DIURNAL VARIATIONS IN VISUALLY OBSERVED BREAKING WAVES 



by 

 Patrioia C. Pritchett 



I. SEA BREEZE 



Local diurnal winds occurring along a coastline as a result o£ 

 different land and water temperatures are called Zand and sea breezes. 

 The land breeze blows during the night from land to sea and is weaker 

 than the sea breeze, which blows from sea to land during the day. Obser- 

 vations of these phenomena are probably as old as the original settlement 

 of the coasts, and their existence has been reported in all coastal areas 

 except the Arctic. One of the earliest reports on the sea breeze (Davis, 

 Schultz, and Ward, 1890) discussed the mechanism that produces sea breeze, 

 and gave an account of a volunteer observer program studying the phenom- 

 enon (observed on 30 days of a 3-month period) in Massachusetts. 



Depending on prevailing pressure patterns, the sea breeze may form 

 at the shoreline and advance inland in a series of pulsations (Wallington, 

 1960) , or the sea breeze may form offshore and then advance toward the 

 shoreline and inland (Wexler, 1946). 



The frequency of sea breeze occurrence, which depends on latitude, 

 season, and other factors, was estimated to be 30 to 40 percent in the 

 summer for the eastern coast of the United States (Wexler, 1946) . 

 Although the typical characteristics of the sea breeze can be modified 

 by prevailing circulation patterns (Moroz , 1967), the typical sea breeze 

 varies from 1 to 7 hours in duration, and has maximum wind gusts of 30 

 knots. In the midlatitudes , maximum speeds are about 20 knots, and the 

 mean speed is about 10 knots . 



Vertical height of the sea breeze ranges from 1,000 to about 3,000 to 

 4,000 feet (Byers and Rodebush, 1948). Although an average value of 20 

 to 25 miles has been estimated for inland penetration of the sea breeze, 

 up to 200 miles was reported in the tropics (Clarke, 1961; Wallington, 

 1961). However, because of the lack of observation stations little is 

 known about the seaward extension of the sea breeze. Findlater's (1963) 

 report on aircraft studies of the sea breeze indicated that the sea breeze 

 was noticeable 10 to 12 miles out to sea. Hsu (1970) detected the sea 

 breeze 18 miles offshore of the Texas coast. 



Although the sea breeze generally has a low windspeed and a short 

 fetch, the sea breeze does have some effect on waves, e.g., a windspeed 

 of 10 knots blowing over a IQ-mile fetch would result in a 1.25-foot- 

 high wave with a 2.4-second period (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1975). Waves of these dimensions, 

 even when superimposed oh a higher and longer period swell, could be 

 visually observed or detected in gage records. Harris (1972) commented 

 on the existence of a diurnal cycle of wave heights in gage data (see 

 also Esteva, 1972) . The Coastal Studies Institute of Louisiana State 



