Glossary & Terms
A ADIABATIC – changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere. ADVECTION – The horizontal transport of air or atmospheric properties. Commonly used with temperatures, i.e., “warm air advection”, or moisture, i.e., “moisture advection”. ADVISORY – 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 AGL – above ground level. AIR MASS – A large body of air having similar horizontal temperature and moisture characteristics. ALBERTA CLIPPER – A low pressure system that forms over Alberta, Canada, and in the winter, tracks eastward across the northern US states bringing snow to the Northern Plains, Midwest, and East Coast. ALTITUDE – a measure of height; can be synonymous with elevation ALTO – mid-altitude ALTOCUMULUS – Mid-altitude clouds with a cumuliform shape. ALTOSTRATUS – Mid-altitude clouds with a flat sheet-like shape. AMPLIFICATION – Building, or sharpening, of an upper-level high-pressure ridge or low-pressure trough. ANABATIC – wind flowing up an incline, such as up a hillside; upslope wind. ANEMOMETER – An instrument that measures wind speed. ANTICYCLONE – A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere. ARCTIC AIR – a mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska. ARCTIC HIGH – a very cold high pressure that originates over the Arctic Ocean. ASOS – Automated Surface Observing System. Observes sky conditions, temperature and dewpoint, wind direction and speed, and barometric pressure. ATMOSPHERE – the mass of air surrounding the earth and bound to it more or less permanently by the earth’s gravitational attraction. AVALANCHE – a large mass of rapidly moving snow down a steep mountain slope. B BACKING – wind changing direction in a counterclockwise way; opposite of veering BAROCLINICITY – A cold air advection/warm air advection couplet that increases atmospheric instability. On analysis and forecast charts it is the isotherms crossing the height contours. BAROTROPIC – Homogeneous atmosphere in which there are neither fronts nor any thermal advection. BAROMETER – An instrument for measuring atmospheric pressure. BEAUFORT SCALE – a scale that indicates the wind speed using the effect wind has on certain familiar objects. BERMUDA HIGH – A large scale high pressure zone over the western North Atlantic Ocean, sometimes extending onto the East Coast, and most prominent during the summer months BLACK ICE – thin, new ice that forms on freshwater or dew-covered surfaces; it is common on roadways during the fall and early winter and appears “black” because of its transparency. BLIZZARD – A winter storm that lasts for at least three hours and includes blowing or falling snow, winds of at least 35 miles per hour, and visibility of a quarter-mile or less. BREEZY – Sustained winds of 15 to 25 mph. BOMB CYCLONE – A large, intense mid-latitude storm that has low pressure at its center and a variety of associated weather, from blizzards to severe thunderstorms to heavy precipitation. BROKEN CLOUDS – Clouds that cover between 6/10 and 9/10 of the sky. C CAA – Cold Air Advection. The movement of colder air toward a fixed point on the earth’s surface; opposite of WAA CAP – Temperature inversion which prevents convection from occurring. CAPE – Convective Available Potential Energy; a measurement of the available energy a thunderstorm has, and is related to the strength of the buoyancy/updrafts of the storm CEILING – The height of the lowest layer of clouds, when the sky is broken or overcast. CELSIUS – a temperature scale in which zero is the freezing point of water and one hundred is the boiling point. CHANCE (CHC) – A 30, 40 or 50 percent chance of occurrence of measurable precipitation. CHINOOK – A warm, dry wind that blows along the east slopes of the northern Rocky Mountains. CIN – convective inhibition; the amount of energy needed for a stable air parcel to become unstable/convective CIRCULATION – the flow/motion of wind CIRRIFORM – High altitude ice clouds with a very thin wispy appearance. CIRROCUMULUS – Cirrus clouds with vertical development. CIRROSTRATUS – Cirrus clouds with a flat sheetlike appearance. CIRRUS (CI) – High clouds, usually above 18,000 feet, composed of ice crystals. CLIMATE – The historical record of average daily and seasonal weather events. CLOUD CONDENSATION NUCLEI – small particles in the air on which water vapor condenses and forms cloud droplets. CLOUDY – the state of the