Wind 



Wind has a considerable influence on the coastal zone. This influence may 

 be direct, since wind is an agent causing erosion and transportation. Near the 

 coast, winds often have a predominant onshore-offshore orientation. If winds 

 are blowing offshore, sediments may be eroded and transported from inland 

 areas and then deposited in the coastal zone. Sediments from within the coast- 

 al zone, particularly the dry sand of dunes and beaches,- may be eroded and 

 transported offshore directly by wind. 



The influence of wind is also indirect because wind stress causes the 

 formation of waves and oceanic circulation. Over large areas and long time 

 scales, predominant wind velocities and durations affect wave climate, 

 whereas over shorter time scales, storm winds generate waves capable of great 

 geomorphic activity. Waves generated by wind, in turn, often approach the 

 coast at an angle, leading to longshore currents. Strong onshore winds, if 

 sustained, may also cause increased water levels or setup. 



Wind is caused by a pressure gradient, or horizontal differences in pressure 

 across an area. This, in turn, may be created by differences in temperature, 

 as the pressure, temperature, and density of gases are interrelated. Depending 

 on the scale of the pressure and temperature variations, atmospheric 

 phenomena range from those of global scale, which are generally persistent, to 

 those of local scale and short duration, such as storms. 



On a global scale, the wind systems occur in characteristic belts, being 

 named for the direction from which they blow. Near the earth's surface, the 

 zone about 5° to 25° poleward of the equator is characterized by the northeast 

 trades in the Northern Hemisphere, and by the southeast trades in the South- 

 ern Hemisphere. Wind direction is predominantly westerly between about 30° 

 and 60° in both hemispheres, whereas easterly winds are predominant pole- 

 ward of 60°. While these belts shift somewhat over the year, these create 

 characteristic wind patterns and intensities, which affect wave energy and 

 predominant wave directions, oceanic circulation, and sediment transport by 

 both wind and water. 



Seasonal shifts in wind also occur in some regions, notably in southeast 

 Asia where monsoon winds occur. The monsoon brings strong seasonal 

 onshore winds, which influence the wave environment and circulation of the 

 Indian Ocean (Davies 1980). The monsoon is accompanied by abundant pre- 

 cipitation caused by moist maritime air masses. Other local winds of note 

 include the katabatic winds in cold climates, mountain and valley winds, and 

 the sea and land breezes (Hsu 1988). 



The sea breeze and land breeze are common phenomena that occur because 

 of diurnal temperature differences between land and water. During the day, 

 especially in the summer, the land warms more rapidly than the water. The 



Chapter 2 Relevant Processes and Factors 



