Boise, Idaho weather

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Advection
The horizontal transfer of any property in the atmosphere by the movement of air. Examples include heat and moisture advection.

Air
This is considered the misture 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%

Airmass
An extensive body of air throughout which the horizontal temperature and moisture characteristics are similar.

ASOS
Acronym for Automated Surace Observing Systen. This system is a collection of automated surface weather instruments that collect data. It performs surface observations in places that either do not have a human observer, or an observer 24-hours a day.

Barometer
An instrument used to measure atmospheric pressure. Examples include the aneroid barometer and the mercurial barometer.

Barometric Pressure
The pressure exerted by the atmophere at a given point. The measurement can be expressed in millibars(mb) or in inches of mercury(Hg).

Blizzard
A severe weather condition characterized by low tempratures, 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.

Blue Norther
Refers to a fast-moving cold front in the southern Great Plains, marked by a dark, blue-black sky, strong north winds and temperatures that may drop 20-30 degrees fahrenheit in a matter of minutes

Calm
Atmospheric conditions devoid of wind or any other air motion.

CAPE
Acronym for Convective Available Potential Energy. The amount of energy available to create convection. Higher values indicate an increasing possibility of severe weather.

Ceiling
The lowest cloud layer that is reported as broken or overcast. If the sky is totally obscured for example by fog, then the ceiling is defined by vertical visibility.

Chinook
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 degrees in a matter of minutes.

Cirrus
High clouds usally above 18,000 feet, composed of ice crystals

Climate
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.

Climate Prediction Center
A branch of the National Centers for Environmental Prediction. The center maintains a continuous watch on short-term climate fluctuations and diagnoses and predicts them.

Climatology
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.

Cloudburst
A sudden, heavy rainfall of a showery nature. Related

Coalescence
The merging of two water drops into a single larger drop.

Cold Air Funnel
Funnel clouds, usually short-lived, that develop from relatively small showers or thunderstorms when the air aloft is very cold. Cold air funnels tend to touch down briefly, but in general are less violent than most other types of tornadoes.

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.

Condensation
The process by which water vapor undergoes a change in state from a gas to a liquid. It's opposite is evaporation.

Convection
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

Convergence
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.

Cyclone
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 extratropical systems.

Data Buoys
Buoys placed throughout the Gulf of Mexice and along the Atlantic and Pacific coasts of the U.S. that relay information on air and water temperature, air pressure, wind speed and wave conditions via radio signals.

Depression
In meteorology it is another name for an area of low pressure, a low or trough. It also applies to the initial stage of a developing tropical cyclone.

Dew
Condensation in the form of small water drops that form on grass and other obect near the ground when the temperature has fallen to the dewpoint. Dew generally forms during the nighttime hours and evaporates by mid to late morning.

Dewpoint
The temperature to which air must be cooled at a constant pressure to become saturated. Example: If the air temperature is 70 degrees and the dewpoint temperature is 70 degrees the air is saturated and dew will form i.e, the relative humidity is 100%

Difluence
A rate at which wind flow spreads apart along an axis oriented normal to the flow in question.

Divergence
Wind movement that results in a horizontal net outlow of air from a particular region. Divergence at lower levels is assiciated 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.

Downburst
A severe localized downdraft from a thunderstorm or shower. This outward burst of cool air creates damaging winds at or near the surface.

Drought
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 druing 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.

Easterlies
Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.

Easterly Wave
An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low level convergence and associated convective weather occur on the eastern side of the wave axis. It is often associated with possible tropical cyclone development and is also known as a tropical wave.

Echo
The energy return of a radar signal after it has hit the target. Related

radar echo.

El Nino
The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial Pacific waters move in and displace the colder waters, cutting off the upwelling process.

Environment
The sum total of all the external conditions that effect an organism, community, materail, or energy.

Equator
The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.

Equinox
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 20 and the autumnal equinox on or about September 22.

Evaporation
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.

Evapotranspiration
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.

Eye
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.

Eyewall
An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds.

Fahrenheit Temperature Scale
A temperature scale where water at sea level has a freezing point of +32 degrees F and a boiling point of +212 degrees 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.

Fair
This is a subjective description. Considered as pleasant weather conditions with regard to the time of year and the physical conditions.

Feeder Bands
In tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. Also known as outer convective bands, a typical hurricane may have three or more of these bands. They occur in advance of the main rain shield and are usually 40 to 80 miles apart. In thunderstorm development, they are the lines or bands of low level clouds that move or feed into the updraft region of a thunderstorm.

Flanking Line
A line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps (usually on the southwest side) of the most active part of a supercell.

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.

Flood
High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.

Foehn
A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. It is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. Classified as a katabatic wind.

Fog
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. It is reported as "FG" in an observation and on the METAR.

Forecast
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. It is reported as "FZDZ" in an observation and on the METAR.

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. It is reported as "FZRA" in an observation and on the METAR.

Front
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.

Frost
The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.

Fujita-Pearson 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.

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.

