CHAPTER 17 Weather and Climate - Mr. Barrow's Science Center

Jet Streams Imbedded in these wind systems are fast andpowerful jet streams that control many weather processes, suchas storm development. Most important for the United States isthe polar front jet stream, a wind maximum in the westerlieslocated about 12 km above the surface. Its speeds can exceed500 km/h. Major storm tracks follow it as it moves north andsouth with the seasons.High and Low Pressure SystemsThe large-scale weather systems that have the most effect onthe United States are the subpolar lows, westerlies, and the subtropicalhighs. Subtropical highs are relatively stable belts of highpressure near latitudes of 30°. In contrast, sub-polar lows and thewesterlies tend to meander as smaller cells of high and low pressuredevelop. The lows generally develop from a disturbance inthe polar front jet and move eastward with the jet stream.Specific patterns of weather are associated with high and lowpressure cells because of the way air flows around them. In thenorthern hemisphere, winds blow counterclockwise around lowsand clockwise around highs, as shown in Figure 7. In the southernhemisphere, the directions reverse. Lows are associated withrainfall and storms, and highs with calm winds and clear skies.Coriolis Effect Airflow around low orhigh pressure areas results from the netforces acting on the air. First, the pressure gradient pulls the airtoward low pressure. Then an apparent force, called the Corioliseffect, deflects the air to the right in the northern hemisphere.When these forces are balanced, air flows perpendicular to linesof equal pressure. Near the surface, friction slows air and modifiesits direction, turning it slightly toward low-pressure centersand slightly away from high-pressure centers. This causes air torise in the center of lows and to sink in the center of highs.Modeling the CoriolisEffectProcedure1. Put a large, round pieceof cardboard or paper ona turnable surface such asa turntable.2. Hold a ruler just above thediameter of the cardboard.3. Ask someone to turn thesurface while you draw aline across the turningcardboard with your penor pencil against the ruler.4. Repeat step 3 but turn thesurface in the oppositedirection.Analysis1. Compare the outlinesdrawn on the cardboard.2. Explain how these outlinesrepresentthe Corioliseffect.RisingairSinkingairLHFigure 7 At Earth’s surface in thenorthern hemisphere, air moves counterclockwisetoward low pressure andclockwise from high pressure.Infer How does air move when it is highabove Earth’s surface, above each pressuresystem?SECTION 2 Weather 525