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 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 Earth high in the atmosphere.

Air currents are caused by the sun's uneven heating of Earth. As sunlight beams down on Earth, it warms some areas, particularly the tropics, more than others. As 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, Earth's 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 river's steepness as it flows toward its destination can affect its currents. The steepness of a river is called its stream gradient. A riverbed's topography also influences its currents. Topography refers to the surface features of an area. A riverbed's topography can include sandbars, basins, and dams.

The Nile flows north from the high elevations of sub-Saharan Africa to the low-lying areas of Egypt near the Mediterranean Sea. The Nile's 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 the Nile's currents.

Ocean currents are great streams of water flowing both near the ocean's 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 planet 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. Water's 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 United States 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 known matter is 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.