A strong localized downdraft less than 4 km wide that occurs beneath severe thunderstorms. A strong downdraft greater than 4 km across is called a downburst.


The climate structure of the air space near the surface of the earth.


The smallest scale of atmospheric motions.


A pressure unit of 1000 dynes cm-2. often used for reporting atmospheric pressure where one millibar equals 1 hectopascal (hPA).


A refraction phenomenon that makes an object appear to be displaced from its true position. When an object appears higher than it actually is. it is called a superior image. When an object appears lower than it actually is. it is an inferior mirage.


Very thin fog in which visibility is greater than 1.0 km (0.62 mi).


A katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast.

Mixing ratio(6)

The ratio of the mass of water vapor in a given volume of air to the mass of dry air.

Model (noun)(6)

A mathematical representation of a process. system. or object developed to understand its behavior or to make predictions. The representation always involves certain simplifications and assumptions.

Moist adiabatic rate(6)

The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of 6C per 1000 m (3.3F/1000 ft) is used. 'Molecular viscosity'


Seasonal winds. They are caused primarily by the greater annual variation in air temperature over large land surfaces compared to ocean surfaces though other factors like land-relief are important.

Montreal Protocol(1)

An international agreement to drastically reduce CFC production. the Protocol was adopted in Montreal in 1987. It was significantly strengthened at a subsequent meeting in London in 1990 that called for a complete elimination of CFCs by the year 2000. The agreement was again amended by a Meeting of the Parties in Copenhagen in November 1992. Consumption of controlled substances--such as CFCs and halons--was greatly reduced or eliminated. and many accountability dates were moved forward. often from 1 January 2000 to 1 January 1996.

Mountain and valley breezes(1)

A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.

Nacreous clouds(6)

Clouds of unknown composition that have a soft. pearly luster and that form at altitudes about 25 to 30 km above the earth's surface. They are also called mother-of-pearl clouds.

NAO Index(3)

This index measures the anomalies in sea level pressure between the Icelandic low-pressure system and the Azores high-pressure system. The nine centers are- Aviation Weather Center. Climate Prediction Center. Environmental Modeling Center. Hydrometeorological Prediction Center. Marine Prediction Center. NCEP Central Operations. Space Environment Center. Storm Prediction Center and Tropical Prediction Center (formerly the National Hurricane Center).

National Centers for Environmental Prediction (NCEP)(3)

The National Centers for Environmental Prediction (NCEP). an arm of the NOAA's National Weather Service (NWS) provides national and international weather and climate guidance products to National Weather Service field offices. government agencies. emergency managers. private sector meteorologists. and meteorological organizations and societies throughout the world. The nine centers are- Aviation Weather Center. Climate Prediction Center. Environmental Modeling Center. Hydrometeorological Prediction Center. Marine Prediction Center. NCEP Central Operations. Space Environment Center. Storm Prediction Center and Tropical Prediction Center (formerly the National Hurricane Center).

National Climatic Data Center (NCDC)(3)

NCDC maintains the world's largest active archive of weather data. NCDC produces numerous climate publications and responds to data requests from all over the world.

National Oceanic and Atmospheric Administration (NOAA)(3)

NOAA's historical role has been to predict environmental changes. protect life and property. provide decision makers with reliable scientific information. and foster global environmental stewardship. Today NOAA�s mission remains unchanged as it describes and predicts changes in the Earth's environment. and conserves and wisely manages the Nation's coastal and marine resources.

National Weather Service (NWS)(3)

National Weather Service.

Negative Feedback(5)

A process that results in a reduction in the response of a system to an external influence. For example. increased plant productivity in response to global warming would be a negative feedback on warming. because the additional growth would act as a sink for CO2. reducing the atmospheric CO2 concentration.


A dark. gray cloud characterized by more or less continuously falling precipitation. It is not accompanied by lightning. thunder. or hail.

Noctilucent clouds(6)

Wavy. thin. bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the surface.

Nocturnal inversion(6)

See Radiation inversion.


To understand whether precipitation and temperature is above or below normal for seasons and longer timescales. normal is defined as the average weather over 30 years. These averages are recalculated every ten years. The National Weather Service has just recalculated the baseline period for normal from 1961 to 1990 to 1971 to 2000. Since the cool decade of the 1960's has been replaced with the mild 1990's. normal temperatures in many areas have increased.

