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The horizontal transfer of any property in the atmosphere by the movement of air. Examples include heat and moisture advection.
This is considered the mixture of gases that make up the earth's atmosphere. The principle gases that compose dry air are Nitrogen at 78.084%, Oxygen at 20.946%, Argon at .93%, and Carbon Dioxide .03%
Air Mass:
An extensive body of air throughout which the horizontal temperature and moisture characteristics are similar.


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An instrument used to measure atmospheric pressure. Examples include the aneroid barometer and the mercurial barometer.
Barometric Pressure:
The pressure exerted by the atmosphere at a given point. The measurement can be expressed in millibars(mb) or in inches of mercury(Hg).
A severe weather condition characterized by low temperatures, winds 35mph of greater, blowing snow that can reduce visibilities to 1/4 mile or less for more than 3 hours. A severe blizzard is characterized by temperatures at or below 10 degrees Fahrenheit, winds exceeding 45mph, and visibility reduced by snow to near zero.


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Atmospheric conditions devoid of wind or any other air motion.
A type of foehn wind. Refers to the warm downslope wind in the Rocky Mountains that may occur after an intense cold spell when the temperatures may rise 20-40° in a matter of minutes.
High clouds usually above 18,000 feet, composed of ice crystals.
The historical record of average daily and seasonal weather events. Statistics are generally drawn over several decades. The word is derived from the Greek “klima” meaning inclination, and reflects the importance early scholars attributed to the sun’s influence.
The study of climate. Includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems.
A sudden, heavy rainfall of a showery nature
The merging of two water drops into a single larger drop.
Cold Front:
The leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, when a cold front passes the temperature and humidity decrease, the pressure rises, and the wind shifts from southwest to northwest. precipitation is usually along or ahead of the front in the form of thunderstorms.
The process by which water vapor undergoes a change in state from a gas to a liquid. Its opposite is evaporation.
Motions in a fluid that transport and mix the properties of the fluid; these properties could be heat and/or moisture. Often the term convection is used to describe upward motion of water vapor(moisture) forced to rise by surface heating in turn creating rain or thunderstorms.
Wind movement that results in a horizontal net inflow of air into a particular region. Convergent winds at lower levels are associated with upward motion.
Coriolis Force:
A force per unit mass that arises solely from the earth’s rotation, acting as a deflecting force; it is dependent on the latitude and speed of the moving object. In the Northern Hemisphere the air is deflected to the right, and in the Southern Hemisphere to the left. The coriolis effect is almost non-existent at the equator.
Cumulonimbus Cloud:
A vertically developed cloud, often capped by an anvil shaped cloud; this cloud is otherwise known as a thundercloud. A cumulonimbus cloud can produce tornadoes, hail, lightning, strong winds and heavy rain.
An area of closed pressure circulation with rotating and converging winds; the circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a low pressure system and the term used for tropical cyclone in the Indian Ocean; other phenomena with cyclonic flow may be referred to as dust devils, tornadoes, tropical and extra tropical systems.


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Condensation in the form of small water drops that form on grass and other object near the ground when the temperature has fallen to the dew point. Dew generally forms during the nighttime hours and evaporates by mid to late morning.
Dew Point:
The temperature to which air must be cooled at a constant pressure to become saturated. Example: If the air temperature is 70° and the dew point temperature is 70° the air is saturated and dew will form i.e., the relative humidity is 100%.
Wind movement that results in a horizontal net outlow of air from a particular region; divergence at lower levels is associated with a downward movement of air from aloft.
Doppler Radar:
Weather radar that measures the direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar.
A severe localized downdraft from a thunderstorm or shower. This outward burst of cool air creates damaging winds at or near the surface.
Abnormal, dry weather for a specific area that is prolonged and causes serious hydrological imbalance.
Dry Line:
The boundary between the dry desert air mass of the southwestern U.S. and moist air mass from the Gulf of Mexico. It usually lies north-south across the central and southern High plains states during the spring and summer months. When a dry line passes it results in a decrease in humidity, clearing skies, and wind shift from east/southeasterly to west/southwesterly. Its presence influences severe weather development in the Great Plains.


