Atmospheric circulation is the flow of the atmosphere around Earth. It is caused by temperature differences in the atmosphere and the rotation of the Earth, which transfers heat from warm zones (the tropics) to cooler zones (toward the poles). The poleward circulation due to heat transference (mostly in the form of convection), gives rise to large-scale eddies such as cyclones and anticyclones, low-pressure troughs and high-pressure ridges. The eddies also take part in the longitudinal atmospheric circulation around the Earth, with the Earth's rotation maintaining easterly winds toward the equator and westerlies toward the poles. Narrow jet streams blow swiftly over middle latitudes, usually horizontally east in the stratosphere, and move farther toward the poles during the summer.