How to read a surf buoy: height, period and direction
Three numbers tell you most of what a swell will do at the beach. Here is what each one means for Southern California.
Offshore buoys measure swell before it reaches the coast. They are the best early read on what the surf will do in the next few hours, and they show whether the forecast is on track.
Height
Buoy height is the significant wave height: the average of the highest third of waves the buoy measured, in open water. It is not the size of the waves at the beach. Surf can be smaller than the buoy reading where the coast is sheltered, and bigger on exposed reefs and sandbars that focus the swell.
Period
Period is the time in seconds between wave crests. It says how much power the swell carries.
- 6 to 9 seconds: local wind swell. Short, weak and often choppy.
- 10 to 13 seconds: mid-period swell from a storm a few days away.
- 14 seconds and up: ground swell from a distant storm. More powerful, better lined up and more likely to wrap into sheltered spots.
At the same height, longer period means bigger surf. A 4 ft swell at 15 seconds usually breaks bigger and cleaner than 4 ft at 8 seconds.
Direction
Direction is where the swell comes from, in compass degrees. Southern California faces different ways along its coast, so direction decides which region lights up.
- West and northwest (270° to 300°): winter swells from the North Pacific. Santa Barbara and Ventura need these, since Point Conception and the Channel Islands block most other angles.
- South and southwest (180° to 220°): summer swells from the Southern Hemisphere. Orange County and San Diego pick up the most.
The Channel Islands shadow parts of the coast from certain angles, which is why two buoys a few miles apart can read very differently.
Timing
Long-period swell travels fast. A 16 second swell moves at about 28 mph in deep water, so a buoy 100 miles offshore gives about 3 to 4 hours of warning. When the period at an outer buoy jumps, a new swell is on the way.
Putting it together
Look for three things: height rising, period long, and direction pointed at your region. Then check the wind, since light or offshore wind keeps it clean. The Buoys page shows all nine Southern California buoys with the last 72 hours, and each region page shows the buoys that matter most for it.