The Weather 



23 



fifty miles an hour may mark its path with ruins of unroofed 

 or overturned buildings and fallen trees. 



The Beaufort wind scale is now employed by the Weather 

 Bureau to indicate velocities in forecasts and warnings. The 

 numbers to 12 are given in such forecasts to represent the 

 velocity of the wind. The meaning of the numbers is given 

 in the table. 



Measuring Wind Velocity. Such practical estimates are 

 valid for many purposes. For the exact purposes of science 

 the velocity of the wind is measured by instruments which 

 indicate miles per hour or pounds pressure upon a unit area of 

 surface. The instrument for measuring the strength of the 

 wind is called the anemometer. The term is made up of two 

 Greek words anemos and metron meaning wind-measure. 

 A common form of the instrument is illustrated in Fig. 9. The 

 hollow cups and axis or spindle are made to revolve by the 

 wind ; each revolution is registered by a system of dials so 

 arranged that five hundred revolutions per hour indicate one 

 mile velocity of wind. It is similar to the speedometer of a 

 motor car. 



The velocity of the winds is not shown directly on weather maps, 



