An explanation of ocean wave types and characteristics.
Wave characteristics
- Crest: the highest point of a wave
- Trough: the lowest point of a wave
- Height: the distance between a wave's crest and trough
- Amplitude: the distance between the crest or the trough to the still water line in between
- Period: the time between successive swell crests
- Frequency: the number of waves that cross a fixed point in a given amount of time
Wind waves
Wind waves are caused by the friction between local winds and surface water.
They are directly caused by the wind in that location.
Below is a 7-day forecast visualized with 2 different Spire WMS layers. The base layer is Significant Wave Height and the contour lines are Wind Gusts in knots. The relationship between wind and waves is particularly evident here, with larger waves (the red areas) often forming in and following concentrated regions of strong winds.
Swell waves
Over time and distance, sustained wind strength and duration build up a large amount of energy beneath the ocean’s surface, forming deeper waves known as swells. Such energy can enable swells to travel thousands of miles across the ocean without changes in height or shape, until they reach a distant shore as breaking waves. Swell waves are an example of gravity waves, or oscillations of matter driven by gravitational force.
(Note: "gravity waves" should not be confused with "gravitational waves". Unlike gravity waves, gravitational waves are disturbances in the curvature of spacetime itself, originally described by Einstein and proven in 2016 by physicists at the LIGO Lab)
Tidal waves
Tidal waves are generated by ocean tides and therefore indirectly by the gravitational forces of the moon and sun. Tidal waves are considered predictable events because ocean tides are predictable events. The terms "tidal waves" and "tsunamis" are sometimes used interchangeably, but this is not technically accurate.
Tsunamis
Tsunamis are long-period oceanic waves driven by gravitational force. They are typically generated by an underwater geological event, such as an earthquake, volcanic eruption, or a submarine landslide. Melting glaciers can also induce landslides which have the potential to generate tsunamis. The resulting abrupt change in sea-surface height from such an event sends a set of long waves propagating outward from the point of origin. As the waves approach the coastline and the water shoals, they are amplified and can be extremely destructive, depending on the shape of the coastline and the bathymetry (the underwater equivalent of topography). In particular, as the tsunami enters the shoaling water, the wavelength shortens, the speed decreases, and the amplitude increases, whilst the period remains constant.
Cross section of a tsunami as its long waves move through the ocean and
compress as they approach the coast. Figure from the National Weather Service
(https://www.weather.gov/jetstream/tsu_prop).
Due to the nature of their creation, tsunamis are not considered to be predictable events. However, Spire Global is actively working on satellite-based solutions that could potentially enable faster detection by providing additional information to augment existing tsunami early warning systems.
Radio occultation being used to detect ionospheric perturbations caused by tsunamis.
Figure from Diane Savastano and Giorgio Savastano.
