What does cotidal mean?
"Cotidal" is a technical adjective used in physical oceanography and navigation to describe places that experience the same stage of the tide at the same time. Its most familiar application is in the phrase "cotidal lines": curves drawn on tidal charts that connect all points where high water (or another given tidal phase) occurs simultaneously. Because the rise and fall of the sea travels across an ocean as an extremely long wave, cotidal lines reveal the wave's progress — sweeping around basins, rotating about amphidromic points, and arriving at neighbouring ports minutes or hours apart. Mariners and coastal engineers rely on such charts to predict tidal behaviour along complex coastlines. Formed transparently from the prefix "co-" ("together") and "tidal," the word is precise and self-explanatory once parsed, yet it remains firmly confined to specialist writing rather than general conversation.
adjectiveHaving or denoting the same tidal phase; used of places that experience high or low water at the same time. In oceanography, a cotidal line on a chart joins points where high tide occurs simultaneously.
- Relating to or connecting points that experience a given phase of the tide at the same time, as in cotidal lines on tidal charts.
"The cotidal lines on the Admiralty chart showed how the tidal wave sweeps up the channel."
"Cotidal lines radiate outward across the North Sea from the amphidromic point near its centre."
"Because the two harbours are nearly cotidal, their high waters occur within minutes of each other."
Draw one line through every port whose high tide arrives at the same moment and you've drawn a cotidal line — the invisible contours of an ocean-wide wave.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cotidal
Cotidal combines the Latin-derived prefix "co-" ("together, jointly") with "tidal," the adjectival form of "tide." The word "tide" itself comes from Old English "tīd," meaning time, season, or hour — the same root that survives in "eventide," "Yuletide," and "tidings." The compound arose in the context of nineteenth- and twentieth-century tidal science, when hydrographers began mapping simultaneous high-water points as lines on nautical charts. It shares its root not with Latin cognates but with native Germanic kin of "tīd," including the archaic "noontide" and the historical sense of "tide" as a fixed time or season.
How cotidal is actually used
A technical term of physical oceanography and navigation, found chiefly in scientific literature, tide tables, and nautical charts; it is rare in everyday speech and always functions attributively or predicatively as an adjective.
Easily confused with cotidal
Intertidal describes the zone between high and low tide marks, whereas cotidal describes places experiencing the same tidal phase at the same time.