NAEP Science Grade 12

Category - Earth Science

The polar jet stream typically has a major effect on the weather of the northern half of the United States. Which of the following is a major factor responsible for increasing the wind speed in the region of the polar jet stream?
  1. The geographical shift in temperature patterns associated with the different phases of the Arctic Oscillation.
  2. The gradual drop in pressure that occurs between the troposphere and stratosphere.
  3. The steep pressure gradient that exists between very cold northern air and warmer southern air.
  4. The seasonal change in atmospheric instability over temperate and tropical oceans.
Explanation
Correct Response: C. Colder and warmer air masses have different densities and consequently different atmospheric pressures. Where cold and warm air masses meet, such as along the polar front, the difference in atmospheric pressure causes wind. The strong pressure gradient along the polar front drives the concentrated winds of the polar jet stream near the altitude of the tropopause. Although fluctuations in the Arctic Oscillation can affect the position and strength of jet stream winds, it is not a major factor driving their development (A). The decrease in atmospheric pressure between the troposphere and stratosphere (B) is also not a major factor driving the polar jet stream. Seasonal changes in atmospheric instability in temperate and tropical regions (D) can be affected by the position of the polar jet stream and subtropical jet stream, but are not a factor in their development.
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