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Arctic sea ice decline
Arctic sea ice decline, also called Arctic ice shrinkage, is the long-term reduction in Arctic Ocean ice cover; September extent has fallen about 12 percent per decade since 1978.
Arctica
Arctica, also called Arctida or Hyperborea, is a hypothesized Precambrian continent proposed to have occupied the Arctic region, first suggested by Nikolai Shatsky in 1935.
Arcus cloud
The arcus cloud, also called an arc cloud, is a dense, low, horizontal roll or wedge cloud forming along a thunderstorm's gust front, as shelf or roll clouds.
Arenal Volcano
Arenal Volcano, also known as Volcán Arenal, is an andesitic stratovolcano in northwestern Costa Rica near La Fortuna, known for its 42-year eruption from 1968 to 2010.
Argo (oceanography)
Argo is an international ocean-observing program that deploys nearly 4000 robotic profiling floats measuring temperature and salinity to 2000 m, supplying most subsurface climate data.
Arhab, Harra of
The Harra of Arhab, also known as the Sana'a volcanic field, is a volcanic field in Yemen north of Sana'a that last erupted around 500 CE.
Aridity index
The aridity index (AI) is a climatic measure, the ratio of average annual precipitation to potential evapotranspiration; low values mean dry conditions, and the UNEP uses it to define drylands.
Armero tragedy
The Armero tragedy was the burial of the Colombian town of Armero by volcanic mudflows from Nevado del Ruiz on 13 November 1985, killing about 23,000 people.
Armoring (stream)
Armoring (also spelled armouring) is the process by which a gravel-bed stream's surface becomes coarser than the underlying bed, measured by the armor ratio of surface to subsurface grain size.
Arnold Dix
Arnold Dix is an Australian geologist, barrister, and tunnelling safety expert who led the 2023 rescue of 41 workers trapped for 17 days in India's collapsed Silkyara tunnel.
AROME
AROME (Application of Research to Operations at Mesoscale) is a limited-area weather forecasting model run by Météo-France, operational since 2008 at resolutions down to 1.3 km.
ARPEGE
ARPEGE is the global numerical weather prediction model operated by the French weather service Météo-France, in service since 1992 and covering forecasts up to about four days ahead.
Array calibration
Array calibration is the family of methods that estimates and corrects errors in sensor position, timing, and gain and phase response across arrays of receivers.
Arsenopyrite
Arsenopyrite, historically called mispickel, is an iron arsenic sulfide mineral (FeAsS), the most abundant ore mineral of arsenic. It occurs in hydrothermal veins and is commonly associated with gold.
Artesian well
An artesian well is a well drilled into an aquifer confined under pressure between impermeable layers, so water rises above the aquifer, sometimes flowing freely.
Artificial upwelling
Artificial upwelling is a proposed marine geoengineering technique that pumps nutrient-rich deep water to the sunlit surface to boost phytoplankton growth and, in principle, sequester CO2.
Asbestos
Asbestos: six naturally occurring fibrous silicate minerals, chiefly chrysotile, used for heat and chemical resistance; all forms cause lung, larynx, ovarian cancer and mesothelioma, banned in over 50 countries and by EPA's 2024 chrysotile rule.
ASCAT (scatterometer)
ASCAT (Advanced Scatterometer) is a C-band real-aperture radar instrument flying on Europe's Metop satellites that measures wind over the global ocean, chiefly for weather forecasting.
Ashikule Volcanic Field
Ashikule Volcanic Field (阿什库勒火山群, Kunlun Volcanic Group): a ~5,000 m-high cluster of ~10 trachyandesitic cones in the western Kunlun Mountains, Xinjiang, site of China's most recent eruption, Ashishan, 27 May 1951.
Asian Dust (황사)
Asian Dust (황사, hwangsa; also yellow dust or yellow sand) is a seasonal phenomenon in which westerly winds carry desert soil from Asia over China, Korea, and Japan each spring.
Association for the Sciences of Limnology and Oceanography
The Association for the Sciences of Limnology and Oceanography (ASLO), formerly the American Society of Limnology and Oceanography, is an international scientific society for freshwater and marine researchers.
Asthenosphere
The asthenosphere is the mechanically weak, ductile layer of the upper mantle directly below the lithosphere, whose low strength lets tectonic plates move over it.
ASTM D698
ASTM D698, commonly called the Standard Proctor test after R. R. Proctor's 1933 method, is an ASTM standard for determining soil compaction curves using standard effort.
Atakor Volcanic Field
Atakor Volcanic Field, Hoggar (Ahaggar) volcanic province, southern Algeria: the province's largest volcanic field, 2,150 km2, Mount Tahat 2,918 m, Quaternary scoria cones, lava domes, and necks with credible but undated recent eruptions.
Atlantic hurricane
An Atlantic hurricane, or Atlantic tropical cyclone, is a storm with winds of at least 74 mph forming in the Atlantic basin, active June through November.
Atlantic hurricane naming lists
The Atlantic hurricane naming lists are six rotating lists of 21 storm names, maintained by the World Meteorological Organization and reused every six years unless a name is retired.
Atlantic hurricane season
The Atlantic hurricane season runs officially from June 1 to November 30, when most tropical cyclones form in the Atlantic, Caribbean, and Gulf of Mexico.
Atlantic hurricane seasons before satellite observation (1850s–1949)
Atlantic hurricane seasons before satellite observation cover Atlantic tropical cyclone activity from 1851 through 1949, recorded from ships and coastal stations and now known to undercount storms at sea.
Atlantification of the Arctic
Atlantification of the Arctic is the growing influence of warm, salty Atlantic Water in the Arctic Ocean, which warms the region and reduces its sea ice.
Atlin Volcanic Field
Atlin Volcanic Field (Surprise Lake and Llangorse volcanic fields) is a late-Pleistocene to Holocene cluster of alkaline cinder cones on the Teslin Plateau, NW British Columbia; largest cone Ruby Mountain, 1,880 m; a reported 1898 eruption is uncertain, with no field evidence.