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Wrestling Winds
Mon May 26, 2003
Listen in RealAudio 
An old folk saying tells us: 'Winds of the day do wrestle and fight, longer and stronger than
those of the night.' That is, daytime winds are stronger and gustier than those occurring at
night. It's true. In the absence of a dominating weather system like a front or a storm, the
industry of wind is strictly a day job. Hi, I'm Bryan Yeaton, and this is The Weather
Notebook.
The 'wrestle and fight' or gustiness, is a characteristic of airflow known as turbulence. A
smooth, steady wind flow like that over a lake has low turbulence. High turbulence is caused
when wind flows over and around objects such as rocks and trees, or buildings in a city. But
our daytime 'wrestle and fight' is caused mostly by the heat from the sun.
On a clear or partly cloudy day, the sun will heat the ground until the air in contact with it
becomes warmer and lighter than the air above. A condition meteorologists call 'unstable.'
Every so often, a bubble of warm surface air breaks away and rises rapidly into the
sky.
The upward movement of the bubble and descent of replacement air from above cause a great
mixing of air, increasing the local surface turbulence and wind speed. Thus, daytime winds
gust to higher speeds, change directions, and then fall back to calm at irregular intervals
throughout the day.
Just before sunset, when the sun no longer heats the ground, the mixing diminishes and then
ceases altogether. The surface wind becomes less gusty and decreases in speed, perhaps
dropping to calm during the night. Puffy clouds vanish. The wrestling match is over.
Our program is a production of the Mount Washington Observatory and is supported by Subaru of
America and the National Science Foundation. Special funding comes from Davis Instruments,
makers of the Vantage Pro wireless weather station.
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