Acid Rain or Fog- Cloud droplets or raindrops combining with gaseous pollutants to make them acidic (pH less than 5.6).
Accretion- The growth of precipitation particles by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon contact.
Advection- The horizontal movement of a large body of air that has the same properties such as temperature or moisture.
Advection fog- Warm, moist air moving over a colder, drier surface, cooling to its dewpoint, and creating fog.
Advisory- A statement issued by the National Weather Service that indicates problematic weather is possible. Advisories do not indicate the dangerous weather coming with weather warnings.
Air mass- A large body of air with the same or similar properties such as temperature or moisture.
Air mass thunderstorm- A thunderstorm that is created through rising air, without the help of a frontal boundary. These storms are typical during hot summer days in the afternoons and early evenings. They lose their strength after sunset as the heat decreases.
Air pressure- The pressure exerted by the weight of air above a given point. Usually expressed in millibars (mb) or inches of mercury (Hg). The International Standards of Measurement now uses hectopascals (hPa) which also equal millibars.
Albedo- The percent of radiation returning from a surface compared to that which strikes it.
Altimeter- An instrument that indicates the altitude of an object above a fixed level. Similar to a barometer but scaled for altitude rather than pressure.
Altocumulus- A middle cloud, usually white or gray that often occurs in layers or patches with wavy, rounded masses or rolls.
Altostratus- A middle cloud composed of gray or bluish sheets or layers of uniform appearance. Sometimes the sun or moon appears dimly visible.
American Meteorological Society- The organization, founded in 1919, that promotes the education and professionalism of the weather industry. The AMS publishes nine atmospheric and related oceanic and hydrologic journals a year. The AMS has more than eleven thousand members.
Amplified flow- See "Meridional flow".
Anemometer- An instrument designed to measure wind speed. A wind vane measures direction. An aerovane measure both wind speed and direction.
Aneroid barometer- An instrument designed to measure atmospheric pressure containing no liquid. It uses a specially designed cell that expands and contracts based on the air pressure.
Anticyclonic- A high pressure area with a clockwise circulation of air in the northern hemisphere.
Arid climate- An extremely dry climate often referred to as a "true desert" climate.
Atmosphere- The envelope of gases (and sometimes liquids and solids) that surround a planet and are held to it by the planet's gravitational attraction. The earth's atmosphere is mainly nitrogen (78%) and oxygen (21%).
Atmospheric Greenhouse Effect- The warming of an atmosphere by its absorbing and reemitting infrared radiation while allowing shortwave radiation to pass on through. The gases mainly responsible for the earth's atmosphere effect are water vapor and carbon dioxide.
Atmospheric models- Simulation of the atmosphere's behavior by mathematical equations or by physical models.
Aurora- Glowing light display in the nighttime sky caused by excited gases in the upper atmosphere giving off light. In the Northern Hemisphere they are known as the aurora borealis and in the Southern Hemisphere as the aurora Australis.
Autumnal (Fall) Equinox- Occurs when the sun approaches the Southern Hemisphere and passes directly over the equator. Usually occurs around September 23rd.
Back-door cold front- A cold front moving south or southwest along the Atlantic seaboard of the United States.
Backing wind- A wind that changes direction in a counter-clockwise fashion (e.g., north to northwest to west).
Backscatter- The radiation that is scattered or reflected at 180 degrees to the direction of the radar beam.
Ball lightning- A rare form of lightning that may consist of a reddish, luminous ball of electricity or charged air.
Barometer- An instrument that measures atmospheric pressure. The three most common barometers are the mercury, aneroid and most recently the electronic. A barograph is a recording barometer.
Bermuda High- A semipermanent high in the subtropical high-pressure belt centered near 30 degrees north latitude in the western Atlantic Ocean off the east coast of North America.
Black ice- See clear ice, below.
Blizzard- A severe weather condition characterized by low temperatures and strong winds (over 32 mph) bearing a great amount of snow. If the snow is not falling, but the winds create a large amount of blowing snow, it is known as a ground blizzard.
Boundaries- Zones in the lower atmosphere characterized by sharp gradients of temperature, pressure or moisture and often having converging winds. Examples include surface fronts, dry lines and outflow boundaries. Thunderstorms tend to develop along these zones, especially at the intersection of two or more boundaries.
Boundary layer- See friction layer
BWER/WER/LEWP- Bounded Weak Echo Region/Weak Echo Region/Line Echo Wave Pattern: weather radar terms that indicate thunderstorms and, possibly, severe weather.
