Encyclopedic Entry

River currents can be influenced by climate, location, and topography.

Photograph by Pearce Adams, MyShot

Streaming Current
Parts of the Gulf Stream ocean current are up to 80 kilometers (50 miles) wide and more than a kilometer (half mile) deep.

A current is the steady, predictable movement of a fluid within a larger body of that fluid. Fluids are materials capable of flowing and easily changing shape. The most familiar natural fluid is water. But air is considered a fluid as well. Electricity can also flow as a current.

Air currents flow in the atmosphere, the layer of air surrounding the Earth. Water currents flow in rivers, lakes, and, oceans. Electric currents flow through power lines or as lightning.

Air Currents

Moving air is called wind. Air currents are winds that move in a riverlike flow in a certain direction. Thermal updrafts are gentle currents caused by warm air rising. Birds like eagles or California condors often ride these updrafts high into the sky. Jet streams are rapidly moving cold currents that circle the Earth high in the atmosphere.

Air currents are caused by the sun's uneven heating of the Earth. As sunlight beams down on the Earth, it warms some areas, particularly the tropics, more than others. As the Earth's surface is heated, it warms the air just above it. The warmed air expands and becomes lighter than the surrounding air. It rises, creating a warm air current. Cooler, heavier air then pushes in to replace the warm air, forming a cool air current.

Some air currents are familiar. Santa Ana winds are seasonal (fall) occurrences in southern California. These warm, dry currents blow from the Mojave Desert and the Great Basin toward the Pacific Ocean. Jet streams are familiar to mountaineers who climb Mount Everest, Earths tallest point.

The summit of Mount Everest actually pierces the jet stream, creating icy winds at the top of the world.

Water Currents

A river current is the water moving in a river. Rivers flow from high points to lower ones and eventually down to a larger body of water. The force of gravity, which makes the water flow downward, creates river currents.


Many factors contribute to the strength of river currents. River currents are influenced by the volume, or amount, of water flowing in a river. A rivers steepness as it flows toward its destination can affect its currents. The steepness of a river is called its stream gradient. A river beds topography also influences its currents. Topography refers to the surface features of an area. A riverbeds topography can include sandbars, basins, and dams.

The Nile River flows north from the high elevations of sub-Saharan Africa to the low-lying areas of Egypt near the Mediterranean Sea. The Niles currents gain strength as the volume of water increases, especially where the Blue Nile (starting in Ethiopia) and the White Nile (starting in Tanzania) merge. The Aswan Dam, in southern Egypt, severely reduces and controls the flow of Nile River currents.

Ocean currents are great streams of water flowing both near the oceans surface and far below it. Prevailing winds (air currents) that blow over parts of the ocean push the water along, creating surface currents. Winds can also contribute to upwelling, or currents that move cold, nutrient-rich water from the bottom of the ocean to the surface.

The spin of the Earth from west to east causes ocean currents to swerve to the right north of the Equator and to the left south of the Equator. This swerving, known as the Coriolis effect, sets surface currents flowing clockwise in a circular pattern in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Differences in seawater density also cause ocean currents. Waters density is affected by its temperature and salinity, or saltiness. The colder and saltier the water is, the denser and heavier it is. Cold, dense water tends to sink and flow under warmer, lighter water, creating a current. The strength of ocean currents is measured in sverdrups (SVAIR-drups), named after a Norwegian oceanographer.

The Gulf Stream is one of the most well-known ocean currents in the world. This warm current flows from the Gulf of Mexico, around the U.S. state of Florida, up the eastern coast of the U.S. and Canada before crossing the Atlantic Ocean. The Gulf Stream is very powerful. Because of the Gulf Stream, northern Europe is warmer than any other area at its latitude, including Alaska and Russia.

Electrical Currents

Electricity is the flow of electrons. Electrons are parts of atoms, of which all things are made. For this reason, almost any surface can be electric under the right conditions.

Electricity needs a conductor. Metals like copper are good conductors for electricity in homes and businesses. Clothes, carpets, and human beings can be conductors of static electricity currents. The strength of electricity is measured in amperes (amps).

The vacuum of space can actually be a conductor. The solar wind is a flow of a type of electricity from the sun. The solar wind flows all the way to the edge of the solar system. On Earth, the solar wind is blocked by the atmosphere. We can see the impact of the solar wind as the Northern Lights and the Southern Lights, bright slashes of color that sometimes appear in the sky near the North and South Poles.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

air

Noun

the layer of gases surrounding Earth.

Encyclopedic Entry: air

air current

Noun

flowing movement of air within a larger body of air.

ampere

Noun

unit measuring electrical current, the amount of electrical charge moving through a conductor in one second. Abbreviated A or amp.

Aswan Dam

Noun

system of two dams in Egypt that control the flow of the Nile River for agricultural, electrical, and sanitary uses.

atmosphere

Noun

layers of gases surrounding a planet or other celestial body.

Encyclopedic Entry: atmosphere

atom

Noun

the basic unit of an element, composed of three major parts: electrons, protons, and neutrons.

basin

Noun

a dip or depression in the surface of the land or ocean floor.

