|
|
|
|
Wrestling Winds
Thu Apr 07, 2005
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.
|
|
