Answer to "wind gust" question

WIND GUSTS-- Ann Detweiler and Mark Leichty of Corvallis, OR wanted to know. "Why the wind blows in gusts." -- 1/1/98

Even though the weather report says the wind is blowing from the north at 7 mph, it's pretty unusual for the wind to actually blow at a steady speed. In fact, according to National Weather Service operating procedures, the reported speed is a two-minute average wind, which includes instantaneous winds that are both above and below the average. It's only when the peak wind is significantly above the weakest wind that "gusts" are reported.

Wind (or air in motion) is created by changes in temperature and/or pressure. Winds generally blow outward from high pressure systems toward low pressure systems. Sea breezes, however, occur when the cooler air over nearby waters blows onto warmer land. Open your refrigerator door and feel how colder air tries to displace warmer air.

If pressure and temperature forces were uniform, the wind would blow steadily. However, this isn't the case. Changes in the character of the Earth's surface (e.g., trees vs fields, oceans vs sand dunes, and buildings of different sizes and shapes) act to create small variations in winds; these are called eddies. When winds of different speeds blow adjacent to each other, the wind shear pattern changes winds even more.

To visualize this, fill a small bowl, a pie plate, or other small container with coffee. Pour in some milk and gently stir the coffee in a more or less uniform manner for a second. Stop stirring. Observe what happens to the milk. First, there is the effect of a high speed "wind" moving through the fluid. Then there is the effect of the fluid (or wind) moving past the spoon.

Although you stirred the coffee-milk mixture with a uniform motion, there are some parts of the coffee where the motion is less (because you aren't stirring there). You'll also see places where the coffee develops tiny swirls with faster or slower motions. Differences in motion (caused by the stirring, friction with the edges of the bowl, etc..) create wind shears. These in turn can generate small swirls or 'eddies" which have greater or lesser motions associated with them. These are analogous to the "gusts" you experience on a windy day.

Try this with anything you stir - for example, water while taking a bath, or a cake batter with various ingredients. Watch the smoke patterns that result from blowing out a candle. Watch how leaves blow around obstacles or the edges of buildings. Watch how water flows past a rock. These all show the turbulent patterns or eddies that can create gusty winds.

Next, line up next to one or more people and hold hands. Then have everyone take a step forward. Assuming a more-or-less uniform step, you'll still be lined up next to each other. Repeat. Still lined up, right? This represents a "constant" wind.

Then, have one person take a small step, while the others take a larger step. This represents different forces and different winds, much like the initial stir of milk in the coffee. Repeat. Notice that with each successive step, the orientation of the initial line starts to pivot.

Take a look back at the Question