Encyclopedic Entry: basin

Blue Nile

Noun

tributary of the Nile River flowing from Lake Tana in Ethiopia and meeting the White Nile at Khartoum, Sudan, to form the Nile River.

coast

Noun

edge of land along the sea or other large body of water.

Encyclopedic Entry: coast

conductor

Noun

material that transfers heat, light, electricity, or sound.

copper

Noun

chemical element with the symbol Cu.

Coriolis effect

Noun

the result of Earth's rotation on weather patterns and ocean currents. The Coriolis effect makes storms swirl clockwise in the Southern hemisphere and counterclockwise in the Northern Hemisphere.

Encyclopedic Entry: Coriolis effect

current

Noun

steady, predictable flow of fluid within a larger body of that fluid.

Encyclopedic Entry: current

dam

Noun

structure built across a river or other waterway to control the flow of water.

density

Noun

number of things of one kind in a given area.

Encyclopedic Entry: density

destination

Noun

place where a person or thing is going.

electric current

Noun

rate of flow of electricity, measured in amperes.

electricity

Noun

set of physical phenomena associated with the presence and flow of electric charge.

electron

Noun

negatively charged particle in an atom.

elevation

Noun

height above or below sea level.

Encyclopedic Entry: elevation

Equator

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

Encyclopedic Entry: equator

fluid

Noun

material that is able to flow and change shape.

gravity

Noun

physical force by which objects attract, or pull toward, each other.

Gulf Stream

Noun

warm current that starts in the Gulf of Mexico and travels along the eastern coast of the U.S. and Canada before crossing the North Atlantic Ocean.

impact

Noun

meaning or effect.

influence

Verb

to encourage or persuade a person or organization to act a certain way.

jet stream

Noun

winds speeding through the upper atmosphere.

Encyclopedic Entry: jet stream

lake

Noun

body of water surrounded by land.

latitude

Noun

distance north or south of the Equator, measured in degrees.

Encyclopedic Entry: latitude

lightning

Noun

sudden electrical discharge from clouds.

Encyclopedic Entry: lightning

merge

Verb

to combine.

metal

Noun

category of elements that are usually solid and shiny at room temperature.

mountaineer

Noun

someone who climbs mountains.

Mount Everest

Noun

highest spot on Earth, 8,850 meters (29,035 feet). Mount Everest is part of the Himalaya range, in Nepal and China.

Nile River

Noun

(5,592 kilometers/3,473 miles) river in East Africa.

northern lights

Noun

also known as the aurora borealis. The bright bands of color around the North Pole caused by the solar wind and the Earth's magnetic field.

nutrient

Noun

substance an organism needs for energy, growth, and life.

Encyclopedic Entry: nutrient

ocean

Noun

large body of salt water that covers most of the Earth.

Encyclopedic Entry: ocean

oceanographer

Noun

person who studies the ocean.

power line

Noun

cable or cord used to transfer electricity from a power plant to a population center. Also called a transmission line.

prevailing wind

Noun

wind that blows from one direction.

reduce

Verb

to lower or lessen.

river

Noun

large stream of flowing fresh water.

Encyclopedic Entry: river

river bed

Noun

material at the bottom of a river.

salinity

Noun

saltiness.

sandbar

Noun

underwater or low-lying mound of sand formed by tides, waves, or currents.

Santa Ana winds

Noun

extremely strong, dry winds flowing from the Mojave Desert or Great Basin through Southern California to the Pacific Ocean.

seawater

Noun

salty water from an ocean or sea.

solar system

Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

solar wind

Noun

flow of charged particles, mainly protons and electrons, from the sun to the edge of the solar system.

southern lights

Noun

the bright bands of color around the South Pole caused by the solar wind and the Earth's magnetic field. Also known as the aurora australis.

South Pole

Noun

fixed point that, along with the North Pole, forms the axis on which the Earth spins.

Encyclopedic Entry: South Pole

static electricity

Noun

motionless electronic charge that builds up on a material.

stream

Noun

body of flowing water.

Encyclopedic Entry: stream

stream gradient

Noun

measurement of how steep a riverbed is.

sub-Saharan Africa

Noun

geographic region located south of the Sahara Desert in Africa.

summit

Noun

highest point of a mountain.

sverdrup

Noun

measurement of the strength of ocean currents.

temperature

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

Encyclopedic Entry: temperature

thermal

Noun

rising current of warm air.

topography

Noun

study of the shape of the surface features of an area.

tropics

Noun

region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

Encyclopedic Entry: tropics

updraft

Noun

rising movement of gas.

upwelling

Noun

process by which currents bring cold, nutrient-rich water to the ocean surface.

Encyclopedic Entry: upwelling

vacuum

Noun

area of empty space.

volume

Noun

space an object occupies.

White Nile

Noun

tributary of the Nile River flowing from the highland rivers of Burundi to Lake Victoria and meeting the Blue Nile to form the Nile River at Khartoum, Sudan.

wind

Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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