What the number means
The drifter reports dominant wave period, the time between crests of the strongest waves in the current sea, in seconds. A short period (a few seconds) means choppy, wind-driven waves. A long period (eight, ten, twelve seconds) means a smoother, more powerful swell that has traveled a distance. Pairing it with wave height gives a fuller picture than either alone: a half-meter sea at three seconds is a different day on the water than a half-meter sea at ten.
It is the dominant period, not an average. It answers which wave rhythm carries the most energy right now, rather than averaging the spacing of every ripple and chop together.
How the drifter measures it
Wave period rides on the same analysis as wave height, so there is no second sensor and no extra step on the water. The drifter senses its own vertical motion, finds true up, and sorts a few minutes of that motion by frequency into a wave spectrum. How we measure wave height walks through those steps in full.
Wave height comes from the total energy in that spectrum. Wave period comes from its shape: the drifter picks out the single frequency carrying the most wave energy, and the period is one divided by that frequency.
The formula. Dominant period is one divided by the peak frequency of the wave spectrum (Tp = 1 ÷ peak frequency). A spectral peak near 0.1 cycles per second is a swell with roughly a 10-second period.
Read the trend, not one reading
The drifter pinpoints the strongest wave frequency precisely rather than rounding it to a coarse step, and reports the period to a tenth of a second. Each reading is a single snapshot of about three and a half minutes of sea, though, and it carries measurement noise: consecutive readings can vary by a second or more even when the swell itself is steady. Read the period from the trend across several readings rather than any single value. If readings hold steady over time, the swell is unusually clean and coherent.
What to expect from a reading
| Property | Behavior |
|---|---|
| Units | Seconds, on the map and in the observation feed. |
| First reading | About three and a half minutes after the drifter starts, the same warm-up as wave height. Before that it reports nothing. |
| Calm water | Reports nothing when there is no clear wave energy to lock onto. A period only appears once there is a dominant swell to measure. |
| Range | Roughly 2 to 12.5 seconds, matching the wave band the drifter is tuned for. |
| Single readings | One reading is noisy; consecutive values can differ by a second or more even in a steady swell. Read the trend across several readings, not one value. |
The companion measurement, from the same motion analysis.
Wave height →