of the high-pressure system is over the area. Windspeed also increases 

 with rising barometric pressure and usually reaches maximum velocity 

 before the high-pressure system is centered over the area (Fig. 45, 19 

 and 25 January). The relationship between barometric pressure and along- 

 shore wind components is shown in Figure 46. As a low-pressure system 

 begins to move into the area (e.g., 13 January), the counterclockwise 

 wind patterns associated with the low result in an increased negative or 

 northerly alongshore wind component. After the low has passed, counter- 

 clockwise winds around the low-pressure center blow onshore creating a 

 positive or southerly alongshore wind component. The mean alongshore 

 wind component for the period was -1.2 miles per hour, with a northerly 

 (positive) maximum of 8.1 miles per hour and a southerly (negative) high 

 of 24.6 miles per hour. The stronger winds associated with polar high- 

 pressure systems result in higher alongshore wind components during the 

 winter than in the summer. Windspeed with onshore-offshore wind com- 

 ponents is compared in Figure 47. As a high-pressure system begins to 

 influence the Plum Island area, the winds shift to the west or northwest. 

 The higher wind velocity associated with rising pressure gives rise to 

 higher offshore wind components. 



Air and water temperatures were more clearly related than any of the 

 variables studied during the winter period (Fig. 48) and were important 

 beach process variables in terms of their influence on beach morphology. 

 The warm temperatures between 9 and 14 January resulted in a lowering of 

 the frost table in the back dune area making a greater thickness of sedi- 

 ment susceptible to eolian erosion. The extremely low temperatures on 

 9, 17, and 26 January caused parts of the beach to become frozen which 

 resulted in lessened erosion on the berm crest and beach face than during 

 warmer periods having similar wave energy. The effects of temperature on 

 beach morphology are discussed in Section VI. 



2. Wave Measurements. 



Wave parameters such as wave period, breaker power, wave height, and 

 breaker depth are determined by local wind conditions to a greater degree 

 during winter than during the summer. The relationship between wind- 

 speed, breaker height, and breaker power is shown in Figure 49. The 

 highest windspeeds measured during the winter period were offshore winds. 

 Breaker-height measurements for 14, 19, and 25 January show an abrupt 

 decrease from previous readings due to strong offshore wind velocities, 

 with gusts measured in excess of 55 miles per hour. Maximum wind veloc- 

 ity measurements were made during the periods immediately preceding the 

 decrease in wave height. Breaker-power calculations for the same dates 

 also show a marked decrease. The effect of these strong offshore winds 

 is considered later. 



Figure 50 shows the relationship between barometric pressure, along- 

 shore wind components, and longshore drift. For example, on the night 

 of 24 January, the barometric pressure began to drop as a low-pressure 

 system moved into the area. The wind circulation on the leading edge 



62 



