The same 4-foot wave can look entirely different depending on whether it came from a storm 3,000 miles away or a windy afternoon 50 miles offshore. Long-period groundswell waves move in deeper water, wrap around headlands, refract cleanly over reefs, and break with predictable shape. Short-period windswell waves have less energy below the surface, don’t wrap as well, and break with less shape — often closing out on open beaches.
Surfers read the period first, then the height. A forecast that says 6ft @ 8s (windswell) is nearly always worse than 4ft @ 16s (groundswell) at the same break, even though the second number is smaller. That’s because period is a proxy for how much energy the wave is carrying.
NOAA NDBC buoys report both. On a typical report:
A DPD of 15+ seconds points to groundswell; a DPD of 8 or less points to windswell. Between 10 and 14 is mixed, and most real surf days are a combination.
LazySurfer pulls DPD, WVHT, and MWD directly from NDBC — for example, NDBC station 46232 for San Diego spots — and feeds them into the on-device ML model. The model doesn’t need to be told "this is a groundswell" — it learns that your 9/10 sessions tend to share a long-period signature, and matches future forecasts against that pattern.