abiotic factor any nonliving part of an ecosystem, such as water, minerals, sunlight, air, or soil
abrasion the process that occurs when sand, stones, and pebbles move and scrape across Earth's surface, acting like sandpaper to pit and polish the surface
absorption the process of taking in radiant energy
abyssal zone the part of the oceanic zone that is found at depths greater than 2,000 meters (6,562 feet), where there is no sunlight, it is very cold, and the water pressure is high
acidity the amount of acid in a substance
acid rain precipitation that contains acidic components as a result of the burning of coal and other fossil fuels
aerial roots roots that reach out above the ground for water and sunlight to nourish the plant
air pressure the force that air molecules exert on the objects beneath them; has a major effect on the weather
alkalinity the amount of base in a substance
alluvial deposit a fan-shaped land deposit at the mouth of a stream
amoeba a protist, found in fresh water, salt water, and soil, that uses pseudopods to move and eat
amplitude the distance from the midpoint to the crest or trough of a wave
anticyclone an area of high pressure that usually brings fair weather
arroyo a small, water-carved channel with steep banks that is located in a dry area
asthenoshpere the layer of semimolten mantle rock that lies directly below the lithosphere
avalanche a landslide that typically occurs in a mountain area with a thick cover of snow
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