Jet streams are currents of air high above Earth. They move eastward at altitudes of about eight to 15 kilometers (five to nine miles). They form where large temperature differences exist in the atmosphere.

An air current is a flowing movement of air within a larger body of air. Air currents flow in the atmosphere, the layers of air surrounding the planet. They form because the sun heats Earth unevenly. As the sun beams down on Earth, it warms some areas, particularly the tropics, more than others, such as the poles. As Earth is heated, it warms the air just above it. The warmed air expands and becomes lighter than the surrounding air. It rises, creating a warm air current. Cooler, heavier air then pushes in to replace the warm air, forming a cool air current. Jet streams are air currents in the highest part of the atmosphere.

The Atmosphere

The atmosphere has a layered structure. From the ground upward, the layers are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere, which merges with thin gases of space. The boundaries between the layers are not sharply defined, and they vary with latitude and season.

Weather occurs in the troposphere. On average, this layer extends to an altitude of about 10 kilometers (six miles), ranging from less than six kilometers (four miles) at the poles to about 20 kilometers (12 miles) at the Equator. The top of the troposphere is higher in summer than in winter. Because the troposphere contains most of the atmospheres water vapor, clouds usually form in this layer. Temperature decreases rapidly in the troposphere as altitude increases.

The sun's rays pass easily through the troposphere. It is not heated directly by the sun, but by Earth. The troposphere absorbs heat that is radiated from Earth into the atmosphere. Various gases in the troposphere, such as carbon dioxide, water vapor, and methane, trap this radiated heat and don't let it escape into space. The warming of the atmosphere through this heat absorption is known as the greenhouse effect.

The boundary between the turbulent troposphere and the calm, cold stratosphere is called the tropopause. Jet streams travel in the tropopause.

Jet Stream

Jet streams are some of the strongest winds in the atmosphere. Their speeds usually range from 129 to 225 kilometers per hour (80 to 140 miles per hour), but they can reach more than 443 kilometers per hour (275 miles per hour). They are faster in winter when the temperature differences between tropical, temperate, and polar air currents are greater.

At most 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. A reverse jet stream blows toward the west in tropical high altitudes during the Northern Hemisphere's summer. It is associated with the heating of the Asian continent and may help bring summer monsoons to the Indian Ocean.