How Windulu works
Windulu doesn’t give you raw wind data but an answer: can you get on the water today, where, and with which size, whether you kitesurf, wing foil or foil. Below is exactly how that advice comes about, so you can judge for yourself what to do with it. Because however good the model is: on the beach, you decide.
Which weather models does Windulu use?
Windulu uses the finest weather model per country, sourced via Open-Meteo: Météo-France AROME HD (~1.3 km) for the Netherlands, Belgium and France, ICON-D2 from the German weather service for Germany and DMI HARMONIE for Denmark. It compares that model with ICON-EU and GFS to gauge how reliable the forecast is. For the current moment a real station measurement (KNMI/Buienradar in the Netherlands, RMI, DWD and DMI elsewhere) beats the model, as long as that reading is fresher than 45 minutes. On coastal spots Windulu deliberately reads the model's sea cell, because kiters ride out on the water.
How does Windulu decide go, borderline or no-go?
Hour by hour, up to 4 days ahead, Windulu checks three things in a fixed order: is there daylight (darkness is always no-go), is the wind direction safe (offshore wind is always no-go, cross-offshore only with rescue), and does the wind strength suit your weight, your level and your gear, whether that is your kites or your wing. Go means all three are in order. Borderline: it is doable, but you are at the light or the strong edge of your gear. If you ride several disciplines, each hour counts the best of your sports that is allowed on that spot.
How does Windulu work out which size to grab?
For each kite in your quiver Windulu computes an ideal wind speed: a level constant times your weight, divided by the kite area in m². Around that ideal sits a comfort band of ±15%. The size choice factors in gusts: when the wind is gusty, Windulu advises a smaller kite, because you get overpowered on the peaks and not on the average. For wing foiling Windulu works out your wing size in m² the same way, matched to your weight, and for kite foiling the wind gets going earlier because a foil is far more efficient. Log your sessions and the bands shift along with what works for you.
Why is offshore wind always no-go?
Offshore wind blows from the land out to sea. If you get into trouble, you drift away from the coast. With gear failure or a misjudgement that can be life-threatening. That is why Windulu always gives no-go for offshore wind, however good the wind speed may look. Cross-offshore gets a warning: only attempt it where rescue is available. This is set per spot based on the orientation of the beach.
What is a go window?
A go window is a continuous block of hours in which wind, wind direction and daylight are together suitable for getting on the water for your profile and your sport. Windulu shows the best window of the next day with usable wind, so you know when to go and not just whether you can go.
How up to date is the data?
Model data is refreshed every hour, in step with how often the models publish new runs, live station measurements every 5 minutes, and the spot pages themselves are rebuilt every 5 minutes. A station measurement older than 45 minutes no longer counts as current and falls back to the model.
Why does Windulu sometimes differ from other apps?
That comes down to two choices. On coastal spots Windulu deliberately reads the weather model's sea cell, because you ride out on the water and the nearest land cell there often reads a few knots lower. And where other apps show raw model data, Windulu gives advice, so the same wind can be go for a light rider with a big kite while it is no-go for a heavy rider.
Honest about the limits
Weather models are sometimes off, especially in showery or thermal conditions. That is why Windulu shows a reliability indication when the models disagree with one another, and always shows the underlying numbers: wind, gusts, direction, per hour. The advice is an aid, not a replacement for your own judgement on the spot.