Welcome to You Ask Andy

Lisa Burns, age 11, of Harrisburg, Penn., for her question:

HOW IS THE SPEED OF WIND MEASURED?

We have wind when the air above the earth's surface moves. Sometimes the wind may blow so gently that we can hardly feel it. Other times it may blow so fast and hard that it will smash buildings and uproot trees.

Wind speed is measured with an instrumental called an anemometer. A number of different kinds are used but the most common has three or four cups attached to spokes on a rotating shaft. The spokes turn the shaft as the wind blows.

Wind speed is then indicated by the speed of the spinning shaft.

In the United States, wind speeds are indicated in miles per hour, or in knots. Knots are nautical miles per hour. In many other countries, however, wind speed is states in kilometers per hour.

You use an instrument called a weather vane to measure wind direction. A weather vane has a broad, flat blade attached to a spoke pivoted at one end. Wind blowing on the blade turns the spoke so that the blade lines up in the wind direction.

Wind direction may be indicated by an arrow fastened to the spoke, or by an electric meter remotely controlled by the weather vane.

Wind directions are often indicated by using the 360 degrees of a circle. On this circle, north is indicated by zero degrees. An east wind blows from 90 degrees while a south wind blows from 180 degrees. A west wind blows from 270 degrees.

Winds at various altitudes often differ in speed and direction. For example, smoke from a chimney may be blown northward while clouds higher in the sky are blown eastward.

Winds high above the earth's surface are measured by sending up helium filled balloons. A balloon moves with the same speed and direction as the wind. The movement is then measured either by sight or by radar.

The Beauford Wind Scale is a series of numbers, ranging from zero to 17, used to indicate wind speeds. The scale was devised in 1805 by British Rear Admiral Sir Francis Beauford.

Beauford defined the numbers in terms of the effect of various winds on sailing vessels. In a standard version published in 1874, for example, the number two indicated a wind defined as "That in which a well conditioned man of war, with all sail set, and clean full, would go in smooth water from one to two knots." Twelve was defined as "That which no canvas could withstand."

Today, the Beauford scale is defined in terms of wind speeds measured 10 meters or 33 feet above the ground. Zero indicates a calm condition with winds blowing less than one mile per hour. Between 12 and 17 indicates hurricane force with wind blowing above 74 miles per hour.

On the Beauford scale, two indicates a light breeze blowing between four and seven miles per hour. Three is called a gentle breeze while four is called a moderate breeze. Five is called a fresh breeze and the wind blows between 19 and 24 miles per hour.

Seven is called a moderate gale with winds blowing between 32 and 38 miles hour. It is a strong gale when a nine is reached on the scale and wind blows between 47 and 54 miles per hour.

 

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