Jet streams are air currents blowing high above the Earth. These air currents are in the highest part of the atmosphere. Their speeds usually range from 129 to 225 kilometers per hour or 80 to 140 miles per hour. Jet streams are faster in winter when the temperature differences between tropical, temperate, and polar air currents are greater.
At most of the times in the Northern and Southern Hemispheres, there are two jet streams: a subtropical jet stream centered at about 30 degrees latitude and a polar-front jet stream whose position varies with the boundary between polar and temperate air. During summer in the Northern Hemispheres, a reverse jet stream blows toward the west in tropical high altitudes.