Geostrophic Wind
A steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.

Graupel
A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together.

Gravity
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.

Green Flash
A brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon.

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.

Greenwich Mean Time (GMT)
The name of the twenty-four hour time scale which is used throughout the scientific and military communities. This is the Prime Meridian of Longitude. The globe is divided into twenty-four time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are numbered from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain Greenwich Time (GMT). To the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain GMT.

Ground Clutter
A pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. This may hide or confuse the proper return echo signifying actual precipitation.

Gulf Stream
The warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of the United States, beginning near Cape Hatteras. The term also applies to the oceanic system of currents that dominate the western and northern Atlantic Ocean: the Florida current, which flows through the Florida Straits between the Florida Keys and Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras and flows northeasterly off the continental slope into the North Atlantic; and the North Atlantic current, which begins around the Grand Banks off Newfoundland and continues east-northeastwards towards the British Isles.

Gust
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 twenty seconds.

Gust Front
The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes confused with an outflow boundary.

Gustnado
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.

Hail
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 caled hailstones.

Haze
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.

Heat
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 Exhaustion
The effect of excessive heat, particularly when combined with high humidity on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

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 Stroke
Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105 degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.

Heat Wave
A period of abnormally and uncomfortably hot weather. It could last from several days to several weeks.

heating degree day
One heating degree day is given for each degree that the daily mean temperature is below 65 degrees Fahrenheit. It is used as an indication of fuel consumption.

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.

Humidity
The amount of water vapor in the air. It is often confused with relative humidity or dew point.

Hurricane
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.

Hurricane Warning
A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are expected in a coastal area or group of islands within a 24 hour period. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement.

Hurricane Watch
A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are a potential threat to a coastal area or group of islands within a 24 to 36 hour period. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.

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.

Hydrometeor
Any form of atmospheric water vapor, including those blown by the wind off the earth's surface. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are examples of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation. Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water vapor that evaporates before reaching the ground is virga. Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spary. Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects.

Hygrometer
An instrument that measures the water vapor content of the atmosphere.

Hypothermia
This situation occurs when the core temperature of one's body falls below normal. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold.

Ice
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.

Infrared
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.

Insolation
Solar radiation or heating received at the earth's surface. The name is derived from INcoming SOLar radiATION.

Instability
It is the condition of the atmosphere when spontaneous convection and severe weather can occur. Air parcels, when displaced vertically, will accelerate upward.

Instrument Flight Rules (IFR)
Refers to the general weather conditions a pilot can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist in take off and landing.

Instrument Shelter
A boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitaion, and condensation, while at the same time time providing adequate ventilation.

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 Hemipshere summer.

Inversion
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.

Ionisphere
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.

Isallobar
The line of equal change in atmospheric pressure during a certain time period. It marks the change in pressure tendedncy.

Isobar
The line drawn on a weather map connecting points of equal barometric pressure.

Isodrosotherm
The line drawn on a weather map connecting points of equal dew point.

Isohyet
THe line drawn through geographic points recording equl amounts of rainfall during a given time or for a give storm.

Isopleth
A line connecting equal points of value.

Isotach
A line connecting equal wind speeds.

Isotherm
The line of equal or constant temperature.

Jetstreak
A region of accelerated wind speed along the axis of a jet stream.

Jetstream
A area of strong winds that are concentrated in a realatively 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.

K Index
The measure of thunderstorm potential based on the vertical temperature lapse rate, the mosture content of the lower atmosphere and the vertical extent of the moist layer.

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.

Knot
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.

Land Breeze
A diurnal coastal breeze that blows offshore, from the land to the sea. It is caused by the termpeature difference when the sea sruface 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.

Latitude
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.

Lee/Leeside
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.

Lifted Index
A measure of atmospheric instability that is obtained by computing the temperature that the air near the ground would have if it were lifted to a higher level and compared to the actual temperature at that altitude. Positive values indicate more stable air and negative values indicate instability.

Lightning
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.

Longitude
The location east or west in reference to the Prime Meridian, which is designated at 0 degrees 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 concertrated 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.

Macroburst
A large downburst iwht an outflow diameter of 2.5miles or larger and damaging winds.

Macroscale
The meteorological scale covering an area ranging from the size of a continent to the entire globe.

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 ninteen year period, determined from hourly height readings measured from a fixed predetermined reference level. It is used as a basis for determining elevations, as the referece for all altitudes in upper air measurements, and as the level above which altitude is measure by a pressure altimeter for aviation. Often referred to as MSL

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.

Mercurial Barometer
An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. First used by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

Mesocyclone
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.

Mesoscale
The scale of meteorological phenomena that range in size from several kilometers to around 100 kilometers. This includes MCCs, MCSs, and squall lines. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale.

Mesoscale Convective Complex (MCC)
A large mesoscale convective system (MCS) which is about the size of the state of Ohio or Iowa and lasts at least 6 hours. Generally forming during the afternoon and evening, the complex normally reaches its peak intensity at night when heavy rainfall and flooding become the primary threat. Severe weather may occur at anytime.

