| A | B |
|---|
| aa | sharp and pointy lava flow |
| abrasion | erosion of rock by water or wind |
| abyssal plain | large flat area of deep ocean floor |
| acid rain | rain that contains acid, caused by pollution |
| aeolian | things carried, deposited, or eroded by the wind |
| aftershock | shock that occurs after a large earthquake |
| air mass | body of air that has similar temperature, pressure, and moisture content |
| air pressure | weight of the air above any point |
| albedo | percentage of the sun's radiation that is reflected off a surface (albedo of snow is very high) |
| alluvial fan | fan-shaped deposits of sediment at the foot of a mountain range (brought by streams) |
| alpine glacier | glacier that forms in the mountains |
| anemometer | instrument to measure speed of wind |
| anticyclone | winds that rotate clockwise in the northern hem. and counterclockwise in the Southern Hem. |
| aphelion | point in the Earth's orbit when Earth is farthest from sun |
| apogee | point in moon's orbit where moon is farthest from sun |
| aquaclude | underground rock layer that groundwater cannot bass through |
| aquifer | rock layer that holds groundwater |
| Arctic Circle | area which is light for 24 hours on June 21 |
| Antarctic Circle | area which is dark for 24 hours on June 21 |
| arid | dry |
| arroyo | dry streambed that fills with water after heave rain |
| artesian well | well from which water rises due to underground pressure |
| ash | tiny particles ejected from a volcano during an eruption |
| atmosphere | layer of gas that surrounds the Earth, where weather and climate take place |
| atoll | ring of coral reef that encloses a lagoon |
| aurora | colorful lights in the night sky |
| avalanche | rapid fall of snow, ice or rocks from a mountainside |
| axis | imaginary line that runs from the North Pole to the South Pole |
