fall and winter months following the first significant rainfall. A leading cause of weather related casualties in California. TURBULENCE: Disrupted flow in the atmosphere that produces gusts and eddies. TYPHOON: A hurricane that forms in the Western Pacific Ocean. UPWELLING: The rising of cold water from the deeper areas of the ocean to the surface. VIRGA: Precipitation falling from the base of a cloud and evaporating before it reaches the ground. VISIBILITY: The horizontal distance an observer can see and identify a prominent object. VORTICITY: A measure of the amount of "spin" (or rotation) in the atmosphere. WARM FRONT: A boundary between a warm airmass that is replacing a cooler airmass. WARNING STAGE: The level of a river or stream which may cause minor flooding, and at which concerned interests should take action. WARNING: Forecast issued when a particular hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning). WATCH: Forecast issued well in advance to alert the public of the possibility of a particular hazard (eg. tornado watch, flash flood watch). WATERSPOUT: A column of rotating air over a body of water (ie. a tornado over the water). WIND ADVISORY: Sustained winds 25 to 39 mph and/or gusts to 57 mph. Issuance is normally site specific. However, winds of this magnitude occurring over an area that frequently experiences such winds, for example the normal strong summertime winds near the San Francisco Bay, would not require the issuance of this product. WIND SHEAR: The change of wind speed or direction with distance, usually vertical. WIND VANE: An instrument that determines the direction from which a wind is blowing. WIND WAVES: Short period and irregular waves created by the flow of air over the water. WIND CHILL FACTOR: The amount of cooling one "feels" due to the combination of wind and temperature. WINTER STORM: A heavy snow event. In the Sierra Nevada below 7000 feet, a snow accumulation of >6 inches/12 hrs or >12 inches/24 hrs; and above 7000 feet, >8 inches/12 hrs or >18 inches/24 hrs. WSFO: Weather Service Forecast Office. The Northern and Central California WSFO is located in Redwood City. WSO: Weather Service Office. The Northern and Central California WSOs are located in Eureka, Redding, Sacramento, Stockton, Fresno, Bakersfield and Santa Maria. -- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + Jan Null + email: jnull@netcom.com + + Lead Forecaster + voice: (415) 876-9382 + + National Weather Service + fax : (415) 364-2599 + + San Francisco + "...opinions all mine!" + +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++