Mesoscale Convective System (MCS)
A large organized convective weather system comprised of a number of individual thunderstorms. It normally persists of several hours and may be rounded or linear in shape. This term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (MCC).

Metar
Acronym for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.

Meteorology/Meteorologist
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.

Microburst
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.

Microscale
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.

Millibar (MB)
The standard unit of measurement for atmospheric pressure used by the National Weather Service. One millibar is equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2 millibars.

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.

Moisture
Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

Monsoon
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.

National Center for Atmospheric Research (NCAR)
A division of the University Corporation for Atmospheric Research, the Center plans, organizes, and conducts atmospheric and related research programs in collaboration with universities. For further information, contact NCAR, located in Boulder, Colorado.

National Centers for Environmental Prediction (NCEP)
As part of the National Weather Service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. Some of the centers include the Aviation Weather Center, the Climate Prediction Center, the Storm Prediction Center, and the Tropical Prediction Center. Formerly known as NMC. For further information, contact the NCEP, with central offices located in Silver Spring, Maryland.

National Climatic Data Center (NCDC)
The agency that archives climatic data from the National Oceanic and Atmospheric Administration as well as other climatological organizations. For further information, contact the NCDC, located in Asheville, North Carolina.

National Hurricane Center (NHC)
A branch of the Tropical Prediction Center, it is the office of the National Weather Service that is responsible for tracking and forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific. For further information, contact the NHC, located in Miami, Florida.

National Meteorological Center (NMC)
Now incorporated into the National Centers for Environmental Prediction, it was the division of the National Weather Service that produced, processed, handled, and distributed meteorological and oceanographic information to users throughout the Northern Hemisphere, specifically U.S. governmental organizations.

National Oceanic and Atmospheric Administration (NOAA)
A branch of the U.S. Department of Commerce, it is the parent organization of the National Weather Service. It promotes global environmental stewardship, emphasizing atmospheric and marine resources. For further information, contact NOAA, located in Silver Spring, Maryland.

National Severe Storms Forecast Center (NSSFC)
As of October 1995, the responsibilities of this Center were divided into two branches, the Storm Prediction Center and the Aviation Weather Center.

National Severe Storms Laboratory (NSSL)
A branch of the National Oceanic and Atmospheric Administration, it provides accurate and timely forecasts and warnings of hazardous weather events, especially flash floods, hail, lightning, tornadoes, and other severe wind storms. For further information, contact the NSSL, headquartered in Norman, Oklahoma.

National Weather Association (NWA)
An organization whose membership promotes excellence in operational meteorology and related activities, recognizing the professional as well as the volunteer. For further information, contact the NWA.

National Weather Service (NWS)
A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings. For further information, contact the NWS.

Nimbostratus
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.

Nitrogen
A colorless, tasteless, odorless gas that is the most abundant constituent of dry air. It comprises 78.09%.

Nor'easter
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.

Normal
The recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. Normals are concerned with the distribution of data within limits of common occurrence. The parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc.

Numerical Forecasting
The use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather.

Observation
In meterology, 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.

Oceanography
The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemisty and physics of sea water, and marine biology.

Omega Block
A warm high aloft which has become displaced and is on the polarward side of the jetstream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts.

Outflow
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.

Overcast
When the sky is completely covered by clouds.

Overrunning
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 tempertures 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 ultaviolet 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.

Palmer Drought Index
A long-term meteorological drought severity index produced by the NOAA/USDA Joint Agricultural Weather Facility. The index depicts prolonged times, in months or years, of abnormal dryness or wetness. It responds slowly. Changing little from week to week, it reflects long-term moisture runoff, recharge and deep percolation, as well as evapotranspiration.

Partly Cloudy
The state of the weather whene clouds are conspicuously present, but do not completely cover the sky at a given time. Sometimes interchanged with mostly sunny.

Pilot Report
A report of in-flight weather by an aircrat pilot or crew member. Often referred to as a PIREP.

Polar Front
A semi-continuous, semi-permanent boundary between polar airmasses and tropical air masses. An integral part of an early meteoroligical theory known as the Polar Front Theory.

Polar-Orbiting Satellite
A satellite whose orbit passes over both of the Earth's poles gathering cloud and temperature data.

Precipitation
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.

Pressure
The force per 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.

Profiler
A type of doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere. Used to monitor winds with height in determining severe weather threat.

Psychrometer
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.

Quasi-stationary Front
A front which is nearly stationary and moves very little since the last synoptic position. Also known as a stationary front.

Radar
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. Related Terms: Doppler Radar and NEXRAD

Radiation
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. Related

ground fog.

Radiosonde
An instrument attached to a weather balloon used to measure pressure, temperature, humidity, and winds aloft. Observations are made when the radiosonde is aloft and emits radio signals as it ascends. May be referred to as a RAOB, an acronym for RAdiosonde OBservation.

Rainbow
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.

Reflectivity
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