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acid rain rain or snow containing high levels of sul- furic or nitric acids, which are produced by fossilfuel- burning industry and internal combustion engines, and have been shown to damage vegetation, foul drinking water, and intensify erosion of buildings and other outdoor structures.

arid describing any area that is very dry and tends to lack precipitation of any kind.

blizzard any storm characterized by steady winds of at least 35 miles per hour (56 km/hr) with heavy snowfall that sharply reduces visibility and lasts for three hours or more.

condensation the change of a substance from a vapor to a liquid; the opposite of evaporation.

corn snow that has melted and refrozen to form a rough, granulated surface.

dew point temperature at which air becomes satu­rated; a further drop in temperature causes condensa­tion followed by precipitation.

downdraft a column of cool air that rapidly descends to ground level, usually accompanied by precipitation.

evaporation the dispersal of moisture from surface water into the atmosphere.

firn old, compacted, and hardened snow; with fur­ther compacting it becomes glacial ice.

graupel falling pellets of snow; also known as soft hail.

humidity moisture content of the air.

hydrologic cycle the process of maintaining a con­stant water vapor content in the atmosphere by sur­face evaporation of oceans, lakes, rivers, and moist soil, and by transforming such moisture into a precip- itable form. The three phases of the hydrologic cycle are evaporation, condensation, and precipitation.

hydrometeors collective term for all types of pre­cipitation.

hyetography the study of rainfall.

hygrometer a device for measuring the amount of humidity in the air.

lake effect snow a heavy snowfall caused by cold, arctic air passing over a large expanse of warmer lake water and pulling up water vapor, freezing it, and turning it into snow, most notably occurring over the U.S. Great Lakes.

precipitation classification drizzle—fine drop­lets barely reaching the ground before evaporation; mist—fine droplets that usually evaporate before reaching the ground; hail—frozen droplets produced from violent convection in thunderstorms; sleet—fro­zen rain; rime—freezing fog.

rain gauge a device for measuring the amount of precipitation in a given time period.

saturation 100 percent humidity, the maximum amount of moisture the air can hold.

snow blindness temporary blindness caused by bright sunlight reflecting off snow.

snowblink a yellowish or whitish glow over a snowfield.

snowflake classification the seven basic types of snowflake are star, hexagonal plate, needle, column, capped column, spatial dendrite, and irregular.

thundersnow a snowstorm, often heavy, with thun­der and lightning.

whiteout zero visibility caused by blizzard conditions.

rivers and streams

Acheron in Greek and Roman mythology, the river of woe, one of the five rivers surrounding Hades.

alluvial fan the debris consisting of silt, gravel, and rocks deposited by rivers along the foot of mountains, creating a fanlike series of ridges.

alluvium any debris eroded by or deposited by a river, such as silt, gravel, rocks, and boulders.

aquifer groundwater, or any natural underground reservoir of water.

bar a ridgelike deposit or accumulation of sand or silt in or along a river.

bed the bottom of a river.

benthos plants and animals inhabiting the bottom of a river.

bight a curve or bend in a shoreline.

billabong an Australian term for a waterway filled with water only during rainy season.

boil a water current that "boils" or upwells into a convex mound.

braided river a river divided into several intertwin­ing branches or "braids" created by a series of built- up sandbars or banks.

branch one division of a forked river or a smaller river joining a larger one; a tributary.

brook a small stream.

cataract a waterfall.

channel the deepest part of a river.

chute a descending and steep and narrow passage of water.

creek a small, shallow stream.

delta a triangular-shaped island of deposited sedi­ment forming downstream at a river's mouth.

detritus particles of decaying plants and animals used as a source of food by many aquatic animals.

eddy the backward-rotating current found behind rocks or other obstructions above the surface.

estuary the body of water affected by tides, where the mouth of a river meets the sea.

feeder any branch that joins into a larger water­course.

fjord a long arm or river of ocean water running between high cliffs or banks.

floodplain any flat area that may be flooded by a river when it overflows its banks.

fluvial referring to rivers or things found in or formed by rivers.