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Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.
The energy return of a radar signal after it has hit the target.
The geographic circle at 0° latitude on the earth's surface. It is equal distance from the North and South poles and divides the Northern Hemisphere from the Southern.
The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20th and the autumnal equinox on or about September 22nd.
The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.
The total amount of water that is transferred from the earth’s surface to the atmosphere. It is made up of the evaporation of liquid or solid water plus the transpiration from plants.
Extratropical Cyclone:
Any cyclone not of tropical origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes.
The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.
An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds.


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Fahrenheit Temperature Scale:
A temperature scale where water at sea level has a freezing point of +32° F and a boiling point of +212° F. More commonly used in areas that observe the English system of measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol and mercury thermometers.
This is a subjective description. Considered as pleasant weather conditions with regard to the time of year and the physical conditions. Also refers to no cloud cover below 10,000 feet.
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. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.
High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low-lying areas.
A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present.
A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist.
Freezing Drizzle:
Drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surfaces.
Freezing Rain:
Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces.
The transition zone or interface between two air masses of different densities, which usually means different temperatures. For Example, the area of convergence between warm, moist air and cool, dry air.
The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.
Fujita Scale:
A scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. It is normally used to identify the most intense damage exhibited by a tornado. developed by T. Theodore Fujita and Allen Pearson.
Funnel Cloud:
A violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonimbus toward the ground, but not in contact with it.


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Geostationary Satellite:
An orbiting weather satellite that maintains the same position over the equator during the earth's rotation. Also known as GOES, an acronym for Geostationary Operational Evnvironmental Satellite.
A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together.
The force of attraction of the earth on an object. The direction is downward relative to the earth, and it decreases with elevation or altitude away from the earth’s surface.
Greenhouse Effect:
The overall warming of the earth’s lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun’s rays to heat the earth, but then restrict some heat-energy from escaping back into space.
A sudden significant increase in or rapid fluctuations of wind speed. peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less than 20-seconds.
Gust Front:
The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes confused with an outflow boundary.
A weak, and usually short-lived, tornado that forms along the gust front of a thunderstorm, appearing as a temporary dust whirl or debris cloud.


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Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. Individual lumps are called hailstones.
A suspension of fine dust and/or smoke particles in the air. Invisible to the naked eye, the particles reduce visibility by being sufficiently numerous to give the air an opalescent appearance.
A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.
Heat Index:
The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.
Heat Lightning:
Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.
Heat Wave:
A period of abnormally and uncomfortably hot weather. It could last from several days to several weeks.
High Clouds:
A term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. The main types of high clouds are cirrus, cirrocumulus, and cirrostratus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.
High Pressure System:
An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposite of an area of low pressure or a cyclone.
Hook Echo:
A radar reflectivity pattern observed in a thunderstorm, appearing like a fish hook and indicating favorable conditions for tornadic development. However, hook echoes and tornadoes do not always accompany each other.
The amount of water vapor in the air. It is often confused with relative humidity or dew point.
The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western pacific and a cyclone in the Indian Ocean.
Hydrologic Cycle:
Often called the water cycle, it is the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.
An instrument that measures the water vapor content of the atmosphere.


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Water in a solid state. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail.
Inches of Mercury:
The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars. First devised in 1644 by Evangelista Torricelli, an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.
The long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can be found between microwave radiation and visible light. Water vapor, ozone, and carbon dioxide are efficient at absorbing or transmitting infrared radiation.
Solar radiation or heating received at the earth's surface. The name is derived from INcoming SOLarar radiATION.
It is the condition of the atmosphere when spontaneous convection and severe weather can occur. Air parcels, when displaced vertically, will accelerate upward.
Intertropical Convergence Zone (ITCZ):
An area where the Northern and Southern Hemispheric trade winds converge. It is a broad area of low pressure where both the Coriolis force and the low-level pressure gradient are weak, occasionally allowing tropical disturbances to form. The ITCZ fluctuates, moving northward over the south Atlantic during the Northern Hemisphere summer.
It refers to an increase in an atmospheric property with height. For Example: A temperature inversion is when the temperature increases with altitude, which is a departure from the usual decrease of temperature with height.
An atmospheric zone of ionized gases that extends between 50 and 400 miles above the surface of the Earth. It is located between the mesosphere and the exosphere.
The line drawn on a weather map connecting points of equal barometric pressure.
The line of equal or constant temperature.


