Upper-Air WindsUpper-Air Winds• • Flow of air at altitudes above 5000 meters   (16,500 ft)Flow of air at altitudes above 5000 meters   (16,500 ft)• • Found in the upper troposphereFound in the upper troposphere• • A response to the temperature difference   between warm tropical A response to the temperature difference   between warm tropical air and cold polar airair and cold polar air• • Affect the formation, movement, and   decay of surface cyclones Affect the formation, movement, and   decay of surface cyclones and anticyclonesand anticyclones• • Far less complex than the surface wind   circulationFar less complex than the surface wind   circulation

• • Poleward of 15˚-20˚ there is a Poleward of 15˚-20˚ there is a pronounced   westward flow of air known as pronounced   westward flow of air known as the upper   air westerliesthe upper   air westerlies· · a broad circumpolara broad circumpolar   flow of   flow of westerly windswesterly winds· move much more· move much more   rapidly than the  rapidly than the  surface counterparts  surface counterparts· strongest in winter (the low-sun season)· strongest in winter (the low-sun season)· much weaker during the summer (the   high-sun season)· much weaker during the summer (the   high-sun season)

Jet streamsJet streams• • very strong air currentsvery strong air currents• • embedded within the upper air westerliesembedded within the upper air westerlies• • polar jet streampolar jet stream      · located where the warm tropical air      meets cold polar air· located where the warm tropical air      meets cold polar air      · ranges from 40 to 160 km (25-100 mi)      wide and…· ranges from 40 to 160 km (25-100 mi)      wide and…

up to 2 or 3 km (1- 2 mi) deepup to 2 or 3 km (1- 2 mi) deep

• • Subtropical jet streamSubtropical jet stream· · flows above the sinking air of the   subtropical highsflows above the sinking air of the   subtropical highs· best developed in winter when their is the   steepest · best developed in winter when their is the   steepest pressure gradientpressure gradient· often the subtropical· often the subtropical   jet stream completely  jet stream completely

  disappears in the  disappears in the   summer  summer

• • Usually the upper air westerlies, and polar Usually the upper air westerlies, and polar jet    stream, flow in a fairly smooth patternjet    stream, flow in a fairly smooth pattern

• • Oscillations termed Rossby waves Oscillations termed Rossby waves   periodically develop  periodically develop· · cold polar air pushes into the lower   latitudes cold polar air pushes into the lower   latitudes forming troughs of low pressureforming troughs of low pressure· surface weather· surface weather  is most influenced  is most influenced   under   under thesethese   conditions  conditions

· · the oscillations can become so extreme that the oscillations can become so extreme that   they become cut off  they become cut off· · the cells eventually dissipate, and the   pattern returns to the cells eventually dissipate, and the   pattern returns to normalnormal· a complete cycle takes 4 to 8 weeks· a complete cycle takes 4 to 8 weeks· unusually warm· unusually warm   or cold ocean  or cold ocean

  currents seem to  currents seem to   trigger the  trigger the  oscillations  oscillations