sky when 9/10ths or more of the sky is covered by clouds. COASTAL FLOODING – Inundation of coastal areas from waves and storm surge. COLD FRONT – The boundary between a cold air mass that is advancing and a relatively warmer air mass. Generally characterized by steady precipitation followed by showery precipitation. CONDENSATION – The process of gas changing to liquid. The process by which water vapor changes into water droplets and clouds. CONDUCTION – the transfer of heat between bodies that are in contact. CONTINENTAL AIR MASS – A dry air mass originating over a large land area. CONTRAIL – condensation trail; pencil-line like clouds formed as a result of excess condensation in the atmosphere emitted from airplanes CONVECTION – the transfer of heat within a gas or liquid by their movement. CONVECTIVE INSTABILITY – Instability caused by very dry air advecting in the mid-levels of the troposphere over a warm and moist lower troposphere. Dynamic lifting causes the mid-levels to cool at a greater rate than the lower troposphere. CONVERGENCE – when wind coming from different directions flows towards a common point CORIOLIS FORCE – a force that comes about because the Earth rotates on an axis; it causes winds to “bend” their flow direction, but does not change the speed of the wind CUMULONIMBUS – A vertically developed cumulus 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. D DAMBREAK FLOODING – Dams may degrade slowly. In these situations, when river forecasts are available, flood warnings and flood statements are used. Dams may also fail catastrophically during a heavy rainfall event or earthquake, resulting in a very dangerous flash flood – type situation. DENSE FOG – A cloud, with its base on the surface, which reduces visibility to 1/4 of a mile or less. DEVELOPING – used to describe a cyclone or anticyclone that is intensifying (see intensify) 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. DIABATIC – a process where a parcel changes state due to temperature differences between the parcel and its environment DISTURBANCE – a disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather. DIVERGENCE – refers to the spreading out of winds. DOPPLER RADAR – A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It uses the Doppler effect to measure the velocity of particles suspended in the atmosphere. DOWNBURST – A strong downdraft of air which induces an outburst of damaging winds on or near the ground. DOWNDRAFT – a small scale vertical wind directed towards the ground; opposite of updraft DOWNWIND – a location farther from where the wind is coming from, relative to a reference point; ex: in a westerly wind, the eastern side of the Rocky Mountains is downwind relative to the western side of the mountains DRIFTING SNOW – An uneven distribution of snowfall/snow depth caused by strong surface winds, but does not reduce surface visibility. DRIZZLE – Fairly uniform precipitation composed exclusively of fine drops very close together. Drizzle appears to float while following air currents; however unlike fog droplets, it falls to the ground. Drizzle drops are too small to disturb appreciably still water puddles. DRY SLOT – Refers to a influx of dry air (especially toward a mid-latitude cyclone) DYNAMICS – Refers to upward forcing caused by jet streak divergence or PDVA 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. DUST STORM – an area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile. E EASTERLY – winds blowing from the east towards the west; prominent winds in the equator/tropics and polar regions of both hemispheres EL NINO – A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years and are characterized by shifts in “normal” weather patterns. ENSO – El Nino – Southern Oscillation. ENTRAINED / ENTRAINMENT – Refers to the drawing in of moisture (or lack of moisture) into a system. Dry air entrainment into the mid-levels of a thunderstorm can enhance the potential for damaging wind gusts. Moisture being entrained into a storm system can enhance precipitation amounts. EQUIVALENT POTENTIAL TEMPERATURE – The temperature of a parcel of air after all moisture and latent heat is condensed out of an air parcel then descended to the 1000 mb level. Also known as Theta-E EVAPORATION – the process of a liquid changing into a vapor or gas. EYEWALL – a distinctive circular band of clouds outlining the center, or “eye”, of a tropical cyclone EXTENDED OUTLOOK – a basic forecast of general weather conditions three to five days in the future. EXTRATROPICAL (CYCLONE) – a cyclone that forms poleward of the tropics; often these systems derive energy from temperature gradients; different from cyclone F FAHRENHEIT – the standard scale used to measure temperature in the United States; in which the freezing point of water is thirty – two degrees and the boiling point is two hundred and twelve degrees. 