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A region of accelerated wind speed along the axis of a jet stream.
A area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the Northern and Southern Hemispheres. Flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving northward toward the poles.


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Katabatic Wind:
A wind that is created by air flowing downhill. When the air is warm, it may be called a foehn wind, and regionally may be known as a Chinook, or Santa Ana. When this air is cool, it is called a drainage wind, mountain breeze or glacier wind.
A nautical unit of wind speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. 1 knot = 1.151 statute miles per hour.


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Land Breeze:
A diurnal coastal breeze that blows offshore, from the land to the sea. It is caused by the temperature difference when the sea surface is warmer than the adjacent land. predominate during the night, it reaches its maximum around dawn.
Lapse Rate:
The change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height. This is a sign of instability.
The location north or south in reference to the equator, which is designated at zero degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.
The side of an object, such as a ship's sail, a mountain, or a hill, furthest away from the wind, and therefore protected from the direct force of the wind.
A rapid, visible discharge of electricity hotter than the surface of the sun. Lightning is caused by the build up of electrical potential between cloud and ground, between clouds, or between clouds and the surrounding air.
The location east or west in reference to the Prime Meridian, which is designated at 0° longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes. Time zones are correlated to longitude.
Low Clouds:
A term used to describe clouds with bases below 6,000 feet. types of low clouds include stratus, stratocumulus, cumulus and cumulonimbus.
Low Level Jet:
Strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. It is often amplified at night.
Low Pressure System:
An area of a relative pressure minimum that has converging winds and rotates in the same direction as the Earth, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as a cyclone.
Lunar Eclipse:
A eclipse of the moon occurs when the Earth is in a direct line between the sun and the moon. The moon does not have any light of its own, instead it reflects the sun's light. During a lunar eclipse the moon is in the earth's shadow.


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Mare's Tail:
The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches of strands, often resembling a horse's tail.
Mean Sea Level:
The average height of the sea surface water level. For the United States, it is computed by averaging the levels of all tide stages over a nineteen year period, determined from hourly height readings measured from a fix, predetermined reference level. It is used as a basis for determining elevations, as the reference for all altitudes in upper air measurements, and as the level above which altitude is measured by a pressure altimeter for aviation. Often referred to as MSL.
An area of rotation of storm size that may often be found on the southwest part of a supercell. Its circulation can be larger than the tornado that may develop within it, but not necessarily. Originally a radar term for a rotation signature that met certain criteria, it is best seen on Doppler radar.
The scale of meteorological phenomena that range in size from several kilometers to around 100 kilometers. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale.
The science and study of the atmosphere and atmospheric phenomena. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few. A scientist who studies the atmosphere and atmospheric phenomena.
A severe localized wind blasting down from a thunderstorm. It covers an area less than 2.5 miles (4 kilometers) in diameter and is of short duration, usually less than 5 minutes.
The smallest scale of meteorological phenomena that range in size from a few centimeters to a few kilometers. Larger phenomena are classified as mesoscale. It also refers to small scale meteorological phenomena with life spans of less than a few minutes that affect very small areas and are strongly influenced by local conditions of temperature and terrain.
Middle Clouds:
A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.
Mixed Precipitation:
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.
Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.
The seasonal shift of winds created by the great annual temperature variation that occurs over large areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the southwestern United States.
Multicell Storm:
A thunderstorm made up of two or more single-cell storms.
Multiple Vortex Tornado:
A tornado which has two or more condensation funnels or debris clouds, often rotating around a common center.


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This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. They are generally associated with fall and winter conditions, but can occur during any season.
A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.


