CLIMATIC FACTORS : THE AIR 101 



be noticeable for several miles inland. This brief summary of fac- 

 tors producing wind should serve to emphasize that air is almost 

 constantly in motion and should suggest that, within limits, the 

 general plan of motion is predictable for seasons and parts of the 

 earth. 



Measurement of Wind.— Wind velocity is measured with some 

 form of anemometer. The cup anemometer used by the United 

 States Weather Bureau has three or four hemispherical or conical 

 cups, each attached to horizontal arms that rotate on a vertical axis 

 and thus drive a gear system, which turns indicator dials. These 

 are readable in miles per unit of time, usually expressed as miles 

 per hour. More elaborate instruments may be equipped with auto- 

 matic recording devices. 



The cup anemometer is inconvenient to carry and operate in 

 the field. In the Biram portable anemometer, a small fan drives the 

 dial indicating air movement. The device is useful in small spaces 

 and for short readings. Since it has no vane, it must be set to face 

 the wind. 



Physiological-Anatomical Effects of Wind.— The movement of 

 air being in general characteristic of all environments, plants are 

 largely unaffected by it under average conditions. In certain situa- 

 tions, however, wind may be an extremely important factor. Plants 

 growing in habitats exposed to continuous winds of moderate 

 velocity transpire more rapidly than unexposed individuals. If the 

 prevailing winds are from one direction, the side of a plant toward 

 the wind may be so desiccated that new growth is killed before it 

 is well begun. Lateral buds taking over the growth may or may 

 not survive, and a scrubby, matted growth develops on the wind- 

 ward side. To leeward, the new shoots are protected by the rest of 

 the plant, and growth goes on there, resulting, over a period of 

 years, in asymmetric growth forms of amazing shape. Such one- 

 sided growth is commonly found in exposed places at high alti- 

 tudes in the mountains where otherwise upright plants may be 

 prostrate and form mats fitting into hollows or behind protecting 

 rocks. Not uncommonly a forest stand on the protected side of a 

 ridge or in a ravine may appear as though every tree had had its 

 tip sheared to an exact height limit. Again, this is due to the desic- 

 cating effect of the prevailing wind. 



