What is a defining characteristic of jetstreams?

Jetstreams are indeed characterized as high-speed winds that occur at high altitudes in the atmosphere, typically within the tropopause layer, which is near the boundary of the troposphere and stratosphere. These winds are generally found between altitudes of approximately 30,000 to 39,000 feet (9,000 to 12,000 meters) and can reach speeds in excess of 200 miles per hour (320 kilometers per hour).

The defining feature of jetstreams is their formation as a result of temperature differences between polar and tropical air masses, which creates a flow of air that moves from west to east. This high-speed wind plays a crucial role in weather patterns and can influence the movement of weather systems across the globe.

The other options do not accurately represent the nature of jetstreams. They are not located at low altitudes, nor are they bands of low-speed winds. Additionally, while ocean currents can have some impact on atmospheric conditions, jetstreams are primarily influenced by the temperature differences and pressure gradients in the atmosphere rather than being solely influenced by ocean currents.