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In meteorology, the evaluation of one or more meteorological elements, like temperature, pressure or wind that describe the state of the atmosphere at a given time. A trained observer is one who records the evaluations of the meteorological records.
Also referred to as an outflow boundary. It is the outward flow of air from a system such as a thunderstorm. It is the result of cold downdrafts and its passage includes a wind shift and most often a temperature drop. Outflow boundaries sometimes help produce thunderstorms as they move into regions of instability.
When the sky is completely covered by clouds.
This occurs when a relatively warm air mass is forced above a cooler air mass of greater density. Weather generally associated with this event include cloudiness, cool temperatures and steady precipitation.
Ozone Layer:
An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere between 9.5 and 12.5 miles above the Earth’s surface. Ozone at the surface is not healthy for humans to breathe.


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Partly Cloudy:
The state of the weather when clouds are conspicuously present, but do not completely cover the sky at a given time. Sometimes interchanged with mostly sunny.
Polar-Orbiting Satellite:
A satellite whose orbit passes over both of the Earth's poles gathering cloud and temperature data.
Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes, drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains.
The force dter unit area exerted by the weight of the atmosphere above a point on or above the earth's surface.
Pressure Gradient:
The amount of pressure change that occurs over a fixed distance at a fixed altitude.
Prevailing Wind:
A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.
An instrument used to measure the water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.


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Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by how they scatter or reflect radio energy. precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back.
The process by which energy is propagated through any medium by virtue of the wave motion of that medium. Electromagnetic radiation, which emits heat and light, is one form, sound waves are another.
Radiation Fog:
Fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its dew point. Formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. This primarily occurs during the night or early morning.
A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.
A measure of the process by which a surface can turn back a portion of incident radiation into the medium through which the radiation approached. It also refers to the degree by which precipitation is able to reflect a radar beam. Related albedo.
Relative Humidity:
A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage.
The rapid freezing of supercooled water droplets as they touch an exposed object, forming a white opaque granular deposit of ice. It is one of the results of an ice storm, and when formed on aircraft it is called rime icing. Related glaze
Roll Cloud:
A relatively rare, low-level, horizontal, tube-shaped cloud. Although they are associated with a thunderstorm, they are completely detached from the base of the cumulonimbus cloud.