Ceiling- The height of the lowest layer of clouds when the sky is more than 6/10ths covered. A ceilometer is the instrument that measures and records cloud heights.
Celsius or Centigrade scale- A temperature scale where Zero is assigned to the temperature where water freezes and 100 to the temperature where water boils at sea level. Designated by a "C".
Chinook wind- A warm, dry wind on the eastern side of the Rocky Mountains. In the Alps, this wind is known as a Foehn.
Cirrocumulus- a high cloud that appears as a white patch without shadows and consists of very small elements in the form of grains or ripples.
Cirrostratus- a high cloud appearing as a whitish veil that may totally cover the sky. Often produces the halo phenomena.
Cirrus- A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.
Clear air turbulence (CAT)- Turbulence encountered by aircraft flying through cloudless skies. Usually produced by thermals, wind shear and jet streams.
Clear ice- a layer of ice that appears transparent because of its homogeneous structure and small number and size of air pockets. Also known on dark surfaces as black ice.
Climate- The accumulation of daily and seasonal weather events over a long period of time, usually 30 years or more.
Cloudburst- Any sudden and heavy rain shower.
Cloud line- A narrow band in which individual cloud elements are connected. Usually, the line is less than one degree of latitude in length. A cloud line often indicates strong winds, usually greater over larger bodies of water.
Cloud seeding- The introduction of artificial substances (usually silver iodide or dry iceZ) into a cloud for the purpose of either modifying its development or increasing its precipitation.
Cloud shield- A broad cloud formation that is not more than four times as long as it is wide. Often it is formed by cirrus clouds associated with a ridge or the jet stream.
Cloud streets- A series of aligned cloud elements that are not connected. Several cloud streets usually line up parallel to each other and each street is not more than ten miles wide.
Cold-core low- A low pressure area that intensifies aloft. The movement of a cold-core low tends to be slow and erratic.
Cold pool- An area in the upper atmosphere where the air temperature is colder than other areas. Air is usually very unstable under cold pools and thunderstorms are more likely to develop.
Cold front- The leading edge of a colder air mass. On weather maps it is depicted in blue with sharp barbs showing the direction of movement.
Cold wave- A rapid fall in temperature within 24 hours that often requires increased protection for agriculture, industry, commerce and human activities. Also a minimum of three (3) consecutive days with temperatures approximately 20 degrees below normal and below freezing; this definition varies in different parts of the country and world.
Comma Cloud System- A cloud system that resembles the comma mark as seen on weather satellite images. The shape is made by the upward and downward movement of air as well as the rotation of the cloud.
Comma Head- The round portion of the comma cloud system that produces most of the heavy precipitation.
Comma Tail- The portion of the comma cloud that usually lies to the right of, and often nearly parallel to, the maximum winds.
Condensation- The process by which water vapor (gas) becomes a liquid.
Conduction- The transfer of heat, from warmer to colder regions, by molecular activity from one substance to another, or through a solid substance.
Contour line- A line that connects points of equal elevation above a reference level, most often sea level.
Convection- In meteorology, usually refers to atmospheric motions that are predominantly vertical, such as rising air currents due to surface heating.
Convergence- An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region.
Cooling degree-day- See degree-day below.
Coriolis force- an apparent force observed on any free-moving object in a rotating system. On the earth this deflective force results from the earth's rotation and causes moving particles to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Crepuscular rays- Alternating light and dark bands of light that appear to fan out from the sun's position, usually at twilight.
Cumulonimbus- An exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail and is also know as a thunderstorm cloud or thunderhead.
Cumulus- a cloud in the form of individual, detached domes or towers that are usually dense and well define. It has a flat base with a bulging upper part that often resembles cauliflower. The fair weather version is called cumulus humilis while those with much vertical growth are called cumulus congestus or towering cumulus.
Cut-off low- A cold upper-level low that has become displaced out of the basic westerly flow and usually lies to the south of this flow.
Cyclogenesis- The development or strengthening of middle latitude (extratropical) cyclones.
Cyclone- An area of low pressure around which the winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also, in the western Pacific Ocean, the name given to tropical systems of hurricane wind strength.
Deposition- A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first. Also called sublimation although that better describes the reverse process of a solid changing directly to a gas without becoming a liquid first.
Dew- Water that has condensed as droplets onto objects near the ground when their temperatures have fallen below the dew point of the surface air.