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. FOEHN – A warm dry wind on the lee side of a mountain range. The heating and drying are due to adiabatic compression as the wind descend downslope. FOG – Water droplets which are suspended in the air near the Earth’s surface and causing reduced visibilities. FORECAST PERIODS – Routine forecasts issued before noon contain three forecast periods: Today (or this Afternoon), Tonight, and the Next Day. Routine forecasts issued after noon contain four forecast periods: Tonight, the Next Day, the Next Night, and the following Day. FREEZE – When the temperature at or near the surface is expected to be 32 or below, during the growing season. Adjectives, such as “killing,’ “severe,” or “hard,” are used when appropriate. A freeze may or may not be accompanied by the formation of frost. FREEZING LEVEL The altitude in the atmosphere where the temperature equals 32F. FREEZING RAIN (DRIZZLE) – Rain (drizzle) which freezes on contact to produce a coating of ice on the ground and other exposed surfaces. FREEZING SPRAY – spray from breaking and splashing waves that has the potential to freeze on contact with a ship FRONT – The transition zone between two distinct airmasses. The basic frontal types are cold fronts, warm fronts and occluded fronts. FROST – 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 are colder. FUJITA SCALE – System developed by Dr. Theodore Fujita to classify tornadoes based on wind damage. Scale is from F0 for weakest to F5 for strongest tornadoes. FUNNEL CLOUD – A rotating column of air forming a pendant from a cumulus/cumulonimbus cloud with circulation not reaching the ground. G GALE – Wind speeds from 39 to 54 mph (34 to 47 knots). GEOSTROPHIC – a balance between the coriolis force and the pressure gradient force; geostrophic winds blow parallel to isobars GLAZE – a layer or coating of ice that is generally smooth and clear, and forms on exposed objects by the freezing of liquid raindrops. GRADIENT – the time rate or spatial rate of change of an atmospheric property. GRAUPEL – the smaller and softer (less frozen outer shell) version of hail GRAVITY – The attraction of two masses to one another. Large masses have higher values of gravitational accelerations than lighter masses. GREENHOUSE EFFECT The warming of the atmosphere by the trapping of earth’s longwave radiation being radiated to space. The gasses 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 parts of California as tule fog. GROWING SEASON – the period of time between the last killing frost of spring and the first killing frost of autumn GUST – A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph. GUST FRONT – The leading edge of the downdraft from a thunderstorm. H HABOOB – a strong sandstorm/duststorm that can lift sand/dust up to 1500m, have a stark leading edge of dust, and can last for minutes to hours. They can be found in the US in the Sonoran Desert, and Great Plains. HAIL – Precipitation of small balls or other pieces of ice (hailstones) falling separately or frozen together in irregular lumps. (Typically associated with thunderstorms and surface temperatures above freezing). 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. HARD FREEZE – freeze where vegetation is killed and the ground surface is frozen solid. HAZE – Fine dry or wet dust or salt particles in the air that reduce visibility. HEAVY SNOW – In the Inland Northwest, mountains above 3000 feet – 8 to 12 inches in 12 hours or 12 – 18 inches or more in 24 hours. For the valleys and Basin below 3000 feet – 4 inches in 12 hours and 6 inches in 24 hours. HEAT INDEX – The HI is the temperature the body feels when the heat and humidity are combined. HEAT LOW – The thermal induced surface low pressure trough that develops during the warm season in the lee of the Cascades. HEAT LIGHTNING – lightning far away that appears to not produce thunder, while the thunder is actually just too far away to be heard HIGH PRESSURE PRESSURE – The center of an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone. HIGH WINDS – Winds of 40 mph or greater lasting for an hour or more , and/or gusts to 58 mph or greater. HORIZONTAL VORTICITY – A rotation of air caused by vertical speed or directional wind shear. HORSE LATITUDES – subtropical regions where anticyclones produce settled weather. 