A jet stream is a fast-flowing air current found in the atmosphere, typically at altitudes of 6 to 12 miles above the Earth's surface. These streams flow from west to east and can influence weather patterns by affecting the movement of high-pressure and low-pressure systems. Jet streams are primarily caused by the temperature differences between the equator and the poles. They can vary in strength and position, impacting flight times for airplanes and playing a crucial role in the development of storms and other weather phenomena.