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Unless otherwise noted, all glossary entries are reproduced from the National Weather Service, Climate Prediction Center’s Climate Glossary. This glossary can be accessed by clicking here.
Sources for the NWS’s Climate Glossary include The American Heritage Dictionary, The New York Times Weather Report, American Meteorological Society's Glossary of Meteorology (2000), and the Internet.
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A | B | C | D | E | F | G | H | I | J | K | L | M | N | O
P | Q | R | S | T | U | V | W | X | Y | Z
A
Anomaly - The deviation of a measurable unit, (e.g., temperature or precipitation) in a given region over a specified period from the long-term average, often the thirty year mean, for the same region.
Arctic Oscillation (AO) - The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. The North Atlantic Oscillation is often considered to be a regional manifestation of the AO.
Atmospheric Circulation Model - A mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs (See GCMs)
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B
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C
Chlorofluorocarbons (CFCs) - Manufactured substances used as coolants and computer-chip cleaners. When these products break down they destroy stratospheric ozone, creating the Antarctic Ozone Hole in the Southern Hemisphere spring (Northern Hemisphere fall). While no longer in use, their long lifetime will lead to a very slow removal from the atmosphere.
Circulation - The flow, or movement, of a fluid (e.g., water or air) in or through a given area or volume.
Climate - The average of weather over at least a 30-year period. Note that the climate taken over different periods of time (30 years, 1000 years) may be different. The old saying is climate is what we expect and weather is what we get.
Climate Change - A non-random change in climate that is measured over several decades or longer. The change may be due to natural or human-induced causes.
Climate Model - Mathematical model for quantitatively describing, simulating, and analyzing the interactions between the atmosphere and underlying surface (e.g., ocean, land, and ice).
Climate Outlook - A climate outlook gives probabilities that conditions, averaged over a specified period, will be below-normal, normal, or above-normal.
Climate System - The system consisting of the atmosphere (gases), hydrosphere (water), lithosphere (solid rocky part of the Earth), and biosphere (living) that determine the Earth's climate.
Climatology - (1) The description and scientific study of climate. (2) A quantitative description of climate showing the characteristic values of climate variables over a region.
Condensation - The physical process by which water vapor in the atmosphere changes to liquid in the form of dew, fog or cloud; the opposite of evaporation.
Convection - Transfer of heat by fluid motion between two areas with different temperatures. In meteorology, the rising and descending air motion caused by heat. Atmospheric convection is almost always turbulent and is the dominant vertical transport process over tropical oceans and during sunny days over continents. The terms "convection" and "thunderstorms" are often use interchangeably, although thunderstorms are only one form of convection. In the ocean, convection is prominent in regions of high heat loss to the atmosphere and is the main mechanism for deep water formation.
Coupled Model (or coupled atmosphere-ocean model) - In the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other.
Cyclone - In general use the term cyclone is applied to any storm, especially violent, small scale circulations such as tornados, waterspouts, and dust devils. In meteorology, the term refers to a type of atmospheric disturbance centered around a low-pressure center that often results in stormy weather. In common practice the term cyclone, and low, are used interchangeably and are frequently referred to as storms. In the Northern Hemisphere the air rapidly circulates counterclockwise and in the Southern Hemisphere clockwise. Tropical cyclones with sustained winds above 73 miles per hour are known as hurricanes in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico and the Eastern North Pacific (east of the date line) and cyclones in the Indian Ocean. They are known as typhoons in other areas or the world. Both mid-latitude and tropical storms serve an important function in transferring warmth away from the tropics to the poles.
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D
Dew Point - The point at which the air at a certain temperature contains all the moisture possible without precipitation occurring. When the dew point is 65oF, one begins to feel the humidity. The higher the temperature associated with the dew point, the more uncomfortable one feels.
Dobson Unit - Unit used to measure the abundance of ozone in the atmosphere. One Dobson unit is the equivalent of 2.69 x 1016 molecules of ozone/cm2.
Drought - Drought is a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area. NOAA together with its partners provides short- and long-term Drought Assessments.