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Saffir-Simpson Damage-Potential Scale:
developed in the early 1970s by Herbert Saffir, and Robert Simpson, it is a measure of hurricane intensity on a scale of 1 to 5. The scale categorizes potential damage based on barometric pressure, wind speeds, and storm-surge.
Any object that orbits a celestial body, such as a moon. However, the term is often used in reference to the manufactured objects that orbit the earth, either in a geostationary or a polar manner. Some of the information that is gathered by weather satellites, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world.
The process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. This is a primary reason for colors, such as blue skies, rainbows, and orange sunsets.
Sea Breeze:
A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. predominate during the day, it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.
Sea Level pressure:
The atmospheric pressure at mean sea level, usually determined from the observed station pressure.
Severe Thunderstorm:
A thunderstorm with winds measuring 50 knots (58 mph) or greater, ¾ inch hail or larger, or tornadoes. Severe thunderstorms may also produce torrential rain and frequent lightning.
Severe Weather:
Generally, any destructive weather event, but usually applies to localized storms, such as blizzards, intense thunderstorms, or tornadoes.
It is the rate of change over a short duration. In wind shear, it can refer to the frequent change in wind speed within a short distance. It can occur vertically or horizontally. Directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. When used in reference to Doppler radar, it describes the change in radial velocity over short distances horizontally.
Sky Cover:
The amount of the celestial dome that is hidden by clouds and/or obscurations.
Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice that rebound after striking the ground or any other hard surface.
Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. It usually appears clustered into snowflakes.
Snow Flurry/Flurries:
Light showers of snow, generally very brief without any measurable accumulation.
Snow Grains:
Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle.
Snow Pellets:
Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail.
An ice crystal or an aggregate of ice crystals which fall from clouds.
Solar Eclipse:
An eclipse of the sun occurs when the moon is in a direct line between the sun and the earth, casting some of the earth's surface in its shadow. The moon's disk shaped outline appears to cover the sun's brighter surface, or photosphere. That part of the earth that is directly in the moon's shadow will see a total eclipse of the sun, while the areas around it will see a partial eclipse.
The point at which the sun is the furthest on the ecliptic from the celestial equator. The point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. The Tropic of Cancer and the Tropic of Capricorn are those parallels of latitude which lies directly beneath a solstice. In the Northern Hemisphere, the winter solstice falls on or about December 21st and the summer solstice on or about June 21nd.
The season of the year which occurs as the sun approaches the summer solstice, and characterized by increasing temperatures in the mid-latitudes. Customarily, this refers to the months of March, April, and May in the Northern Hemisphere, and the months of September, October, and November in the Southern Hemisphere. Astronomically, this is the period between the vernal equinox and the summer solstice.
A sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more knots (25 miles per hour) for at least one minute. The intensity and duration is longer than that of a gust.
Squall Line:
A narrow band of line of active thunderstorms that is not associated with a cold front. It may form from an outflow boundary or the leading edge of a meso-high.
Stationary Front:
A front which is nearly stationary or moves very little since the last synoptic position. May be known as quasi-stationary front.
Straight-Line Winds:
Any surface wind that is not associated with rotation. An Example is the first gust from a thunderstorm, as opposed to tornadic winds.
Clouds composed of water droplets that exhibit no or have very little vertical development. The density of the droplets often blocks sunlight, casting shadows on the earth’s surface. Bases of these clouds are generally no more than 6,000 feet above the ground. They are classified as low clouds, and include all varieties of stratus and stratocumulus.
A low cloud composed of layers or patches of cloud elements. It can form from cumulus clouds becoming more stratiformed and often appears as regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. It is light or dark gray in color, depending on the size of the water droplets and the amount of sunlight that is passing through them.
The boundary zone or transition layer between the stratosphere and the mesosphere. Characterized by a decrease in temperature with increasing altitude.
The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase and absence of clouds. It extends between 11 and 31 miles (17 to 50 kilometers) above the earth’s surface. It is the location of the earth’s ozone layer.
One of the three basic cloud forms (the others are cirrus and cumulus). It is also one of the two low cloud types. It is a sheet-like cloud that does not exhibit individual elements, and is, perhaps, the most common of all low clouds. Thick and gray, it is seen in low, uniform layers and rarely extends higher than 5,000 feet above the earth’s surface.
The process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water).
A sinking or downward motion of air, often seen in anticyclones. It is most prevalent when there is colder, denser air aloft. It is often used to imply the opposite of atmospheric convection.
The region between the tropical and temperate regions, an area between 35° and 40° North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.
Subtropical Jet:
Marked by a concentration of isotherms and vertical shear, this jet is the boundary between the subtropical air and the tropical air. It is found approximately between 25° and 35° North latitude and usually above an altitude of 40,000 feet. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer.
Astronomically, this is the period between the summer solstice and the autumnal equinox. It is characterized as having the warmest temperatures of the year, except in some tropical regions. Customarily, this refers to the months of June, July, and August in the North Hemisphere, and the months of December, January, and February in the South Hemisphere.
A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes.
The reduction of the temperature of any liquid below the melting point of that substance’s solid phase. Cooling a substance beyond its nominal freezing point. supercooled water is water that remains in a liquid state when it is at a temperature that is well below freezing. The smaller and purer the water droplets, the more likely they can become supercooled.
Surface Boundary Layer:
The lowest layer of the earth’s atmosphere, usually up to 3,300 feet, or one kilometer, from the earth’s surface, where the wind is influenced by the friction of the earth’s surface and the objects on it.
The increase in seawater height from the level that would normally occur were there no storm. Although the most dramatic surges are associated with hurricanes, even smaller low-pressure systems can cause a slight increase in the sea level if the wind and fetch is just right. It is estimated by subtracting the normal astronomic tide from the observed storm tide.
Synodttic Scale:
The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more.