Dew point (Dew-point temperature)- The temperature at which the air is saturated. The dewpoint temperature is a better indicator than relative humidity to show how moist or dry the air is. As the air gets close to being saturated or reaching the dewpoint, fog and/or clouds will start forming. The closer the atmosphere is to being saturated the higher the humidity, while lower humidity indicates drier air. Often can be used as an indicator of the minimum temperature on clear nights.
Diffraction- The bending of light around objects such as cloud and fog droplets, producing fringes of light and dark or colored bands.
Divergence- Descending air diverges or spreads at the surface. Divergence is a downward motion that tends to stabilize the atmosphere.
Doldrums- The region near the equator that is characterized by low pressure and light, shifting winds.
Doppler radar- A radar (RAdio Detection And Ranging) that determines the velocity of falling precipitation either toward or away from the radar unit using the Doppler shift or effect (the change in the frequency of waves that occurs when the emitter or the observer is moving toward or away from the other; often heard in the change in pitch of trains moving towards or away from the observer).
Downburst- A severe localized downdraft beneath a severe thunderstorm. See also Microburst and Macroburst.
Drizzle- Small drops between 0.2 and 0/5 mm in diameter that fall slowly and reduce visibility more than light rain.
Drought- A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities.
Dry climate- A climate deficient in precipitation where annual evaporation and transpiration exceed precipitation.
Dryline- An area where two air masses come together that are very different in moisture even though there is not a big difference in temperature. The boundary between the dry and moist air mass causes lift that often triggers showers and thunderstorms.
Dust devil (whirlwind)- A small but rapidly rotating wind made visible by the dust, sand, and debris it picks up from the surface. It usually develops on clear, dry hot afternoons.
Easterly wave- A migratory wavelike disturbance in the tropical easterlies. Occasionally they intensity into tropical cyclones.
Eddy- A small volume of air or other fluid that behaves differently from the larger flow in which it exists.
El Nino- An extensive ocean warming that begins along the coast of Peru and Ecuador. Major El Nino events occur once every 3 to 7 years. See also "La Nina" and "Southern Oscillation".
Embedded thunderstorm- A thunderstorm that occurs within non-convective precipitation or within an area of rain.
Evaporation- The process by which a liquid changes into a gas.
Evaporation Fog- Fog produced when sufficient water vapor is added to the air by evaporation. The two common types are "steam fog", which forms when cold air moves over warm water, and "frontal fog", which forms as warm raindrops evaporate in a cool air mass.
Exosphere- The outermost portion of the atmosphere.
Extratropical cyclone- A storm that most often forms along a front in middle and high latitudes. Also called a depression, wave or low. Since it does not have tropical characteristics, it is not a tropical storm or hurricane, although some develop "eyes" and winds over 75 mph.
Eye- The area in the center of a hurricane where skies are often mostly clear and winds are light. Sometimes birds will find refuge in the eye and fly with the storm for hundreds of miles. Hurricane eyes can grow and shrink. Sometimes more than one eye can occur simultaneously.
Eyewall- An area or wall around the eye of a hurricane that has intense thunderstorms.
Fahrenheit scale- A temperature scale where 32 is assigned to the temperature where water freezes and 212 to the temperature where water boils at sea level. Designated by an "F".
Fall streaks- Falling ice crystals that evaporate before reaching the ground. Also now know as "virga".
Feeder Bands- Spiralling bands of thunderstorms that surround the center of a tropical storm. A hurricane may have numerous feeder bands that are in advance of the main rain shield and are 40 to 80 miles apart.
Fetch- The distance that the wind travels over open water.
Flash flood- A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area.
Fog- Small water droplets suspended in the atmosphere at or near the surface which reduces visibility to below one km. It is created when the air temperature and the dew point temperature have become the same or nearly the same. In simple terms a cloud at the ground. See also "Advection fog", "Evaporation fog" and "Radiation fog".
Fog Bank- A fairly well-defined fog mass observed in the distance usually at sea or along a coastal area.
Freeze- A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops.
Freezing drizzle- Drizzle that falls in liquid form but freezes on contact with a surface. It can form a glaze of ice on the ground or other surfaces such as power lines.
Freezing fog- Fog that is present when the air temperature is below 32 degrees Fahrenheit/ 0 degrees Celsius.
Freezing Rain- Rain that falls in liquid form but freezes on contact with the ground or a surface. It can form a glaze of ice on the ground or other surfaces such as power lines.
Friction layer- The atmospheric layer near the surface usually extending up to about one km (3300 ft) where the wind is influenced by friction of the earth's surface and objects on it. Also known as boundary layer.
Front- The transition zone between two distinct air masses. See also cold front, occluded front, stationary front and warm front.
