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Autumn
Autumn, also called fall, is the season of transition from summer to winter, associated in the Northern Hemisphere with harvest, shortening days, and the color change of leaves.
Available potential energy
Available potential energy (APE) is the portion of the atmosphere's potential energy convertible into kinetic energy, formally defined by Edward Lorenz at MIT in 1955.
Available water capacity
Available water capacity (AWC) is the amount of water in soil that plants can take up, held between field capacity and the permanent wilting point.
Avalanche
An avalanche, or snow avalanche, is a rapid flow of snow down a slope. It occurs in mountain areas worldwide and kills about 25 people per winter in the US.
Avalanche dynamics
Avalanche dynamics is the branch of snow science that studies how avalanches move and exert destructive force, from dense flowing slides to powder clouds, informing hazard maps.
Avalanche hazard assessment
Avalanche hazard assessment is the process of judging how likely, how large, and where avalanches threaten people, communicated as a five-level danger rating for an area and time.
Aventurine
Aventurine, also known as aventurine quartz, is a translucent quartz containing plate-like mineral inclusions that sparkle, an effect called aventurescence; it is most often green from fuchsite mica.
AWOS
AWOS, the Automated Weather Observing System, is a computerized airport weather station that measures weather parameters and broadcasts minute-by-minute reports to pilots, typically by VHF radio.
Azores High
The Azores High, also known as the Bermuda High or North Atlantic Subtropical High, is a semipermanent high-pressure system over the subtropical North Atlantic that steers rain-bearing storms and shapes winter precipitation across western Europe.
Azumayama
Azumayama (吾妻山) is an active volcanic group of stratovolcanoes, shields, lava domes, and pyroclastic cones on the Yamagata–Fukushima border, Japan; its highest point reaches 2,035 m.
Azurite
Azurite, also known as blue malachite or lapis armenius, is a deep blue copper carbonate mineral found in oxidized copper deposits, used as copper ore and a Renaissance pigment.
b-value
The b-value is the slope parameter in the Gutenberg–Richter law, which relates earthquake counts to magnitude; typically 0.8 to 1.2, it measures the ratio of small to large earthquakes.
Back-arc basin
A back-arc basin is a region of rifting and seafloor spreading behind a volcanic arc at a convergent plate margin, first proposed for western Pacific marginal seas in 1971.
Bacterioplankton
Bacterioplankton are bacteria and archaea suspended in the water column of oceans, estuaries, and lakes, which recover dissolved organic matter and drive nutrient cycling and carbon sequestration.
Badlands
Badlands are dissected, sparsely vegetated landscapes on soft sediment, with rills, gullies and very high erosion rates; the White River Badlands of South Dakota are the type locality.
Bagdad Mine
The Bagdad Mine is an open-pit copper and molybdenum mine in Yavapai County, Arizona, owned by Freeport-McMoRan and among the first major U.S. mines with a fully autonomous haulage fleet.
Baikal Rift Zone
The Baikal Rift Zone is the largest active continental rift system in Eurasia, a roughly 2,000 km network of basins in southeastern Siberia that contains Lake Baikal and still opens a few millimeters per year.
Baku to Belém Roadmap
The Baku to Belém Roadmap is a climate finance initiative launched at COP29 in 2024 to scale up funding to USD 1.3 trillion a year by 2035.
Balang Formation
Balang Formation (杷榔组), Cambrian Stage 3-4 shale unit in Guizhou and Hunan, South China, host of the Burgess Shale-type Balang Biota, transitional between the Chengjiang and Kaili biotas, with oryctocephalid trilobites, chancelloriids, and the Huayuan biota.
Bald Range fire
The Bald Range fire was a 2026 wildfire near Summerland, British Columbia that burned about 25,000 hectares, destroyed roughly 150 structures, and killed one person.
Bali Road Map
The Bali Road Map is a set of UN climate decisions adopted at COP13 in Bali in December 2007, launching two-year negotiations toward a post-2012 climate agreement.
Ball lightning
Ball lightning, also known as globular lightning, is a rarely observed phenomenon in which luminous spheres, typically 20 to 50 cm wide, appear during thunderstorms.
Baltica
Baltica was a paleocontinent centered on the East European Craton, covering most of northern Europe as far east as the Ural Mountains, with crust over 3 billion years old.
Banded iron formation
Banded iron formation is a marine sedimentary rock with alternating iron-rich and silica-rich layers, deposited mostly in the Precambrian and the main source of iron ore.
Bankfull discharge
Bankfull discharge is the water flow that completely fills a stream's active channel just before spilling onto the floodplain, and it is widely treated as the river's dominant channel-forming flow.
Barchan
A barchan is a crescent-shaped sand dune with horns pointing downwind that forms where sand is limited, winds blow from one direction, and the ground is bare.
Bardarbunga
Bardarbunga is a subglacial basaltic stratovolcano beneath the Vatnajökull ice cap in central Iceland, best known for the 2014–2015 Holuhraun eruption, Iceland's largest since the 1783 Laki event.
Barnett Shale
The Barnett Shale is a hydrocarbon-producing rock formation in north-central Texas covering about 5,000 square miles, the play where commercial shale gas production was first worked out.
Baroclinic instability
Baroclinic instability is a fluid-dynamical instability of rotating, stratified flow that converts available potential energy from a horizontal temperature gradient into kinetic energy, and it is the primary cause of synoptic-scale midlatitude storms.
Barometric formula
The barometric formula, or barometric law, is the equation giving atmospheric pressure as a function of altitude, derived from hydrostatic balance and the ideal gas law.