Often imbedded in upper tropospheric jet streams are elongated regions of very strong winds called "jet streaks". This term is used because of the streaky appearance of these regions on upper air weather charts. These streaks may be several hundred miles long, but only a hundred miles wide. Winds within these streaks may reach 200 mph. Meteorologists have found that these regions help explain how surface weather systems can develop and be maintained.
During the early part of the 20th century, meteorologists had developed elaborate models that attempted to explain why mid latitude low pressure systems developed along frontal boundaries. Most of their observational data was from surface weather reports, since sparse upper air measurements were available to altitudes that were usually below 20,000 feet. However, during the late 1920's, a British meteorologist (Henry Dines) proposed a theoretical model that suggested a link between surface wind flow and upper tropospheric wind flow. The horizontal convergence of surface wind flow into the surface low pressure systems could be maintained if a compensating horizontal divergence in the upper troposphere existed. Likewise, near surface divergence of the wind flow from a surface high pressure cell could be maintained if upper tropospheric convergence were sufficient to provide the compensation. Vertical cross sections of a version of this theoretical model appear in Figures 8.14 and 8.15 of Part A: Narrative.
Following the discovery of the jet stream near the end of World War II and the accumulation of a sufficient data set of upper tropospheric wind observations, the proposed theoretical model was verified. Meteorologists found that quite often surface low pressure systems in the mid latitudes developed below regions where an upper tropospheric jet streak was detected. These observations provided the needed compensating mechanisms for the development and maintenance of surface weather features.
Air parcels that move along with the upper tropospheric winds in the jet stream will accelerate when they enter the rear of a jet streak and then decelerate when leaving the front end of the jet streak. As a result of this acceleration and deceleration, regions of horizontal divergence and convergence develop within this jet streak as the air winds its way through the streak. Figure 9.13 of Part A: Narrative provides a graphical depiction of the distribution of such features in the core of the jet streak. Vertical circulation regimes develop that link these upper level features with the surface pressure patterns. Specifically, a region of upper level horizontal convergence of air produces sinking motion of the air in the atmospheric column below the jet streak, contributing to the surface divergence of air from a surface high pressure cell. Conversely, rising motion from a surface low pressure cell would appear below a region of upper level divergence. This model extends the hand twist model that you have applied to the surface weather features in order to explain why fair skies and little precipitation typically occur with surface highs, while clouds and precipitation tend to appear with lows.
Often, if you look at a surface weather analysis and find a distinct surface low pressure feature - such as a mid latitude storm system, you may be able to see a jet streak above that location on an upper level chart, such as the 300 mb chart that can be accessed from the Online Weather Homepage.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.