Glossary

Tidal coefficient

The tidal coefficient is a single number, between roughly 20 and 120, that French and Iberian tide tables print next to each day's predictions. It summarises how big the tide is on that day, on that coast, in one quick label. A coefficient of 20 is the smallest neap of the year. Around 45 is a typical neap. 70 is an average tide. 95 is a normal spring. 120 is the largest spring of the year — sometimes called marées d'équinoxe in French because the biggest values cluster around the equinoxes.

The convention is most prominent on the French Atlantic coast, where the Service Hydrographique et Océanographique de la Marine (SHOM) publishes coefficients alongside times and heights, and on the Portuguese and Spanish coasts where the same notation carries through. British and US tide tables generally don't show coefficients — they show heights and ranges directly. Both approaches encode the same physics; the coefficient is just a faster label.

For most TideTurtle users the coefficient is a useful piece of context if you've moved to or from France, Portugal, or Spain and have noticed the unfamiliar number on local tide boards. It maps directly to the spring/neap cycle: high coefficient is a spring day, low is a neap, and everything in between scales smoothly. The Atlantic coefficient for a given day is the same number from Brest to Lisbon to Cádiz, because the underlying astronomy is the same — only the local height that 100 represents differs.