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E
El Niño - El Niño, a phase of ENSO, is a periodic warming of surface ocean waters in the eastern tropical Pacific along with a shift in convection in the western Pacific further east than the climatological average. These conditions affect weather patterns around the world. El Niño episodes occur roughly every four-to-five years and can last up to 12-to-18 months. The preliminary CPC definition of El Niño is a phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the 1971-2000 base period), averaged over three months, greater than or equal in magnitude to 0.5oC in a region defined by 120oW-170oW and 5oN-5oS (commonly referred to as Niño 3.4). El Niño, which would appear off the coast of Peru around Christmas time, is Spanish for "the boy" referring to the Christ child.
ENSO (El Niño-Southern Oscillation) - Originally, ENSO referred to El Niño/ Southern Oscillation, or the combined atmosphere/ocean system during an El Niño warm event. The ENSO cycle includes La Niña and El Niño phases as well as neutral phases, or ENSO cycle, of the coupled atmosphere/ocean system though sometimes it is still used as originally defined. The Southern Oscillation is quantified by the Southern Oscillation Index (SOI).
Evaporation - The physical process by which a liquid or solid is changed to a gas; the opposite of condensation.
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F
Forecasts (Synonymous with predictions and outlooks) - A weather forecast, or prediction, is an estimation based on special knowledge of the future state of the atmosphere with respect to temperature, precipitation, and wind. Weather forecasts are now routinely provided for up to 14 days in advance and outlooks for seasonal and longer timescales.
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G
GCMs (General Circulation Models) - These computer simulations reproduce the Earth's weather patterns and can be used to predict change in the weather and climate.
Global Warming - Certain natural and human-produced gases prevent the sun's energy from escaping back to space leading to an overall rise in the temperature of the Earth's atmosphere.
Greenhouse Effect - The atmosphere allows solar radiation to reach the earth relatively easily. The atmosphere absorbs the infrared radiation emitted by the Earth's surface and radiates it back to the Earth in much the same way a greenhouse traps heat as the sun's rays pass through the glass, and the heat generated does not pass back through the glass. The "greenhouse effect" causes the surface of the Earth to be much warmer that it would be without the atmosphere 60oF). Without the greenhouse effect, life as we know it might not exist on Earth.
Greenhouse Gas - Certain gases, such as water vapor, carbon dioxide, and methane, that more effectively trap heat affecting the Earth's surface temperature.
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H
Hurricane - See Cyclone
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I
Intraseasonal Oscillations - Variability on a timescale less than a season. One example is the Madden-Julian Oscillation.
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J
Jet Stream - Strong winds concentrated within a narrow zone in the atmosphere in the upper troposphere, about 30,000 feet aloft that generally move in an easterly direction that drive weather systems around the globe. In North America jet streams are more pronounced in winter.
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K
Kelvin Waves - Fluctuations in wind speed at the ocean surface at the Equator result in eastward propagating waves, known as Kelvin Waves. Kelvin Waves cause variations in the depth of the oceanic thermocline, the boundary between warm waters in the upper ocean and cold waters in the deep ocean. They play an important role in monitoring and predicting El Niño episodes.
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L
La Niña - La Niña, a phase of ENSO, is a periodic cooling of surface ocean waters in the eastern tropical Pacific along with a shift in convection in the western Pacific further west than the climatological average. These conditions affect weather patterns around the world. The preliminary CPC definition of La Niña is a phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal (for the 1971-2000 base period), averaged over three months, greater than or equal in magnitude to 0.5oC in a region defined by 150oW-160oE and 5oN-5oS (commonly referred to as Niño 4).
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M
Madden-Julian Oscillation (MJO) - Tropical rainfall exhibits strong variability on time scales shorter than the seasonal El Niño-Southern Oscillation (ENSO). These fluctuations in tropical rainfall often go through an entire cycle in 30-60 days, and are referred to as the Madden-Julian Oscillation or intraseasonal oscillations. The intraseasonal oscillations are a naturally occurring component of our coupled ocean-atmosphere system. They significantly affect the atmospheric circulation throughout the global Tropics and subtropics, and also strongly affect the wintertime jet stream and atmospheric circulation features over the North Pacific and western North America. As a result, they have an important impact on storminess and temperatures over the United States. During the summer these oscillations have a modulating effect on hurricane activity in both the Pacific and Atlantic basins.
