CHAPTER 3 



WIND EQUIPMENT 



The usual point for taking observations is 

 at the surface of the earth. When we refer 

 to the weather element "wind," we are speaking 

 of "surface wind." This chapter will deal with 

 various types of equipment used in measuring 

 wind, methods of observing, procedures for 

 recording, requirements for maintenance, and 

 wind computers. You will find a discussion on 

 the various facets of wind, in chapters 12, 13, 

 14, and 15. Upper winds and their associated 

 measuring equipments are briefly discussed in 

 chapters 9 and 10 of this manual. 



WIND 



Wind is air in motion. As air in motion, 

 the wind has four important properties of vital 

 interest to us: direction, speed, character, 

 and shifts. The character of the wind refers 

 to its gustiness and the like; the shifts of the 

 wind refer to its steadiness or unsteadiness 

 in direction. 



DEFINITIONS 



Wind definitions are as follows: 



1. Wind direction. Wind direction is the 

 direction FROM which the wind is blowing. It 

 is reported with reference to true north, and is 

 expressed to the nearest 10 degrees or to 16 

 points of the compass. 



2. Wind speed. Wind speed is the rate of 

 motion of the air in a unit of time. Wind speed 

 can therefore be measured in a number of 

 ways. The Naval Weather Service measures the 

 speed of the wind in knots; that is, it measures 

 the wind in nautical miles per hour. 



3. Gust. Gust is defined as rapid fluctuations 

 in wind speed with a variation of 10 knots or 

 more between peaks and lulls. 



4. Squall. Squalls are defined as a sudden 

 increase in wind speed of at least 15 knots and 

 sustained at 20 knots or more for at least 

 1 m'nute. The occurrence of squalls is indicative 

 of turbulence near the surface. 



5. Peak gust. The highest instantaneous 

 wind speed observed or recorded. 



6. Wind shifts. "Wind shift" is a term 

 applied to a change in wind direction of 45° 

 or more which takes place in less than 15 

 minutes. Wind shifts are normally associated 

 with some or all of the phenomena characteristic 

 of a cold-frontal passage. These phenomena are: 



a. Gusty winds shifting in a clockwise 

 manner in the Northern Hemisphere and 

 counterclockwise in the Southern Hemisphere. 



b. Rapid drop in dewpoint. 



c. Rapid drop in temperature. 



d. Rapid rise in pressure. 



e. In summer: lightning, thunder, heavy 

 rain, and possibly hail. 



f. In winter: frequent rain or snow 

 showers. 



Changes of wind direction may also result 

 from other causes such as katabatic or foehn 

 winds, sea breezes, and thunderstorms. In 

 such cases, the change of direction may be 

 gradual or abrupt, and may or may not be 

 accompanied by significant changes of other 

 weather elements. Wind shifts are reported when 

 believed to be associated with frontal movement, 

 or when considered important for the safety 

 of aircraft operations. 



7. Variable wind direction. Wind direction 

 is considered to be variable when it fluctuates 

 by 60° or more during the period of observation. 



33 