Frontogenesis- The formation, strengthening, or regeneration of a front.
Frontolysis- The weakening or dissipation of a front.
Frost- A covering of ice produced by deposition (sublimation) on exposed surfaces when the air temperature falls below the frost point (the dew point when below freezing). Also known as hoarfrost. See also rime.
Frostbite- The partial freezing of exposed parts of the body, causing injury to the skin and sometimes to deeper tissues.
Frozen dew- The transformation of liquid dew into tiny beads of ice when the air temperature drops below freezing.
Fujita scale. A scale developed by T. Theodore Fujita for classifying tornadoes according to the damage they cause and their rotational wind speed. The scale runs from F0 (minor) to F6 (super-catastrophic).
Funnel Cloud- A rotating conelike cloud that extends from the base of a thunderstorm. When it reaches the ground, or kicks up debris, it is called a tornado; over water it is called a waterspout.
Geostationary satellite- A satellite that orbits the earth at the same rate that the earth rotates and thus remains over a fixed place above the equator. The satellite images seen of the United States and nearby locations are primarily from the visible and infrared pictures taken by one Geostationary satellite in the western Atlantic Ocean and one in the eastern Pacific Ocean.
Glory- Colored rings that appear around the shadow of an object.
Graupel- Ice particles between 2 and 5 mm in diameter that form in a cloudy often by the process of accretion. Snowflakes that become rounded pellets due to riming are called graupel or snow pellets.
Green flash- A small, green color that occasionally appears on the upper part of the sun as it rises or sets.
Greenhouse effect- See Atmospheric greenhouse effect.
Growing degree-day- A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates.
Gulf Stream- A warm, swift, narrow ocean current flowing along the east coast of the United States.
Gust front- A windshift line that is caused by thunderstorms and other convection. The line acts like a front in that it can cause pressure changes, gusty winds and wind shear.
Hail/Hailstones- Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls or more.
Halo- Ring or arc that encircles the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.
Haze- Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulative they will diminish visibility.
Heat- A form of energy transferred between systems by virtual of their temperature differences.
Heat Index (HI)- An index that combines air temperature and relative humidity to determine an apparent temperature--how hot it actually feels. Accuweather, Inc. has developed a "real feel" temperature that also includes wind, humidity and sun angle and is used throughout the year.
Heating degree-day- A form of the degree-day used as an index for fuel consumption.
Heat lightning- Distant lightning that illuminates the sky but is too far away for its thunder to be heard.
Heat stroke- A physical condition induced by a person's overexposure to high air temperatures, especially when accompanied by high humidity. Can cause serious injury or death.
High pressure- An area of the maximum of highest atmospheric pressure. In a high pressure area the air descends or compresses, generally creating dry weather. A high pressure area is sometimes referred to as an anticyclone since it is associated with anticyclonic (clockwise) circulation.
Horse latitudes- The belt of latitude at about 30-35 degrees where winds are predominantly light and weather is hot and dry.
Humidity- A general term that refers to the air's water vapor content (See Relative humidity).
Hurricane-A severe tropical cyclone in the Atlantic, Caribbean, Gulf of Mexico, or along the west coast of Mexico. To be considered hurricane status, winds must reach at least 74 miles an hour. Hurricanes go through a process of becoming stronger. They start out over warm water (>80 degrees F) as a cluster of thunderstorms with a slight wind motion called a tropical disturbance. As the winds increase they move through the states of tropical depression and tropical storm before becoming a hurricane. Major hurricanes produce heavy rain (over ten inches an hour is possible) high winds (over 155 miles an hour) and high storm surge (over 18-20 feet high). Hurricanes are divided into five categories on the Saffir-Simpson scale depending on their strength. In the western North Pacific they are called typhoons, cyclones in the Indian Ocean and Australia (which also calls them "silly-willies".
Hurricane warning- A warning is given when it is likely that a hurricane will strike an area within 24 hours. A watch indicates a threat to an area (often within several days) and residents of the watch area should be prepared or evacuate.
Hydrologic cycle- A model that illustrates the movement and exchange of water among the earth, atmosphere, and oceans.
Hygrometer- An instrument designed to measure the air's water vapor content. The sensing ;part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).
Hypothermia- The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature.
Ice Age- Periods in the history of the Earth characterized by a growth of the ice caps towards the equator and a general lowering of global surface temperatures, especially in temperate mid–latitudes. The most recent ice age ended about 10,000 years ago. Ice advances in this period are known to have altered the whole pattern of global atmospheric and ocean circulation.
