Wrestling Winds
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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 Dave Thurlow for the Mount Washington Observatory 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 decrease in speed, perhaps dropping to calm during the night. Puffy clouds vanish. The wrestling match is over. As is the Weather Notebook, for today. Thanks to contributing writer Keith Heidorn and to Subaru, the beauty of all wheel drive.