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). HURRICANE HUNTERS – Pilots and scientists from the National Hurricane Center that fly special hurricane – equipped planes into tropical cyclones HYDROLOGIC CYCLE – the composite picture of the interchange of water substance between the earth, the atmosphere and the seas which includes the change of state and vertical and horizontal transport. HYGROMETER – An instrument used to measure humidity. I ICE JAM – an accumulation of broken river ice caught in a narrow channel that frequently produces local floods during a spring break – up. ICE STORM – liquid rain falling and freezing on contact with cold objects creating ice build – ups of 1/4th inch or more that can cause severe damage. INFLOW – Wind speed, in knots, of the average PBL windspeed. INDIAN SUMMER – An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather. INTENSITY – the strength of a system usually measured by wind speed, central pressure, and change in central pressure over time INVERSION – An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere. ISALLOBAR – A line of equal surface pressure change. ISENTROPIC LIFT/DECENT – Lifting or sinking of air along constant potential temperature (theta) surfaces. WAA (especially over shallow frontal boundaries) leads to lift while CAA leads to descent. ISOBAR – A line of equal barometric pressure on a weather map. ISODROSOTHERM – A line of constant dewpoint temperature. ISOHYET – A line of equal rainfall. ISOLATED – Showers covering less than 15 percent of an area. ISOPLETH – A line of a constant meteorological value. ISOTACH – A line of constant wind speed. ISOTHERM – the line of equal temperature denoted on surface weather maps. ITCZ – Inter – tropical Convergence Zone. The region where the northeasterly and southeasterly tradewinds converge, forming an often continuous band of clouds or thunderstorms near the equator. J JET STREAK – A relative maximum of windflow within the jet stream. JET STREAM – Strong winds concentrated within a narrow band in the atmosphere. The jet stream often “steers” surface features such as fronts and low pressure systems. K KATABATIC WIND – Local winds a result of cold, dense air flowing downhill. KELVIN – A ratio temperature scale using 0 as the theoretical coldest temperature. KELVIN HELMHOLTZ CLOUDS – clouds that appear like breaking waves due to rapid changes in wind shear over the height of the cloud KNOT – One nautical mile per hour (1.15 mph). L LAKE EFFECT SNOW – snow that falls downwind of a lake due to upstream cold air blowing over warmer moist air above lakes, which forms clouds downstream that precipitate snow over downstream land LAND BREEZE – A wind that blows from the land towards a body of water. Also known as an offshore breeze. LA NINA – The opposite of El Nino, when a major cooling occurs in the equatorial waters in the Pacific Ocean which is characterized by shifts in “normal” weather patterns. LAPSE RATE The change in temperature with altitude in the atmosphere. LATENT HEAT – the heat energy that must be absorbed when a substance changes from solid to liquid and liquid to gas, and which is released when a gas condenses and a liquid solidifies. LATITUDE – a measurement of a position on the earth relative to the equator; the equator is at 0° latitude, while the North Pole is at 90° (or 90°N), and the South Pole is at- 90° (or 90°S). LONGITUDE – a measurement of a position on the earth relative to East or West of the Prime Meridian; the Prime Meridian is at 0° longitude; Boston, USA is at 71°W, and Beijing, China is at-116°E. LEE TROUGH – Low pressure which develops just east of the Rocky Mountains or Cascades. Often lee troughs become organized into mature cyclones as they develop eastward. LEEWARD – a downwind region relative to a region or location upwind; opposite of windward; ex: in a westerly wind, the eastern side of the Rocky Mountains is the leeward side of the mountain LIFTED INDEX – The environmental temperature at 500 millibars