Glossary of surf terms: G

GMT
Greenwich Mean Time. The Greenwich Meridian is located at 0 degrees longitude, over a town named Greenwich in England. GMT is World Time and the basis of every time zone in the world. It is fixed all year and does not switch to daylight savings time. All other time zones have a GMT correction to determine local time. For Standard time, New York is minus 5 hours from GMT, and California is minus 8 hours. So if GMT were 1200, California would be 0400 Pacific Standard Time. GMT is also sometimes called Zulu Time, especially on weather charts which may display 12Z for 1200 GMT (Noon), or 00Z for 0000 GMT (Midnight).

Gale Warning
A warning when sustained surface winds are reported or forecasted to be in the range of 34 to 47 knots over the water.

Glassy
A windless surf condition in which the texture of the ocean surface is ultra-smooth, like glass.

Going Off
When the surf is very good and firing or pumping. Also refers to a surfer who is surfing particularly well, i.e., "Kelly Slater's going off."

Great Circle
The shortest distance between two points on a curved or spherical surface like the Earth, which is actually a curved line when projected on a flat surface like on a Mercator chart. These lines are called Great Circles. Swells travel in Great Circles around the Earth. As an example, if you take a string and extend it between two points on a globe, you can see a good representation of a Great Circle.

Groundswell
A swell with a swell period over 11 seconds between successive waves. As a rule, the harder the wind blows in a storm, and the longer it blows over a longer distance of ocean, the bigger the swell will be and the longer the swell period will be between successive waves. The longer the swell period, the deeper the swell energy extends below the ocean surface, which interacts more with the ocean floor, or the "ground" so to speak. This is contrary to a windswell, which has a shorter swell period, and is always generated by local winds with brief duration and over a limited distance of ocean. Groundswells with longer swell periods can wrap (refract) greatly into many spots due to deeper interaction with the ocean floor compared to shorter period wind swells.

Group Velocity
The forward speed of a swell, or wave group. In deep water, it is equal to 1.5 times the swell period between successive waves in the wave group. The waves within the wave group move twice as fast as the overall wave group at 3 times the swell period. If a swell or wave group has a swell period of 20 seconds, the individual waves will be moving at 60 knots, while the group as a whole will be moving forward at 30 knots. As each wave moves forward within the wave group and reaches the front of the group, it will fall back to the rear and repeat the cycle.