A swell is a cluster of wind-generated waves. Winds blowing hundreds of miles off the coast may generate some of the finest waves on the globe, and surfers can't wait to check them out. Swells, on the other hand, are despised by sailors because they capsize vessels. There are two types of swells: long waves and short waves.
Long waves are formed when winds blow over large bodies of water. These waves travel far across the sea before breaking. The most famous long wave swells come from hurricanes or typhoons. They can be very dangerous because their power is spread out over such a wide area that even small boats can be overwhelmed by them.
Short waves are smaller versions of long-wave swells. They're created by winds hitting shallow waters like bays and inlets. Because they move so quickly over hard surfaces, they're much more powerful than long-wave swells of the same size. They can also carry much closer to shore because there's less distance for them to go before hitting land. Short-wave swells are found everywhere winds blow near the surface of the water, but they're particularly common in areas where there are steep cliffs or where there's significant precipitation right after a storm.
When waves of both types reach shore, they combine to form a swell. The height of this combined wave is called its peak height. The larger the swell, the farther it can travel before breaking.
If the ocean surf approaches the beach at an angle, as it usually does, the waves that reach the shallow water first will break. As the rest of the wave approaches shallower water, the break will move parallel to the coast. The first wave that breaks is called the forebreak.
The reason why the first wave breaks is because it has reached the shore first and its energy is still high. The next one would be less powerful because some of its energy has been lost through friction with the sand. The last one would be the weakest because even more of its energy has been lost already.
When a wave reaches the shore, part of its energy is transferred to the surface of the water and part is lost in friction. The lost part is called the wave's energy budget. Energy is also lost due to evaporation if the water is humid or not. At very low temperatures (below 15 degrees Celsius) ice can form on top of the wave which increases its height but decreases its period (the time it takes for one wave crest to follow another). This phenomenon is called "frosting" and the ice that forms is called "sleet".
Foremost among the factors that determine how far a wave can travel before breaking are its wavelength and its frequency.
Low-pressure systems, in principle, are responsible for producing nice and forceful waves. If winds generated by low-pressure systems continue to blow over the ocean's surface for an extended period of time, the swells will be larger because the energy from the wind is collected in the waves being produced. These large swells are called storm waves.
Winds that are too weak to produce significant waves are called calm winds. Waves are also affected by other factors such as water temperature, depth, and shoreline contours.
Large waves can be dangerous if a person is not careful. If a swimmer does not know how to react to changing conditions in the water, they may have a bad experience. Swimmers should always check local surf conditions and use their own judgment when deciding whether or not to go in the water.
Here are some basic rules for swimming in big waves:
If there is a lifeguard on duty, always let him or her know your plan to go in the water.
Make sure someone knows where you are going in the water.
Have a plan in case something goes wrong.
Know your limits. Don't try to swim in conditions beyond your ability.
Use caution around rocks, reefs, and other hazards.