minus the 500 millibar parcel temperature on a Skew – T diagram. Negative LI values are unstable. LIGHTNING – An electrical discharge from a thunderstorm. LIKELY – In probability of precipitation statements, the equivalent of a 60 or 70 percent chance. LOW – The center of an area of low pressure, accompanied by cyclonic and inward wind flow in the northern hemisphere. Also known as a cyclone. M MACROBURST – Large downbursts with 2.5 miles or larger outflow diameter and damaging winds lasting 5 to 20 minutes. Intense macrobursts could cause tornado-force damage. MARINE PUSH – A regional phenomenon where the heat low shifts east across the Pacific Northwest, along an onshore flow of cool, marine air to spill over the Cascades. It is characterized by gusty winds and the potential of convection across the Inland Northwest. MARITIME AIR MASS – Moist air mass originating over the ocean. MESOSCALE – atmospheric phenomena smaller than synoptic scale systems (cyclones), but larger than microscale processes (condensation); ex: thunderstorms, fronts, sea breezes Mesoscale Convective System – A large cluster of thunderstorms and rain. Can be a squall line, multi – cells or a mesoscale convective complex. MESOCYCLONE – The rotating updraft in a supercell thunderstorm METEOROLOGIST – A scientist who studies atmospheric circulation and weather phenomena METEOROLOGY – The study of the atmosphere and atmospheric phenomena. MERIDIONAL – motion in the north/south direction; opposite of zonal MICROSCALE – atmospheric phenomena that occur at scales smaller than 2 kilometers; ex: condensation, downdrafts, breaking waves MICROBURST – Small downbursts, less than 2.5 miles in outflow diameter, with peak winds lasting 2 to 5 minutes. They may induce dangerous wind and downflow wind shears which can affect aircraft performance. MICROWAVE RADIATION – electromagnetic radiation which comprises the highest frequency radio energy. MILLIBAR – A unit of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013 millibars. MIST – very fine water droplets at ground level MIXING DEPTH – The vertical distance the process of convection mixes the air from the surface to aloft. The mixing depth is often the same depth as the PBL. The mixing depth will increase with solar warming of the surface and increased low level wind speed. Could also be in reference to the depth of the “transition zone” between two air masses that are horizontally differentially advecting one over the other. MOISTURE AXIS / RIDGE – : An area of higher moisture values, usually in the form of a ridge of higher dew points at the surface or 850 mb. Low level moisture axes enhance atmospheric instability, which in turn promotes thunderstorm development. Existing storms can intensify by moving into moisture axes. The concept is similar to dewpoint pooling. MONSOON – A persistent seasonal wind, often responsible for seasonal precipitation regime. MOS – Model Output Statistics (usually in reference to NGM model). These are numerical representations of expected weather such as forecasted temperatures and precipitation chances. MOSTLY CLOUDY – Between 7/10 and 9/10 cloud cover. MOSTLY SUNNY (MOSTLY CLEAR) – Between 1/10 and 3/10 cloud cover. N NCEP – National Centers for Environmental Prediction. Central computer and communications facility of the National Weather Service; located in Washington, DC. NEGATIVE TILTED TROUGH – A trough which tilts from the northwest toward the southeast. This situation creates instability since cold temperatures in the mid and upper levels advect over warmer air at the surface. A negative tilt is a sign a trough is mature. NEXRAD – NEXt Generation RADar. A NWS network of about 140 Doppler radars operating nationwide. NIMBOSTRATUS – stratus clouds producing rain NHC – National Hurricane Center. The office of the National Weather Service in Miami that is responsible for tracking and forecasting tropical cyclones. NOAA – National Oceanic and Atmospheric Administration. A branch of the US Department of Commerce, NOAA is the parent organization of the National Weather Service. NOR’EASTER – a winter storm that originates in warm waters near Florida and travels northeast along the East Coast bringing strong northeasterly winds and heavy snow to New England. NOTHERLY (NORTHEASTERLY, NORTHWESTERLY) – winds blowing from the north towards the south (or from the northeast towards the southwest, or from the northwest towards the southeast) NOWCAST – a weather forecast valid for 1-3 hours in the future 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 Centers for Environmental Prediction (NCEP). NWS – National Weather Service. O OCCLUDED FRONT – A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion. OFFSHORE WATERS – Ocean waters from 60 nm to 250 nm. OPEN WAVE – A wave of low pressure that does not have a complete circulation around it; also called a short wave trough. OUTFLOW – Air that flows outward from a thunderstorm. 