Ice Crystals- Slowly falling, very small crystals of ice that seem to float in the air. They may fall from either a cloud or a cloudless sky. Ice crystals often form sun pillars or other halos.
Ice Fog- Fog that is made up of minute ice particles. It forms in very cold air and is found in polar areas in areas near water vapor where the temperature falls below minus 50 degrees F.
Ice Pellets- See sleet.
Indian Summer- An unseasonably warm spell with clear skies near the middle of autumn that follows a substantial period of cool weather (usually including frost).
Insolation- The INcoming SOLar radiATION that reaches the earth and the atmosphere.
Instrument shelter- A boxlike wooden structure designed to protect weather instruments from direct sunshine and precipitation but has louvered walls to allow air circulation.
Intertropical Convergence Zone (ITCZ)- The dividing line between the southeast trade winds and the northeast trade winds of the southern and northern hemispheres.
Inversion- An increase in air temperature with height; usually temperature decreases with height.
Ionosphere- An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist.
Iridescence- Brilliant spots or borders of colors, most often red and green, observed in clouds up to about 30 degrees from the sun.
Isobar- A line connecting points of equal pressure.
Isotach- A line connecting points of equal wind speed.
Isotherm- A line connecting points of equal temperature.
Jetstream- Narrow bands of strong winds in the atmosphere. These maximum wind speeds are embedded in the mid-latitude westerlies. The predominant jet stream is the polar front jet in the middle and upper-middle latitudes and the secondary is the subtropical jet which is found around 20 and 30 degrees latitude. See also "Low-level jet stream".
Katabatic (fall) wind- Any wind blowing downslope, usually cold.
Kelvin Scale- A temperature scale with zero degrees equal to the theoretical temperature at which all molecular motion ceases. Also called the "Absolute Scale". Designated by a "K".
Kinetic energy- The energy within a body that is a result of its motion.
Knot- A unit of speed equal to one nautical mile per hour. One knot equals 1.15 mid/hr.
Lake breeze- A wind blowing onshore from the surface of a lake.
Lake-effect snows- Localized snowstorms that form on the downwind side of a larger lake. Such storms are common in late fall and early winter near the Great Lakes as cold, dry air picks up moisture and warmth from the unfrozen bodies of water. On occasions the same phenomenon will occur in bays and along the ocean edge.
Land breeze- A coastal breeze that blows from land to sea, usually at night. See also "Sea breeze".
Latent heat- The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, sublimation or deposition.
Leeside low- Storm systems (extratropical cyclones) that form on the downwind (lee) side of a mountain changing. In the United States leeside lows frequently form on the eastern side of the Rockies and Sierra Nevadas.
Lenticular cloud- A cloud in the shape of a lens.
Lidar- (LIght Detection And Ranging) An instrument that uses a laser to generate intense pulses that are reflected from atmospheric ;particles of dust and smoke.
Lightning- A visible electrical discharge produced by thunderstorms.
Little Ice Age- The period from about 1550 to 1850 when average global temperatures were lower, and alpine glaciers increased in size and advanced down mountain canyons.
Longwave radiation- A term most often used to describe the infrared energy emitted b by the earth and the atmosphere.
Long wave- A disturbance or wave in the major belt of westerlies characterized by a long length (thousands of kilometers) and significant amplitude. Also called "Rossby Waves".
Low pressure- An area of the lowest atmospheric pressure created by rising air. A low is sometimes called a depression and the term is used interchangeably with cyclone since it is associated with cyclonic (counterclockwise) circulation. See also "Extratropical cyclone".
Low-level jet streams- Jet streams that typically form near the earth's surface below an altitude of about 2 km and usually attain speeds of less than 60 knots. These can be instrumental in the formation of tornadoes as they create significant wind shear.
Macroburst- A strong downdraft or downburst greater than 4 km wide that can occur beneath thunderstorms. A downburst less than 4 km across is called a microburst.
Macroclimate- The general climate of a large areas, such as a country.
Magnetic storm- A worldwide disturbance of the earth's magnetic field caused by solar disturbances.
Magnetosphere- The region around the earth in which the earth's magnetic field plays a dominant part in controlling the physical processes that take place.
Mammatus clouds- Clouds that look like pouches or cow's udders hanging from the underside of a cloud.
Marine climate- A climate controlled largely by the ocean. The ocean's influence keep winters relatively mild and summers cool.
Maritime air- Moist air whose characteristics were developed over an extensive body of water.
Maximum/Minimum thermometer- A thermometer designed to measure the heist and lowest temperatures for a time period such as a day.
