Frequently Asked Questions
Common questions about surf forecasts and ocean conditions.
What is swell period in surfing?
Swell period is the time in seconds between successive wave crests passing a fixed point. Longer periods indicate more powerful, organized waves that have traveled farther from the generating storm. A period above 14 seconds is powerful groundswell; 10–13 seconds is solid mid-range swell; below 8 seconds is short-period wind swell that breaks quickly and lacks power. Swell period is one of the most important numbers in a surf forecast.
What does significant wave height (Hs) mean?
Significant wave height (Hs) is the average height of the highest one-third of waves over a given time window. It is the standard measurement used in marine and surf forecasts. Because it represents the average of the largest waves, individual waves at a surf spot can be 20–30% higher than the reported Hs. Offshore Hs values are typically lower than the wave face height a surfer sees at the beach, which is shaped by local bathymetry and breaking angle.
What does offshore wind mean in surfing?
Offshore wind blows from the land out to sea, holding up the face of breaking waves and creating clean, hollow, well-shaped surf. It is the opposite of onshore wind, which blows into the wave face and makes waves crumble and close out. Light offshore winds (under 15 km/h) are ideal for most surf spots. Cross-shore winds blow parallel to the beach and have a mixed effect depending on the angle.
What is the difference between groundswell and wind swell?
Groundswell is generated by distant storms hundreds or thousands of kilometres away. It travels long distances across the ocean, organizing into long-period (14+ seconds), powerful, evenly-spaced waves. Wind swell is created by local winds near the coast and produces shorter-period (under 10 seconds), choppier, less powerful waves. Groundswell produces the most desirable surf conditions and is what surfers look for when planning big sessions.
How do you read a surf forecast?
A surf forecast shows wave height (Hs), swell period, swell direction, and wind. For good surf, look for: wave height above 0.5m; swell period of 10 seconds or more for power; a swell direction that aligns with your break's orientation; and offshore or light cross-shore winds for clean conditions. Tide stage matters too — many spots break best at low or mid tide. The combination of these factors determines the overall surf quality rating.
What is surf climatology?
Surf climatology uses years of historical wave data to identify seasonal patterns at a surf spot. It shows which months have the highest average swell height, the longest swell periods, and the most consistent conditions. Surf climatology helps plan surf trips by identifying peak season — when conditions are reliably good — versus off-season, when swells are rare or weak, at any given location.
How is wave height measured in surf forecasts?
Surf forecast wave heights come from physics-based ocean wave models that compute how wind energy at sea translates into waves. The main global models are ECMWF (European Centre for Medium-Range Weather Forecasts) and GFS (NOAA Global Forecast System). Groundswell uses Open-Meteo's ECMWF-based marine wave model for offshore significant wave height, and where available compares model output against real-time NOAA NDBC buoy measurements to verify accuracy.
What makes a good surf day?
A good surf day combines the right wave size for your level (0.5–1.5m Hs is ideal for most surfers), a swell period of 10 seconds or more for organized, powerful waves, offshore or light winds for clean conditions, and a tide stage that suits your local break. Swell direction matters — the same swell can be perfect at one beach and flat at another 5 km away. Checking all these factors together gives you a complete picture of the day.
What is swell direction and why does it matter?
Swell direction is the compass bearing from which waves are travelling, expressed in degrees (315° means the swell is coming from the northwest). Direction matters because every surf break has an optimal window of directions that allow the swell to reach it, stand up cleanly, and break correctly. A swell from the wrong direction may be blocked by a headland, arrive at the wrong angle for the reef, or pass without producing any surf at all. Checking the swell direction against your break's exposure is as important as checking the size.
How do tides affect surf conditions?
Tides change the depth of water over a surf break, directly affecting how waves break. A reef break that produces hollow barrels at low tide may become a flat, unsurfable wall at high tide when there is too much water over the reef. Conversely, an exposed reef can be dangerously shallow at extreme low tide. Beach breaks are more flexible but still have preferred tide stages. Most experienced surfers learn which tides work best at their local spots and plan sessions around the tide chart. Groundswell displays tide predictions for every surf spot in the forecast.
What is the difference between wave height and wave face height?