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. P P3 AIRPLANE – a military style, propeller airplane used by Hurricane Hunters PACIFIC HIGH – A semi permanent anticyclone located in the Eastern North Pacific. PACKAGE – A compilation of analysis and forecast charts and their interpretation. PARTLY CLOUDY (PARTLY SUNNY) – Sky condition when between 3/10 and 7/10 of the sky is covered. Planetary Boundary Layer (PBL) – The lowest level of the atmosphere where friction is an important force and vertical mixing is common. PERMAFROST – a soil layer below the surface of tundra regions that remains frozen permanently. POTENTIAL TEMPERATURE – the temperature a parcel of dry air would have if it was lifted (adiabatically) to 100 kPa POTENTIAL VORTICITY – i think this is too complicated for the glossary but could be an interesting advanced topic for a video ATMOSPHERIC PRESSURE – the downward force of air we feel as a result of the weight of the column of air above the surface of the Earth PRESSURE GRADIENT – Tightening pressure gradient indicates stronger winds. PROBABILITY OF PRECIPITATION – the chance (expressed as a percentage) that at least 0.01 inches of liquid equivalent precipitation (i.e. rain, snow melted to liquid, etc.) will fall at a point over a specific amount of time. PHASING – When two separate short waves come together to form one wave. Also, when upper and lower level features are positioned so that each provides energy to the other, it is said that the features are in phase with one another. 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. PROGRESSIVE FLOW or PROGRESSIVE PATTERN – An upper level flow pattern in which storm systems move along at a fairly regular pace. Q QUASI – GEOSTROPHIC – again, i think this is too complicated for the glossary but could be an interesting advanced topic for a video QPF – quantitative precipitation forecast; a forecast of the total amount of precipitation that will fall at a location over a specified amount of time. R RADAR – An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging) RADIOSONDE – An instrument attached to a weather balloon that measures and transmits pressure, humidity, temperature, and winds as it ascends. RAIN – Precipitation of liquid water particles which, in contrast to drizzle, are widely separated and larger in size. RAINBOW – Optical phenomenon when light is refracted and reflected by moisture in the air into concentric arcs of color. RELATIVE HUMIDITY (RH) – 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). RETROGRADE – To move backward. It relates to a ridge or trough moving from east to west. Typically in the mid-latitude, troughs and ridges move west to east. Retrograde motion is the opposite movement of normal. RIDGE – An elongated area of high pressure in the atmosphere. RING OF FIRE – This refers to thunderstorms and rain across the edges of a high – pressure ridge. If a ridge occurs across the central US in summer, the ring of fire will extend across the west coast, up into Canada and across the east coast. Underneath the main core of the ridge weather will be stable. S SAFFIR – SIMPSON HURRICANE WIND SCALE – Used to translate wind speed into the level of potential property damage a storm with that wind speed can inflict. Ratings between 1 to 5 are used and based only on a hurricane’s maximum sustained wind speed. SCATTERED CLOUDS – Sky conditions when between 1/10 and 5/10 are covered. SCATTERED SHOWERS – Showers covering 25 to 54 percent of an area. SEA BREEZE – A wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. SEA SURFACE TEMPERATURE (SST) – Surface temperature data collected using IR satellite imagery, buoy and ship data. SEASONS – a period of the year characterized by astronomical and climatic characteristics. SEVERE THUNDERSTORM – A thunderstorm with winds of 58 mph or more or hail 3/4 – inch diameter or