Mean annual temperature- The average temperature at any given location for the entire year.
Mean daily temperature- The average of the highest and lowest temperature for a 24-hour period.
Meridional flow- A type of atmospheric circulation;pattern in which the north-south component of the wind is pronounced. Also know as "Amplified flow".
Mesoclimate- The climate of an area ranging in size from a few acres to several square kilometers.
Mesocyclone- A vertical column of cyclonically rotating air within a severe thunderstorm.
Mesoscale Convective Complex (MCC)- A large organized convective weather system comprised of a number on individual thunderstorms. The size of an MCC can be 1000 times larger than an individual air-mass thunderstorm.
Mesosphere- The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80 km above the earth's surface.
Meteorology- The study of the atmosphere and atmospheric phenomena as well as the atmosphere's interaction with the earth's surface, oceans, and life in general.
Microburst- See "macroburst".
Microclimate- The climate structure of the air space near the surface of the earth.
Microscale- The smallest scale of atmospheric motions.
Middle latitudes- The region of the world typically described as being between 30 and 50 degrees latitude. Delmarva falls in this region.
Millibar- A unit for expressing atmospheric pressure. Sea level pressure is normally close to 1013 mb or 29.92 inches.
Mirage- A refraction phenomenon that makes an object appear to be displaced from its true position. "When an object appears higher than it actually is, it is called a "superior mirage"; when lower, an "inferior mirage".
Monsoon- A wind system that reverses direction between winter and summer. Usually the wind blows from land to sea in winter and from sea to land in summer.
Nacreous clouds- Clouds of unknown composition that have a soft, pearly luster ("mother-of-pearl") that form at altitudes about 25 to 30 km above the earth's surface.
National Weather Association- This organization has as its goals the promotion of excellence in operational meteorology and related activities. The NWA was founded in 1975 and, like the AMS, offers seals of approval for excellence in broadcast meteorology.
Nimbostratus- A dark, gray cloud characterized by more or less continuously falling precipitation of varying intensity.
Noctilucent clouds. Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the surface.
Northeaster, Nor'easter or Northeast Winter Storm- A low pressure or cyclonic storm that moves up the eastern coast of the United States. It is named a nor'easter because the winds ahead of the storm come in from the northeast thanks to the circulation of the cyclone. Wind-driven waves can batter the coast from the Carolinas to Maine causing flooding and severe beach erosion. The storm brings in moisture from the Atlantic and can dump heavy snow along coastal areas and inland. Nor'easters can also bring in heavy rain and ice storms. They can happen anytime but occur the most between September and April.
Numerical weather prediction (NWP)- Forecasting the weather based on the solutions of mathematical equations by high-speed computers.
Occluded Front- A composite of two fronts or when one front overtakes another. There are three types of occlusions. A cold occlusion is when colder air replaces warm air. A warm occlusion is when a warm front replaces a cold front and pushes it upward. A neutral occlusion results when there is not a big temperature difference between the air masses of the warm and cold fronts.
Offshore/Onshore breeze- An offshore breeze is one that blows from the land out over the water. The opposite is an onshore breeze.
Orographic uplift- The lifting of air over a topographic barrier, such as hilly or mountainous terrain. Clouds that form in this lifting process are "orographic clouds".
Overrunning-When one air mass moves over another one that has greater density. It usually applies to warmer air rising over colder air, making it unstable and possibly triggering heavy rain.
Ozone- An almost colorless gaseous form of oxygen with an odor similar to weak chlorine. The highest natural concentration is found in the stratosphere. Low level ozone can be harmful especially to children, the elderly and persons with respiratory ailments.
Permafrost- In polar regions, a layer of soil beneath the earth's surface that remains frozen throughout the year.
Plate tectonics- The theory that the earth's surface down to about 100 km is divided into a number of plates that move relative to one another across the surface of the earth. When plates slip over and under each other, earthquakes often occur; when they collide, mountain ranges can form. Also known as "continental drift".
Polar front- A semipermanent, semicontinuous front that separates tropical air masses from polar air masses.
Pollutants- Any gaseous, chemical or organic matter that contaminates the atmosphere, soil or water.
Precession (of the earth's axis of rotation)- The wobble of the earth's axis of rotation that traces out the path of a cone over a period of about 26,000 years.
Precipitation- Any form of water particles--liquid or solid--that falls from the atmosphere and reaches the ground.
Pressure gradient- The rate of decrease of pressure per unit of distance. On the same chart, when the isobars are close together the pressure gradient is steep and the winds are strong; when the isobars are far apart, the pressure gradient is weak and the winds are light.