Meteorology - The scientific study of the physics, chemistry, and dynamics of the Earth's atmosphere, especially weather and climate.
Monsoons - 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.
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N
NAO - North Atlantic Oscillation. The NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high pressure system 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 (Also called mathematical forecasting, dynamical forecasting, physical forecasting, and numerical weather prediction) - 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.
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O
OLR - Outgoing Longwave Radiation is a polar satellite derived measurement of the radiative character of energy radiated from the warmer earth surface to cooler space. This measurement provides information on cloud-top temperature which can be used to estimate tropical precipitation amounts which is important in forecasting weather and climate.
Oscillations - 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 Niño-Southern Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation).
Ozone - 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.
Ozone Hole - 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.
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P
Pacific Decadal Oscillation - 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 Niño/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 are not known.
Palmer Drought Severity Index (PDSI) - An index that compares the actual amount of precipitation received in an area during a specified period with the normal or average amount expected during that same period. It was developed to measure lack of moisture over a relatively long period of time and is based on the supply and demand concept of a water balance equation. Included in the equation are amount of evaporation, soil recharge, and runoff and temperature and precipitation data.
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Q
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R
Relative humidity - An estimate of the amount of moisture in the air relative to the amount of moisture that the air can hold at a specific temperature. For example, if it's 70oF near dawn on a foggy summer morning, the relative humidity is near 100%. During the afternoon the temperature soars to 95oF and the fog disappears. The moisture in the atmosphere has not changed appreciably, but the relative humidity drops to 44% because the air has the capacity to hold much more moisture at a temperature of 95oF than it does at 70oF. But even when the relative humidity is "low" at 44%, it's a very humid day when the temperature is 95oF. For this reason, a better measure of comfort is dew point.
Retrogression or retrograde motion - motion that is backwards from the usual way things move in the Northern Hemisphere extratropics - which is from west to east. In meterology, the term is used in relation to atmospheric waves or pressure systems. When meterologists say that a pattern will retrograde, they mean that the troughs and ridges will end up further west than they were previously. Normal motion (over the United States) is progressive, or prograde, which means (weather systems move) from west to east.
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S
SOI (Southern Oscillation Index) - SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface temperature anomaly indices recorded in Niño3.
Stratosphere - The region of the atmosphere extending from the top of the troposphere to the base of the mesosphere, an important area for monitoring stratospheric ozone.
Stratospheric ozone - In the stratosphere, ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently, it was discovered that in certain parts of the world, especially over the poles, stratospheric ozone was disappearing creating an ozone hole.
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T
Teleconnection - A strong statistical relationship between weather in different parts of the globe. For example, there appears to be a teleconnection between the tropics and North America during El Niño.
Thermocline - As one descends from the surface of the ocean, the temperature remains nearly the same as it was at the surface, but at a certain depth temperature starts decreasing rapidly with depth. This boundary is called the thermocline. In studying the tropical Pacific Ocean, the depth of 20oC water ("the 20oC isotherm") is often used as a proxy for the depth of the thermocline. Along the equator, the 20oC isotherm is typically located at about 50m depth in the eastern Pacific, sloping downwards to about 150 m in the western Pacific.
Troposphere - The lowest portion of the atmosphere which lies next to the earth's surface where most weather occurs.
Typhoon - See Cyclone
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U
Ultraviolet (UV) (or Ultraviolet Radiation) - Ultraviolet radiation from the sun plays a role in the formation of the ozone layer by acting as a catalyst for a chemical reaction that breaks apart oxygen molecules which then recombine to form ozone. The absorption of UV by stratospheric ozone and atmospheric oxygen prevents very little ultraviolet radiation to reach earth's surfaces where it can detrimental effects on human health and property.
Upper-level (or upper air) - In weather observing, the term applies to the portion of the atmosphere that is above the lower troposphere, generally 850 hPa and above.
Upwelling - In ocean dynamics, the upward motion of sub-surface water toward the surface of the ocean. This is often a source of cold, nutrient-rich water. Strong upwelling occurs along the equator where easterly winds are present. Upwelling also can occur along coastlines, and is important to fisheries and birds in California and Peru.
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V
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W
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X
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Y
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Z
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