North Atlantic Oscillation (NAO)(3)

The NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high-pressure systems located near the Azores in the Atlantic Ocean and the sub-polar low-pressure system near Iceland and is quantified in the NAO Index. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to Western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa

Numerical Forecasting(3)

A computer forecast or prediction based on equations governing the motions and the forces affecting motion of fluids. The equations are based. or initialized. on specified weather or climate conditions at a certain place and time. (Also called mathematical forecasting. dynamical forecasting. physical forecasting. and numerical weather prediction).


The angle between the planes of the celestial equator and the ecliptic. currently the earth has a 23.4 degree obliquity cycle.

Offshore breeze(6)

A breeze that blows from the land out over the water. Opposite of an onshore breeze.

Onshore breeze(6)

A breeze that blows from the water onto the land. Opposite of an offshore breeze.

Orbital forcing(2)

Theory that proposes large scale climate changes are due in part to the variations in precession. eccentricity and obliquity that affects the amount of solar radiation received by the earth.

Orographic precipitation(6)

Rainfall or snowfall from clouds. induced by topographic uplift.

Orographic uplift(6)

The lifting of air over a topographic barrier. Clouds that form in this lifting process are called orographic clouds.


A shift in position of various high and low pressure systems that in climate terms is usually defined as an index (i.e.. a single numerically-derived number. that represents the distribution of temperature and pressure over a wide ocean area. such as the El Ni�o-Southern Oscillation. North Atlantic Oscillation. and Pacific Decadal Oscillation).

Ozone Hole(3)

A severe depletion of stratospheric ozone over Antarctica that occurs each spring. The possibility exists that a hole could form over the Arctic as well. The depletion is caused by a chemical reaction involving ozone and chlorine. primarily from human produced sources. cloud particles. and low temperatures.

Ozone Layer(1)

The layer of ozone that begins approximately 15 km above Earth and thins to an almost negligible amount at about 50 km. shields the Earth from harmful ultraviolet radiation from the sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 25 km above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcanoes and solar flares can produce changes in ozone concentration. but man-made changes are of the greatest concern.


A molecule consisting of three oxygen atoms that is formed by a reaction of oxygen and ultraviolet radiation. In the stratosphere. ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Closer to the planet�s surface. ozone is considered an air pollutant that adversely affects humans. plants and animals as well as a greenhouse gas.

Pacific Decadal Oscillation(3)

A recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons. It is characterized by SST anomalies of one sign in the north-central Pacific and SST anomalies of another sign to the north and east near the Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface temperatures in the Pacific Northwest. Alaska. and northern Pacific Islands. Two main characteristics distinguish PDO from El Ni�o/Southern Oscillation (ENSO)- first. 20th century PDO 'events' persisted for 20-to-30 years. while typical ENSO events persisted for 6 to 18 months. second. the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector. while secondary signatures exist in the tropics- the opposite is true for ENSO. Several independent studies found evidence of just two full PDO cycles in the past century- cool' PDO regimes prevailed from 1890-1924 and again from 1947-1976. while warm PDO regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990's. Causes for the PDO are not currently known. Likewise. the potential predictability for this climate oscillation is not known.


Past or ancient climates.


One who studies ancient (paleo-) climate. For example. increased atmospheric water vapor in response to global warming would be a positive feedback on warming. because water vapor is a GHG.

Paleoenvironmental proxy(2)

An environmental remnant of the past (pollen grains. tree rings. lake sediments. pack rat middens. ice cores. coral skeletons) that assist researchers in deciphering past climate conditions through the use of scientifically proven dating techniques.

Palmer Drought Severity Index (PDSI)(8)

An indicator. based on temperature. precipitation. and soil type. of long-term deficits or surpluses of soil moisture.

Particulate matter (PM)(4)

Solid particles or liquid droplets suspended or carried in the air (e.g.. soot. dust. fumes. mist). See aerosol. sulfate aerosols.


See Palmer Drought Severity Index.


Perennially frozen ground that occurs wherever the temperature remains below 0�C for several years.


A branch of science dealing with the relations between climate and periodic biological phenomena. such as bird migration or plant flowering.


The splitting of a molecule by a photon.


A discrete quantity of energy that can be thought of as a packet of electromagnetic radiation traveling at the speed of light.

Pileus cloud(6)

A smooth cloud in the form of a cap. Occurs above. or is attached to. the top of a cumuliform cloud.

Planetary albedo(1)

The fraction of incident solar radiation that is reflected by a planet and returned to space. The planetary albedo of the Earth-atmosphere system is approximately 30 percent. most of which is due to backscatter from clouds in the atmosphere.

Polar air mass(6)

A cold air mass that forms in a high-latitude source region.
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