Pressure tendency- The rate of change of atmospheric pressure within a specified period of time, most often three hours. Also known as "barometric tendency".
Prevailing westerlies- The dominant westerly winds that blow in middle latitudes on the poleward side of the subtropical high pressure areas.
Prevailing wind- The wind direction most frequently observed during a given period.
Prognostic chart (Prog)- A chart showering expected or forecasted conditions, such as pressure patterns, frontal positions, contour height patters, and so on. WBOC uses Futurecast as a prognostic graphic on the air.
Psychrometer- An instrument used to measure the water vapor content of the air. It consists of two thermometers: dry bulb and wet bulb. After whirling the instrument, the dewpoint and relative humidity can be obtained with the air of tables.
Radar- See "Doppler radar".
Radiant energy- Energy propagated in the form of electomagnetic waves. These waves do not need molecules to propagate them, and in a vacuum they travel at nearly 300,000 km per sec.
Radiation fog- Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also know as "ground fog" or "valley fog".
Radiosonde- A balloon-borne instrument that measures and transmits pressure, temperature and humidity to a ground-based receiving station.
Rain- Precipitation in the form of liquid water drops that have a diameter greater than 0.5 mm. Liquid precipitation less than 0.5 mm and greater than 0.2 mm is drizzle. Intensity of rainfall is considered as the following:
1. "very light" scattered drops which do not wet an exposed surface.
2. "light" the rate of fall between a trace and a .10 inch per hour.
3. "moderate" from .11 to .30 inch an hour.
4. "heavy" over .30 inches per hour or more than 0.03 inch in six minutes.
Rainbow- An arc of concentric colored bands that spans a sec tion of the sky when rain is present and the sun is positioned at the observer's back.
Rain gauge- An instrument designed to measure the amount of rain that falls during a given time period.
Rain shadow- The region on the leeside of a mountain where the precipitation is noticeably less than on the windward side.
Reflection- The [process whereby a surface turns back a portion of the radiation that strikes it.
Refraction. The bending of light as it passes from one medium to another, such as air to water.
Relative humidity- The ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at that particular temperature and pressure. Also the ratio of the airs' actual vapor pressure to its saturation vapor pressure.
Ridge- An elongated area of high atmospheric pressure.
Rime ice- A while,granular deposit of ice formed by the freezing of water drops when they come in contact with an object.
Roll cloud- A dense, roll-shaped cloud attached to the lower front part of the main cloud. It often forms with thunderstorms along the leading edge of a gust from. Also called an "arcus cloud".
Saffir-Simpson scale- A scale relating a hurricane's central pressure and winds to the possible damage it is capable of inflicting. The categories run from 1 to 5 with 3 or higher considered a "major" hurricane.
St. Elmo's fire- A bright electric discharge that is projected from objects (usually pointed) when they are in a high electric field, such as during a thunderstorm
Saturation of air- An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure. This usually leads to the formation of dew, fog, clouds, etc., and when cold enough of frost, ice crystals.
Scattering- The process by which small particles in the atmosphere deflect radiation from its path into different directions. Often the cause of visible optic phenomena such as sun dogs, crepuscular rays and others.
Sea breeze- A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is termed a "sea breeze front". See also "land breeze".
Sea level pressure- The atmospheric pressure at mean sea level. Station pressure is adjusted to sea level pressure in order for standardization for analytical and climatological use.
Seiches- Standing waves that oscillate back and forth over an open body of water. Similar to the waves one makes when shuffling in a bath tub.
Severe thunderstorm- Intense thunderstorms capable of producing heavy showers, flash floods, hail over 3/4" in diameter, strong and gusty surface winds over 58 mph and tornadoes. See also "Thunderstorm".
Sheet lightning- A fairly bright lightning flash from distant thunderstorms that illuminates a portion of the cloud. Sometimes called "heat lightning".
Shortwave radiation- A term most often used to describe the radiant energy emitted from the sun, in the visible and near ultraviolet wavelengths.
Shower- Intermittent precipitation from a cumuliform cloud, usually of short duration but sometimes heavy.
Sleet- A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Formed as rain drops fall into subfreezing air and condense before hitting the ground. Also known as "ice pellets". In Great Britain, sleet is snow and rain mixed.
Smog- Originally a mixture of smoke and fog. Now air that has restricted visibility due to pollution, or the pollution itself that is formed in the presence of sunlight (photochemical smog).
Snow: A solid form of precipitation composed of ice crystals in complex hexagonal (6-sided) form.
