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Acoustic source localization
Acoustic source localization is a signal processing method that estimates a sound source's position from measurements at multiple sensors, underpinning passive sonar and underwater acoustic positioning.
Acoustic tomography
Acoustic tomography is a remote-sensing method that reconstructs ocean temperature and currents from sound travel times between moored transducers, proposed by Walter Munk and Carl Wunsch in 1979.
Actinolite
Actinolite is a green calcium magnesium iron silicate mineral of the amphibole group, forming the middle of the tremolite–actinolite–ferro-actinolite series and common in low-grade metamorphic rocks such as greenschist.
ACTIVATE (field campaign)
ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment) was a NASA airborne field campaign studying aerosols, clouds, and meteorology over the western North Atlantic.
Active flux
Active flux, also called active transport, is the downward export of carbon and nutrients from the ocean's surface layer to depth by the metabolism and movement of vertically migrating zooplankton.
Active layer (permafrost)
The active layer is the top layer of ground in permafrost terrain that thaws each summer and freezes again each winter, also called seasonal frost.
ACTRIS
ACTRIS (Aerosol, Clouds, and Trace Gases Research Infrastructure) is a pan-European research infrastructure observing aerosols, clouds, and reactive trace gases, established as a consortium in 2023.
Adakite
Adakite is a group of igneous rocks defined by trace-element chemistry, named in 1990 after Adak Island magnesian andesites, and associated with porphyry copper deposits.
Adamant
Adamant is an archaic English word for an impenetrably hard substance, historically identified with the diamond, that survives today mainly in the adjective adamantine and figurative use.
Adaptation Fund
The Adaptation Fund is a multilateral climate fund established under the Kyoto Protocol in 2001 to finance adaptation projects in developing countries vulnerable to climate change.
Adaptive capacity
Adaptive capacity is the ability of systems, institutions, humans, and other organisms to adjust to potential damage or respond to consequences of climate change and other environmental change.
Admiralty chart
An Admiralty chart is a nautical chart issued by the United Kingdom Hydrographic Office since 1800, showing depths, dangers, and aids to navigation for safe voyages.
Advanced Baseline Imager
The Advanced Baseline Imager (ABI) is the primary Earth-observing instrument on NOAA's GOES-R geostationary weather satellites, a 16-channel multispectral radiometer flown on GOES-16, GOES-17, GOES-18, and GOES-19.
Advanced Dvorak Technique
The Advanced Dvorak Technique (ADT) is a computer-based method for estimating tropical cyclone intensity from geostationary satellite imagery, developed at the University of Wisconsin and used worldwide.
Advanced Himawari Imager
The Advanced Himawari Imager (AHI) is a 16-band visible and infrared imager on Japan's Himawari-8 and Himawari-9 weather satellites, imaging the full Earth disk every 10 minutes.
Advanced National Seismic System
The Advanced National Seismic System (ANSS) is a United States earthquake monitoring program led by the U.S. Geological Survey, coordinating regional networks since Congress endorsed it in 2000.
Advanced Research WRF model
The Advanced Research WRF model (ARW) is the research dynamical core of the open-source Weather Research and Forecasting system, used to simulate weather from cloud scales to the globe.
Advection-dispersion equation
The advection-dispersion equation is the classical governing equation for solute transport in groundwater, describing how dissolved substances move by advection, which carries them with the fluid, and hydrodynamic dispersion, which spreads them.
Advection–diffusion model
The advection–diffusion model is a mathematical model describing how a quantity such as heat, salinity, or pollutant concentration is carried by fluid flow while spreading by diffusion.
Aeolian processes
Aeolian processes are the erosion, transportation, and deposition of sediment by wind, active where vegetation is sparse, including deserts and coasts, and also shaping dunes on Mars.
Aeon
An aeon, also spelled eon, is the longest formal division of geologic time, ranking above eras and periods. Earth's history is divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic.
Aerial photogrammetry
Aerial photogrammetry is a remote sensing technique that derives measurements and maps of the Earth's surface from overlapping aerial photographs, producing digital elevation models, orthophotos, and dense 3D point clouds.
Aerial survey
An aerial survey is a remote sensing method that collects imagery or sensor data from aircraft or drones to map terrain, vegetation, infrastructure, or property, producing orthomosaics, terrain models, and 3D point clouds.
Aeromagnetic survey
An aeromagnetic survey is a geophysical method in which a low-flying aircraft with a magnetometer records magnetic anomalies from rocks below, used for geological mapping and mineral exploration.
AERONET
AERONET (AErosol RObotic NETwork) is a federation of ground-based aerosol networks established by NASA and the French PHOTONS program in 1993, operating robotically controlled sun photometers worldwide.
Aerosol and Chemistry Model Intercomparison Project
The Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) was a set of CMIP6 climate model experiments quantifying the climate and air quality impacts of aerosols and chemically reactive gases.
Aerosol indirect effect
The aerosol indirect effect is the change in Earth's radiation balance caused when airborne particles modify clouds, a cooling that is the most uncertain climate forcing.
Aerosol-cloud interaction
Aerosol-cloud interaction is the process by which atmospheric aerosol particles alter cloud microphysics, radiation, and precipitation, assessed by the IPCC as the largest single uncertainty in near-term climate projections.
African easterly jet
The African easterly jet (AEJ) is a band of strong mid-tropospheric easterly winds over northern Africa during summer, formed by the temperature contrast between the Sahara and the Gulf of Guinea.
African humid period
The African humid period, also called the Green Sahara, was a wet interval from about 14,800 to 5,500 years ago when the Sahara supported grasslands and lakes.