Wave height in a surf forecast is significant wave height (Hs) — the average of the highest one-third of waves measured offshore. Wave face height is what a surfer sees and rides at the break: the vertical measurement from trough to crest as the wave breaks. Face height is almost always larger than offshore Hs because the energy is amplified as the wave enters shallow water. The relationship varies by break type, swell period, and angle. A 1.5 m Hs swell at 15 seconds on a steep reef can produce 3–4 m faces.
How accurate are surf forecasts?
At 1–3 days ahead, modern ocean wave models are highly reliable — typically within 10–15% of actual conditions for open-ocean swells. Accuracy decreases at 5–7 days and beyond, particularly for rapidly developing storm systems. Local factors such as sandbar movements and harbour effects are not captured by global models and require local knowledge to interpret. Groundswell publishes its own forecast accuracy data at groundswell.surf/accuracy so you can see exactly how model predictions compared with real buoy measurements.
How far in advance can surf be forecast?
Global ocean wave models produce data up to 16 days ahead, but reliability drops significantly beyond 5–7 days. At 1–3 days out, swell arrival timing can be predicted within a few hours and swell height within around 10–15%. At 7 days, forecast uncertainty roughly doubles. At 14 days, forecasts are best understood as a probabilistic indication of whether a significant swell is likely — not a precise prediction of size or timing. Groundswell provides 10-day forecasts for standard accounts and 16-day forecasts for premium subscribers.
What is a surf rating score?
The surf rating on Groundswell is a composite quality index from 0 (flat or unsurfable) to 10 (epic). It combines swell height, swell period, swell direction relative to the break's optimal window, and wind direction relative to the shore. For the 27 world-class breaks in Groundswell's calibration database, the rating also accounts for each spot's specific directional preferences, minimum swell size, and minimum period requirements. The rating is a quick decision-making tool, not a replacement for reading the full forecast.
What is a swell window?
A swell window is the range of directions that a surf break is exposed to and able to receive cleanly. A point break on the north side of a headland might only receive northwest to north swells (290°–350°) and be completely blocked from southerly swells. A beach break on an open coast might have a much wider window of 200°–360°. Knowing your break's swell window tells you immediately whether an incoming swell will produce surf there at all.
What causes waves to close out?
Waves close out when the entire length of wave breaks simultaneously rather than peeling progressively from one end. Closeouts are caused by the swell period being too short for the beach's shape, the sandbar being too straight or too shallow, the swell arriving at too direct an angle, or the swell being too large for the break to handle cleanly. Closeouts are unsurfable because there is no open face to ride. Tide stage, swell direction, and swell size all influence whether a break closes out on a given day.
What is the best time of day to surf?
Early morning is usually the best time to surf. Sea breezes (onshore winds) typically build through the day as land heats faster than the ocean. Mornings are calmer, winds are lighter, and the surface is often glassy. In many surf destinations the ideal window is dawn to mid-morning, before the sea breeze fills in. This is a general pattern that varies by location and season. Groundswell's hourly wind forecast lets you identify the exact calm window for any given day and spot.
What is a set wave?
A set wave is one of a group of larger waves that arrive periodically between quieter periods called lulls. Ocean swells travel in groups — the energy from a storm arrives in pulses rather than a continuous stream. A typical set contains 3–7 waves, with lulls of a few minutes between sets. The biggest waves of the day are usually the last waves in a set. Experienced surfers position themselves further out to catch set waves, while smaller waves break closer to shore during lulls.
What is the difference between a reef break, beach break, and point break?
A reef break is a wave breaking over a submerged rock or coral reef. The hard, fixed bottom produces consistent, often hollow and powerful waves — Pipeline and Teahupo'o are classic examples. A beach break is a wave breaking over a sandy seafloor. Sandy bottoms shift with tides and currents, so beach breaks are less predictable but accessible for all skill levels — Hossegor and Huntington Beach are beach breaks. A point break wraps around a headland and breaks along it, producing long, peeling rides — Rincon, Jeffreys Bay, and Anchor Point are classic point breaks.
Does Groundswell work for any surf spot in the world?
Yes. Groundswell provides surf forecasts for any coastal location on earth by querying open ocean wave model data in real time. In addition to the 220+ curated surf spots in the directory — with hand-verified coordinates and calibrated surf quality ratings — you can search for any beach, town, or coordinates using the search bar. Whether you are surfing a well-known break or an unnamed beach on a remote coast, Groundswell returns a 10-day wave, wind, and tide forecast for that location.