Marine observations are published in units and conventions that assume you already know them. This page is the glossary this site would otherwise be pretending you do not need.
Wave height is not the wave you see
The figure labeled wave height is significant wave height — the average height of the highest one third of waves in the sampling window. It is a statistic, not a measurement of any individual wave. The practical consequence is that roughly one wave in ten will be noticeably larger than the number shown, and the largest wave in a few hours may be close to twice it.
Period matters more than height
Dominant period is the time in seconds between wave crests, and it says more about where the energy came from than height does. Short periods — under about eight seconds — usually mean local wind chop generated nearby. Long periods, thirteen seconds and up, mean groundswell that has travelled from a distant storm, arriving organized and carrying far more energy per foot of height.
Four feet at seven seconds and four feet at sixteen seconds are not the same ocean.
Tide is measured from an agreed zero
Every water level on this site is referenced to MLLW — mean lower low water, the average of the lower of each day’s two low tides over a nineteen-year period. It is a bookkeeping datum, not the sea floor. A reading of zero means the water is at that long-run average, not that the water is gone, and negative readings are ordinary.
Nineteen years is not arbitrary: it is close to the period over which the moon’s orbital geometry repeats, so it averages out the full cycle rather than a convenient slice of it.
A buoy reading is not a beach reading
Every buoy here sits in open water, some of it many miles offshore. Between the buoy and any particular stretch of sand, swell refracts around headlands, shoals over reefs, and is blocked entirely by islands. Port San Luis and Santa Monica are the clearest cases on this coast: both sit in shadows that reduce what actually arrives to a fraction of what is recorded offshore.