air expands and rises as it warms, forming a belt of lower pressure along the 

 coast. The pressure gradient between the water and land causes a sea breeze 

 in which air from the sea blows landward. The opposite happens at night. 

 The land cools more rapidly because it is less efficient than water at storing 

 heat. The air over the sea has lower pressure, resulting in vertical 

 convection. The convection causes the land breeze to flow seaward. 



Cyclonic Disturbances 



Centers of relatively low atmospheric pressure, also known as cyclones, 

 are associated with windy, cloudy, and wet weather. In contrast, 

 anticyclones, or high pressure weather patterns, are generally associated with 

 calm, dry weather. Cyclones occur in many sizes and forms, including 

 continental-size extratropical cyclones, tropical cyclones of varying intensity, 

 and tornadoes. All are capable of causing significant geomorphic change, 

 because of the winds, waves, and storm surges associated with them. The 

 intensity of cyclonic winds is controlled by the pressure gradient, or change in 

 pressure measured along a line at a right angle to the isobars. Low pressure 

 may also occur as elongated troughs, which range in scale from a few hundred 

 kilometers to continental. 



Extratropical cyclones, also known as wave or mid-latitude cyclones, are 

 transient features that develop in various stages. Initially, they develop along 

 the polar front, a narrow zone separating the cold, polar easterly winds from 

 the warmer, mid -latitude westerly winds. A wave or indentation forms 

 between the cold and warm air masses, causing the cold air to invade warmer 

 territory and the warm air to enter colder territory along sharply defined 

 fronts. The cold air masses, being denser, force up the warm air as they 

 move along the surface, whereas the lighter warm air masses move over the 

 colder air masses. The cold air masses eventually dominate the surface after 

 all of the warm air is forced off the ground. This cuts off the source of 

 moisture and energy, causing the system to die. 



Central pressures in extratropical cyclones vary greatly, with lows of 

 940 mb' (compared to average sea level pressure of 1013.2 mb), although 

 typically they do not fall below 980 mb. In both hemispheres, the most 

 intense extratropical cyclones occur in the winter, with less intense systems 

 developing in fall and spring. Cyclonic winds blow counterclockwise in the 

 Northern Hemisphere, and clockwise in the Southern Hemisphere. Thus, 

 depending upon the relationship of the storm orientation and track to the 

 coast, there are several possible wind sequences (e.g., onshore winds followed 

 by offshore winds) that affect wave, current, and sediment transport patterns 

 (Niedoroda et al. 1984). Typically, the systems are transported with the 

 westerly flow and move toward the east, although their tracks depend on 



A table of factors for converting non-SI units of measurement to SI (metric) units is presented on page x. 



Chapter 2 Relevant Processes and Factors 