Snowflake-an ice crystal or a collection of ice crystals that fall from the clouds. Snowflakes made up of several crystals or crystal fragments may grow as large as three to four inches in diameter. In extremely still air, flakes with diameters as large as ten inches have been reported.
Snow fence- An open slatted board fence usually four to ten feet high, placed about fifty feet on the windward side of a railroad track or highway. The fence works to change the flow of the wind so the snow is deposited onto the fence and not the highway or railroad track.
Snow eater- Any warm wind which blows over a snow surface. Sometimes a fog over a snow surface which makes the snow cover disappear.
Snowflake- An aggregate of ice crystals that falls from a cloud.
Snow flurries- Light showers of snow that fall intermittently usually without accumulation and sometimes accompanied by gusty winds.
Snow grains- Precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.
Snow pellets- While, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion.
Snow rollers- A cylindrical spiral of show shaped somewhat like a muff and produced by the wind.
Snow squall- An intermittent heavy shower of snow that greatly reduces visibility and is usually accompanied by gusty winds.
Solar flare- A rapid eruption from the sun's surface that emits high energy radiation and energized charged particles. If large enough and in the right direction can cause disruption of satellite and radio transmissions and/or actually damage the instrumentation.
Solar wind- An outflow of charged particles from the sun that escapes the sun's outer atmosphere at high speed.
Sonic boom- A loud explosive-like sound caused by a shock wave emanating from an aircraft (or any object) traveling at or above the speed of sound.
Source regions- Regions where air masses originate and acquire their properties of temperature and moisture.
Southern Oscillation- The reversal of surface air pressure at opposite ends of the tropical Pacific Ocean that occur during major El Nino events. The combination is noted as ENSO. See also "El Nino".
Squall line- Any nonfrontal line or band of active thunderstorms or heavy rain or snow showers, usually accompanied by gusty winds.
Stationary front- A front that is stationary or nearly so (quasi-stationary) with winds blowing almost parallel and from opposite directions on each side of the front.
Station pressure- The actual asir pressure computed at the observing station. Station pressure is converted to sea level pressure for standardization.
Storm surge- An abnormal rise of the sea along a shore. Primarily due to the winds of a storm, especially a hurricane.
Stratocumulus- A low cloud, predominantly stratiform with low, lumpy, rounded masses, often with blue sky between them.
Stratosphere- The layer of the atmosphere above the troposphere and below the mesosphere (between 10 km and 50 km, generally characterized by an increase in temperature with height.
Stratus- A low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.
Sreamline- A line on a weather chart that shows the wind flow pattern.
Sublimation- The process whereby ice changes directly into water vapor without melting. In meteorology, sublimation can also mean the transformation of water vapor into ice crystals also called "deposition".
Subsidence- The slow sinking of air, usually associated with high-pressure areas.
Subtropical jet stream- The jet stream typically found between 20 deg. and 30 deg latitude at altitudes between 12 and 14 km. See also "jet stream".
Summer Solstice- Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23 1/2 degrees N, the Tropic of Cancer.
Sundog- A colored luminous spot produced by refraction of light through ice crystals that appears on either side of the sun. Also called "parhelia".
Sun pillar- A vertical streak of light extending above or below the sun. It is produced by the reflection of sunlight off ice crystals.
Sunspots- Relatively cooler areas on the sun's surface. They represent regions of extremely high magnetic field.
Supercell storm- An enormous severe thunderstorm whose updrafts and downdrafts are nearly i balance, allowing it to maintain itself for several hours. It can produce large hail and tornadoes.
Supercooled cloud (or cloud droplets)- A cloud composed of liquid droplets at temperatures below 0 deg C (32 deg F).
Supersaturated air- A condition that occurs in the atmosphere when the relative humidity is greater than 100 percent.
Synoptic scale- The typical weather map scale that shows features such as high- and low-pressure areas and fronts over a distance spanning a continent.
Virga-Water or ice particles falling out of a cloud but evaporating before reaching the earth as precipitation.
Warm front-The leading edge of a warmer air mass.
Wave-At the surface, waves are breaks in frontal boundaries where low pressure areas form. In the upper atmosphere waves are disturbances that move in the horizontal wind flow pattern. Waves moving through the atmosphere can bring a change to the weather at the surface.
Wet Snow-Snow that contains a lot of liquid water.
Wind shear-a rapid change in the direction or velocity of the wind.
Zonal flow-a wind flow that is pretty much in a west to east direction. Temperatures stay fairly close to normal in a zonal flow.