A jetstream is a fast-flowing air current found in the atmosphere, typically at altitudes of 6 to 12 miles above the Earth's surface. These narrow bands of strong winds can reach speeds of over 200 miles per hour and play a crucial role in influencing weather patterns. Jetstreams are primarily caused by the temperature differences between polar and tropical air masses. There are two main types of jetstreams: the polar jetstream and the subtropical jetstream. The polar jetstream occurs near the poles and is associated with stormy weather, while the subtropical jetstream is found closer to the equator and generally brings clearer skies. Both types are important for aviation and meteorology.