Springs and Neaps

Around new and full Moon, the tide-producing effects of the Sun and Moon reinforce one another and spring ranges occur. Around first and third quarter, the effects partly oppose one another and neap ranges occur. 'Spring' describes the range springing higher and lower; it is unrelated to the season.

A similar spring-neap sequence repeats roughly fortnightly, but the largest range does not occur at one fixed number of days after alignment everywhere. UKHO guidance notes that local spring-tide age can vary from zero to four days. Weather and non-astronomical residuals can also change the observed level without changing the spring or neap label.

For a yacht passage, range matters because it affects available depth, drying height coverage, tidal-stream strength and sometimes the width of a safe time window. Read the day's predicted heights and the applicable spring or neap stream rate instead of inferring them from the calendar alone.