Photograph by Barry Bishop, National Geographic
  • Jet streams are currents of air high above the Earth. They move eastward at altitudes of about 8 to 15 kilometers (5 to 9 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 Earth. They form because the sun heats the Earth unevenly. As the sun beams down on the Earth, it warms some areas, particularly the tropics, more than others, such as the poles. As the 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 Earth 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 (6 miles), ranging from less than 6 kilometers (4 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 suns rays pass easily through the troposphere. It is not heated directly by the sun, but by the Earth. The troposphere absorbs heat that is radiated from the Earth into the atmosphere. Various gases in the troposphere, such as carbon dioxide, water vapor, and methane, trap this radiated heat and dont 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 Hemispheres summer. It is associated with the heating of the Asian continent and may help bring summer monsoons to the Indian Ocean.

    jet stream
    A jet stream is a very cold, fast-moving wind found high in the atmosphere.

    Pilots Go With the Flow
    Jet streams are so fast and powerful that airplanes have difficulty flying against them. Pilots either fly with the jet stream or above it; they do not attempt to fly against it.

    Into Thin Air
    The freezing, powerful winds that whip the top of Mt. Everest, the world's tallest mountain, are actually jet streams. Jet streams can be so cold, and so strong, that climbers cannot leave the shelter of their tents.

  • Term Part of Speech Definition Encyclopedic Entry
    air current Noun

    flowing movement of air within a larger body of air.

    altitude Noun

    the distance above sea level.

    Encyclopedic Entry: altitude
    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    boundary Noun

    line separating geographical areas.

    Encyclopedic Entry: boundary
    carbon dioxide Noun

    greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

    cloud Noun

    visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

    Encyclopedic Entry: cloud
    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    exosphere Noun

    outermost layer of Earth's atmosphere, beginning at an altitude of about 550 kilometers (341 miles) above the Earth's surface.

    greenhouse effect Noun

    phenomenon where gases allow sunlight to enter Earth's atmosphere but make it difficult for heat to escape.

    Encyclopedic Entry: greenhouse effect
    jet stream Noun

    winds speeding through the upper atmosphere.

    Encyclopedic Entry: jet stream
    latitude Noun

    distance north or south of the Equator, measured in degrees.

    Encyclopedic Entry: latitude
    meander Verb

    to wander aimlessly.

    mesosphere Noun

    region in Earth's atmosphere between the stratosphere and the thermosphere, about 50-80 kilometers (31-50 miles) above the Earth's surface.

    methane Noun

    chemical compound that is the basic ingredient of natural gas.

    monsoon Noun

    seasonal change in the direction of the prevailing winds of a region. Monsoon usually refers to the winds of the Indian Ocean and South Asia, which often bring heavy rains.

    Encyclopedic Entry: monsoon
    polar Adjective

    having to do with the North and/or South Pole.

    polar-front jet stream Noun

    fast-moving winds moving high in the atmosphere above polar and temperate regions.

    pole Noun

    extreme north or south point of the Earth's axis.

    radiate Verb

    to move outward from a central spot.

    reverse jet stream Noun

    fast-moving winds moving west high in the atmosphere above tropical regions.

    season Noun

    period of the year distinguished by special climatic conditions.

    Encyclopedic Entry: season
    stratosphere Noun

    level of Earth's atmosphere, extending from 10 kilometers (6 miles) to 50 kilometers (31 miles) above the surface of the Earth.

    subtropical jet stream Noun

    fast-moving winds moving high in the atmosphere in latitudes of between 30 and 40 degrees.

    temperate Adjective


    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    thermosphere Noun

    layer of the Earth's atmosphere located between 80 kilometers (50 miles) and 550 kilometers (341 miles) above the Earth's surface.

    tropical Adjective

    existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

    tropics Plural Noun

    region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

    Encyclopedic Entry: tropics
    tropopause Noun

    boundary between the troposphere and the stratosphere layers in the Earth's atmosphere.

    troposphere Noun

    lowest layer of the Earth's atmosphere, extending from the surface to about 16 kilometers (10 miles) above.

    turbulent Adjective

    violent or chaotic.

    vapor Noun

    visible liquid suspended in the air, such as fog.