larger. Structural wind damage may imply the occurrence of a severe thunderstorm. SHEAR (WIND SHEAR) – how a wind’s speed changes over a certain direction; can be in the vertical or horizontal direction SHELF CLOUD – Long, wedge – shaped clouds associated with the gust front. Shelf clouds indicate the downdraft or outflow of a thunderstorm. SHORTWAVE – a progressive wave in a horizontal air flow that leads to lower air pressure and possibly unsettled weather. SHOWER – Precipitation that is intermittent, in space, time, or intensity and is falling from a cumuliform cloud. SLEET – Solid grains of ice that form from the freezing of raindrops or the refreezing of melted snowflakes. These small, transparent ice pellets usually bounce when they hit a hard surface. SLIGHT CHANCE – In probability of precipitation statements, usually equivalent to a 20 percent chance. SMALL CRAFT ADVISORY – an advisory issued by local weather forecast offices for coastal areas that are forecast to receive weather that may be particularly hazardous to small ships and boats; specific criteria of hazardous weather varies by location. SMOG – Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health. SNOW – Unless qualified by such words as ” occasional” or ” intermittent” , a prediction of snow indicates a steady fall of a few hours or more. 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 – Short duration of intermittent light snowfall with little if any accumulations. SOUTHERLY – (SOUTHWESTERLY, SOUTHEASTERLY) SOUTHERN OSCILLATION – A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events. SPRING TIDE – Semi – monthly tide of increased height due to the new or full moon. SQUALL LINE – A line of thunderstorms or squalls extending over several hundred miles. STABLE AIR – air with little or no tendency to rise, that is usually accompanied by clear dry weather. STACKED – A pressure system that shows up in the same general location on every height analysis or forecast chart. This is an indication low pressure is weakening (filling) or a pressure system is stalling. STATIONARY FRONT – A transition zone between air masses, with neither advancing upon the other. STRAIGHT LINE WINDS – Damaging thunderstorm winds produced as air rushes downward from a thunderstorm. Often called gust fronts, downburst, or microburst winds these winds originate from thunderstorm downdrafts and may produce damage similar to that of a tornado. STRATOSPHERE – The layer of the atmosphere above the troposphere, where temperature increases with height. STRATUS – Flat low – level clouds. SUBSIDENCE – Sinking air that is associated with warming air and little cloud formation. SUBTROPICAL JET – The branch of the jet stream that is found in the lower latitudes. SUN DOG – a halo of white/rainbow light around the sun with prominent bright spots on the sides and top of the halo; also called parhelion SUPERCELL – A highly organized thunderstorm with a rotating updraft, known as a mesocyclone. It poses an inordinately high threat to life and property. Often produces large hail, strong winds, and tornadoes. SUPERCOOLED WATER – water that stays in liquid form if undisturbed even though it has been cooled to a temperature below its normal freezing point. SUSTAINED WINDS – The wind speed obtained by averaging the observed values over a one minute period. SYNOPTIC SCALE – a scale for weather phenomena larger than mesoscale; ex: tropical cyclone, extratropical cyclone, anticyclone SWELLS – Ocean waves of regular and longer duration than wind waves. T TEMPERATURE – a measure of the warmth or coldness of an object or substance with reference to a standard value. TEMPEST – a violent, windy storm THERMAL – Small rising column of air due to surface heating. v THERMAL RIDGE or THERMAL TROUGH – A ridge of warmer temperatures or a trough of colder temperature. THERMODYNAMICS – In reference to the (in)stability of the atmosphere. Important thermodynamic information includes lapse rates, CAPE, changes in temperature / moisture with height, and cap strength. THERMOMETER – an instrument used for measuring temperature THUNDER – The sound wave produced as a lightning stroke heats the air causing it to rapidly expand. THUNDERSTORM – A rain shower accompanied by lightning and thunder, and occasionally by strong gusty winds, hail, and or heavy rain. TORNADO – A rotating column of air, usually forming a pendant from a cumulonimbus