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The measure of molecular motion or the degree of heat of a substance.
An instrument used for measuring temperature. The different scales used in meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.
A thermal classification, it is the layer of the atmosphere located between the mesosphere and out space. It is a region of steadily increasing temperature with altitude.
The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning’s electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. temperatures can rise to over 10,000° Celsius in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one’s ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.
Thunder Snow:
A wintertime thunderstorm from which falls snow instead of rain.
produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes.
A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.
Tornado Alley:
A geographic corridor in the United States which stretches north from Texas to Nebraska and Iowa. In terms of sheer numbers, this section of the United States receives more tornadoes than any other.
Towering Cumulus:
Another name for cumulus congestus, it is a rapidly growing cumulus or an individual dome-shaped clouds whose height exceeds its width. Its distinctive cauliflower top often mean showers below, but lacking the characteristic anvil of a cumulonimbus, it is not a thunderstorm.
Generally, a non-measurable or insignificant quantity. A precipitation amount of less than 0.005 inch.
Trade Winds:
Two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. primarily lower level winds, they are characterized by their great consistency of direction. In the Northern Hemisphere, the trades blow from the northeast, and in the Southern Hemisphere, the trades blow from the southeast.
The process by which water in plants is transferred as water vapor to the atmosphere.
Tropical Air Mass:
An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry.
Tropical Cyclone:
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A wa, rm-core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.
Tropical Depression:
A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize.
Tropical Disturbance:
An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.
Tropical Storm:
A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the system is given a name to identify and track it.
Tropical Wave:
Another name for an easterly wave, it is an area of relatively low pressure moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.
The region of the earth located between the Tropic of Cancer, at 23.5° North latitude, and the Tropic of Capricorn, at 23.5° South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitation during part of the year.
The boundary zone or transition layer between the troposphere and the stratosphere. This is characterized by little or no increase or decrease in temperature or change in lapse rate with increasing altitude.
The lowest layer of the atmosphere located between the earth’s surface to approximately 11 miles (17 kilometers) into the atmosphere. Characterized by clouds and weather, temperature generally decreases with increasing altitude.
An elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic circulation. The opposite of a ridge.
Often called dusk, it is the evening period of waning light from the time of sunset to dark.
A slang term used in the United States for a tornado.
The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western North Pacific Ocean. This same tropical cyclone is known as a hurricane in the eastern North Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean.


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Electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. Although it accounts for only 4% to 5% of the total energy of insolation, it is responsible for many complex photochemical reactions, such as fluorescence and the formation of ozone.
Universal Time Coordinate:
One of several names for the 24-hour time which is used throughout the scientific and military communities.
Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own.
A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud, the first step towards thunderstorm development.
Upper Air/Upper Level:
The portion of the atmosphere which is above the lower troposphere. It is generally applied to the levels above 850 millibars. Therefore, upper level lows and highs, troughs, winds, observations, and charts all apply to atmospheric phenomena above the surface.
Upslope Effect:
The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and the air is stable, stratiform clouds and precipitation may form. If the air is unstable, there might be an increased chance of thunderstorm development.


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Vernal Equinox:
Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20th and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground.
A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight.
Visible Light:
The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 miles per second. It has a wave length longer than ultraviolet light and shorter than x-rays.


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Wall Cloud:
An abrupt lowering of a cloud from its parent cloud base, a cumulonimbus or supercell, with no visible precipitation underneath. Forming in the area of a thunderstorm updraft, or inflow area, it exhibits rapid upward movement and cyclonic rotation. It often develops before strong or violent tornadoes.
Warm Advection:
The horizontal movement of warmer air into a location.
Warm Front:
The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage.
A forecast issued when severe weather has developed, is already occurring and reported, or is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.
A forecast issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, or heavy snows.
A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or towering cumulus clouds and is the water equivalent of a landspout.
The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes.
Usually applied to the broad patterns of persistent winds with a westerly component. It is the dominant persistent atmospheric motion, centered over the midlatitudes of each Hemisphere.
Air that flows in relation to the earth’s surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.
Wind Chill Index:
The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.
Wind Direction:
The direction from which the wind is blowing.
Wind Shear:
The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change of the wind with respect to altitude. Horizontal wind shear is the rate of change on a horizontal plane.
Wind Vane:
An instrument that indicates the wind direction. The end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position.
The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the downwind or leeward side.
Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere.


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Zonal Flow:
The flow of air along a latitudinal component of existing flow, normally from west to east.
Zulu Time:
One of several names for the 24-hour time which is used throughout the scientific and military communities.
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