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Salem Clock Shop - 1085 Broadway Street NE, Salem, OR 97301 - (503) 581-3803 Fax: (503) 581-3331 |
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An explanation of how the winds influence weather forecasts. |
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Wind |
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Differences in air pressure cause the winds to blow. These imbalances in force cause air to begin moving from an area of high air pressure to an area with lower pressure at the same altitude, whether it's at the Earth's surface or 50,000 feet up. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the pressure gradient force.
High and low pressure are relative. There's no set number that divides high and low pressure. |
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Once air begins moving, the Earth's rotation causes it to follow a curved path; this force produced by the Earth’s rotation is called the Coriolis force. |
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The Jet Stream: The jet stream is a current of fast moving air found in the upper levels of the atmosphere. This rapid current is typically thousands of miles long, a few hundred miles wide, and only 1-2 miles thick. Jet streams are usually found some-where between 6 to 9 miles above the earth's surface. The position of this upper-level jet stream denotes the location of the strongest SURFACE temperature contrast between the warm and cool air masses which shape the weather.
They generally move west to east, and are strongest in the winter with core wind speeds as high as 250 mph. Changes in the jet stream indicate changes in the motion of the atmosphere and weather. |
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In 1835, Gustave-Gaspard Coriolis, a French scientist, first described the effect mathematically. In simple terms, what's going on is similar to what would happen if you tried to draw a straight line on a rotating turntable. Unless you were infinitely quick, the resulting line would be curved. As air begins flowing from higher to lower pressure, the Earth rotates underneath it, and the wind follows a curved path over the Earth’s surface. In the Northern Hemisphere, the wind curves to the right . |
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Low pressure areas are steered by the jet streams. In the example at right, the low pressure area currently over Idaho has been moving steadily southeast toward southern Utah & Colorado. It is highly likely, however, that the low pressure area and it’s associated weather, will veer northeast toward Montana and Wyoming instead. A home weather forecaster in St. George, Utah, knowing this, might confidently predict continued fair weather. |
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Jet stream crossing Cape Breton Island—Maritime Provinces, Canada. Photographed from space (NASA) |
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Is it possible for the home weatherman to find the position of the jet streams with hand-built instruments? Nope! No one even knew they were there until recently.
As World War II was approaching its end, the United States introduced the first high-altitude bomber, the B-29. It could fly at altitudes well above 20,000 feet. B-29 pilots reported wind speeds of 170 knots from the west. The jet stream, as it would come to be known, was discovered. |
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Actually, atmospheric scientists had theorized the existence of jet streams as early as 1937. Today, almost every weathercast mentions the positions of jet streams and their impact on weather events. So, unless a rare view (such as the one in the photo above) allows you to see the effects of the jet stream directly, you will need to rely on the weather pros. |