cloud with the circulation reaching the ground. It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. On a local scale, it is the most destructive of all atmospheric phenomena. TRADE WINDS – Persistent tropical winds that blow from the subtropical high-pressure centers towards the equatorial low. TROPICAL DEPRESSION – Tropical mass of thunderstorms with 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. TROPOSPHERE – The lowest layer of the atmosphere where the temperature decreases with height. Most of earth’s weather occurs in this layer. TROPOPAUSE – the upper limit of the troposphere and the lower limit of the stratosphere 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). TURBULENCE – Disrupted flow in the atmosphere that produces gusts and eddies. TYPHOON – A hurricane that forms in the Western Pacific Ocean. U UPDRAFT – a small scale vertical wind directed upwards; opposite of downdraft UPWELLING – The rising of cold water from the deeper areas of the ocean to the surface. This phenomenon often occurs along the California coast during the summer. UPWIND – a location closer to where the wind is coming from, relative to a reference point; ex: in a westerly wind, the western side of the Rocky Mountains is upwind relative to the eastern side of the mountains UNORGANIZED – a system lacking characteristics needed to induce development (see: developing, intensity) V VEERING WINDS – A clockwise change in wind direction. Veering winds with height are indicative of warm air advection (WAA). VELOCITY – the speed and direction of wind; ex: today’s wind velocity is 10 knots from the southwest VERY WINDY – Sustained winds of 30 to 40 mph. 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” (rotation) and “shear” in the atmosphere. W WALL CLOUD – An isolated lowering of a cloud that is attached to the rain – free base of a thunderstorm, generally to the rear of the visible precipitation area. Wall clouds indicate the updraft of or the inflow to a thunderstorm. WARM FRONT – A boundary between a warm airmass that is replacing a cooler airmass. WARM AIR ADVECTION – WAA. The movement of warmer air toward a fixed point on the earth’s surface; opposite of CAA WARNING – is issued when severe or hazardous weather has already developed and has been reported by spotters or indicated by radar. Warnings are statements of imminent danger and are issued for relatively small areas near and downstream from the severe storm or flood. For example, Tornado Warning, Severe Thunderstorm Warning, Flash Flood Warning, Winter Storm Warning. WATCH – identifies a relatively large area in which hazardous or severe weather might occur. The watch is only an indication of where and when the severe weather probabilities are the highest, and should not be confused with a warning. Examples include Tornado Watch, Severe Thunderstorm Watch, Flash Flood Watch, Winter Storm Watch. WATERSPOUT – A violently rotating column of air usually forming a pendant from a cumulus/cumulonimbus cloud, occurring over a body of water, and having circulation reaching the water. WATER VAPOR – water substance in a gaseous state that comprises one of the most important of all the constituents of the atmosphere. WESTERLY – winds blowing from the west towards the east WIND – air in motion relative to the surface of the earth. WINDWARD – an upwind region relative to a region or location downwind; opposite of leeward; ex: in a westerly wind, the western side of the Rocky Mountains is the windward side of the mountain 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 (e.g., the normal strong summertime winds near the San Francisco Bay, would not require the issuance of this product). WIND CHILL – The apparent temperature that describes the cooling effect on exposed skin by the combination of temperature and wind, expressed as a loss of body heat. An increase in wind speed or decrease in temperature will accelerate the effect. A wind chill factor of 30 degrees or lower on exposed skin will result in frostbite in a short period of time. WIND SHEAR – The change of wind speed or direction with distance or height. WIND VANE – An instrument that determines the direction from which a wind is blowing. WINDY – Sustained winds of 20 to 30 mph. X X – BAND – a frequency band of microwave radiation in which radars operate. Z ZONAL WIND – the wind or wind component along the local parallel of latitude.