Cold, dense water tends to sink and flow under warmer, lighter water, creating a current. The colder and saltier the water is, the denser and heavier it is. Water's density is affected by its temperature and salinity, or saltiness. Differences in seawater density also cause ocean currents. 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. 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. Winds can also contribute to upwelling, or currents that move cold, nutrient-rich water from the bottom of the ocean to the surface. Prevailing winds (air currents) that blow over parts of the ocean push the water along, creating surface currents. Ocean currents are great streams of water flowing both near the ocean's surface and far below it. The Aswan Dam, in southern Egypt, severely reduces and controls the flow of the Nile's currents. 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 Nile flows north from the high elevations of sub-Saharan Africa to the low-lying areas of Egypt near the Mediterranean Sea. A riverbed's topography can include sandbars, basins, and dams. Topography refers to the surface features of an area. A riverbed's topography also influences its currents. The steepness of a river is called its stream gradient. A river's steepness as it flows toward its destination can affect its currents. River currents are influenced by the volume, or amount, of water flowing in a river. Many factors contribute to the strength of river currents. The force of gravity, which makes the water flow downward, creates river currents. Rivers flow from high points to lower ones and eventually down to a larger body of water. Water Currents A river current is the water moving in a river. The summit of Mount Everest actually pierces the jet stream, creating icy winds at the top of the world. Jet streams are familiar to mountaineers who climb Mount Everest, Earth's tallest point. These warm, dry currents blow from the Mojave Desert and the Great Basin toward the Pacific Ocean. Santa Ana winds are seasonal (fall) occurrences in Southern California. Cooler, heavier air then pushes in to replace the warm air, forming a cool air current. The warmed air expands and becomes lighter than the surrounding air. As Earth's surface is heated, it warms the air just above it. As sunlight beams down on Earth, it warms some areas, particularly the tropics, more than others. Air currents are caused by the sun's uneven heating of Earth. Jet streams are rapidly moving cold currents that circle Earth high in the atmosphere. Birds, like eagles or California condors, often ride these updrafts high into the sky. Thermal updrafts are gentle currents caused by warm air rising. Air currents are winds that move in a riverlike flow in a certain direction. Electric currents flow through power lines or as lightning. Water currents flow in rivers, lakes, and, oceans. Air currents flow in the atmosphere, the layer of air surrounding Earth. The most familiar natural fluid is water. Fluids are materials capable of flowing and easily changing shape. A current is the steady, predictable movement of a fluid within a larger body of that fluid.
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