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How Come Lows Don't Fill and Highs Smooth as the Winds Blow

It would seem logical that the winds would blow directly from Highs to Lows and the Lows would fill up. But, one of the first things people notice about weather maps is that the winds don't seem to blow directly from High pressure areas to Low pressure areas. And the Lows don't fill up. Check it out on this piece of a weather map. (The arrows with feathers on one side are the way we plot the winds. The arrow points to a station in the direction the wind is flowing.)

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This map is not too clear because of all the data plotted... but the winds certainly aren't blowing directly from the H to the L. They blow sort of towards the Lows but not directly there. The problem is surface friction. Hills, fields, and rivers affect the winds near the ground. Higher up, the surface friction doesn't affect the winds as much, so look at this map.

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This map is a contour map showing height above mean sea level (in tens of meters) where, if there were no mountains, fifty percent of the air molecules would be below the height contours and 50% of the air molecules are above the contours. This is mathematically equivalent to isobars on a constant altitude chart. The Low is over upstate New York and the High pressure areas are off the southwest and southeast corners of the map. ( Also shown are temperatures (oC), dew point depressions, and heights (x10m) of the location where the pressure is 500 mb.)

If this were a topographic map and you put a marble on it in Georgia, it would roll towards Upstate New York.

The important thing to notice is that the winds are parallel to the contour lines and definitely not toward the Low. So winds don't blow from High to Low pressure areas when viewed from a stationary Earth.

The problem is that the wind's target keeps changing because the Earth is turning on its axis. In the hour a hundred mile an hour wind takes to move from Buffalo to Rochester, NY, (about one hundred miles) the Earth has turned 15 degrees on its axis. Imagine, for a few moments, that you have taken a trip into nearby space and you are poised between the Sun and the Earth, looking at the Earth....

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Last Updated: March 26, 2004