Definition and Units of Wind

Natural wind in the open air is a three-dimensional vector that has the directions of north, south, east and west in addition to vertical components and magnitude (i.e., wind speed). As the vertical component is ignored for most operational meteorological purposes, surface wind is practically considered as a two-dimensional vector.

Wind blowing over the earth’s surface is turbulent, and is characterized by random fluctuations of speed and direction. This can be seen in smoke drifting from a chimney, for example, as it fluctuates from quick to slow and backward, right, left, up and down.   This rapid fluctuation is called gusting.

Wind speed is classified into instantaneous and average types. The average wind speed is the average of the instantaneous wind speed over a ten-minute period. As described above, however, wind speed fluctuates continuously, and measured values of instantaneous wind speed are affected by anemometer response characteristics. Defined below are some units used in wind measurement, with a focus on those related to response characteristics that affect anemometer performance.

A number of different units are used to indicate wind speed, including meters per second (m/s), kilometer per hour (km/h), miles per hour (mph), feet per second (ft/s) and knots (kt). In synoptic reports, the average wind speed measured over a period of 10 minutes is reported every 0.5 meters per second (m/s) or in knots (kt). Table below shows the conversion for these units.

Speed Conversion table

Wind is described in terms of the direction from which it blows, and is given as compass-point expressions graduated into 8 or 16 directions clockwise from true north (Figure below).

Wind direction scale

In synoptic reports, the average wind direction over 10 minutes is reported in the same way as for wind speed in degrees to the nearest 10 degrees using a code number from 01 to 36. By way of example, 02 means that the wind direction is between 15° and 25°. Wind with an average speed of less than 1 kt is termed calm, and its direction and speed are both reported as “00.”

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