Definition
In maritime law, any atmospheric condition that thickens the air, obstructs visibility, and thereby increases the perils of navigation. The legal definition is deliberately broader than the meteorological one: it encompasses not only fog in the strict sense but also mist, falling snow, and other conditions that produce the same navigational hazard. Courts adopted this expansive reading to give effect to the practical purpose of rules requiring vessels to reduce speed, sound signals, and exercise heightened caution whenever visibility is impaired.
Common Language
Modern common usage (Wiktionary): A thick cloud that forms near the ground; the obscurity of such a cloud. Also used to describe a mist or film clouding a surface, a state of mental lethargy or confusion, or a blur on a photographic image.
Historical common usage (Webster's 1913): A second growth of grass; aftergrass. Dead or decaying grass remaining on land through the winter — called also foggage.
Editorial note: The gap here runs in two directions. The maritime legal definition expands on the everyday atmospheric meaning by treating fog as a category of hazard rather than a precise meteorological phenomenon. The Webster's 1913 agricultural meaning — dead winter grass — is entirely unrelated to the legal term and reflects a now-archaic regional usage that has no legal significance; a researcher encountering "fog" or "foggage" in older property or agricultural records should not assume any maritime or navigational context.
Why It Matters in Research
The primary research significance of this term is that courts declined to apply its scientific or meteorological definition. When reading older maritime insurance policies, admiralty decisions, or collision regulations, "fog" should be understood as a legal term of art meaning any condition of reduced visibility — not a narrowly technical atmospheric state. A policy clause excusing liability during "fog" or a navigation rule requiring precautions "in fog" will be construed broadly.
Researchers working in the Law Mind corpus should watch for two things. First, the term appears most heavily in maritime insurance disputes and collision cases, where the question is usually whether a vessel was navigating with sufficient caution given conditions. Second, in older agricultural and property documents — particularly English sources and early American land records — "fog" or "foggage" carries the entirely separate meaning of dead winter grass or aftergrowth, sometimes appearing in disputes over grazing rights or pasturage. The two senses never overlap legally, but the shared spelling can create confusion when sources are not clearly contextualized.
Jurisdictional variation is minimal for the core maritime meaning, which has been applied consistently in federal admiralty courts. The agricultural sense is primarily an English and early American regional usage and is unlikely to appear in modern American legal materials.
Historical Dictionary Support
Both Bouvier and Black agree on the essential point: the legal meaning of fog in maritime contexts is broader than its common or scientific meaning. Bouvier cites Webster's meteorological definition only to reject it as too narrow, noting that fog has been treated as a generic term covering all atmospheric conditions that increase navigational peril and should not be limited to exclude mist. Black's is more specific, listing fog properly so called, mist, and falling snow as all falling within the term's legal scope, and supports the proposition with federal case authority.
Neither dictionary addresses the agricultural meaning of the word, though Webster's 1913 — as a general dictionary rather than a legal one — preserves it. This is appropriate: the agricultural sense had no foothold in legal usage by the time these dictionaries were compiled. Researchers should not expect historical legal dictionaries to flag the agricultural meaning, and should rely on general historical dictionaries when that context arises.
What the historical sources do not address is any statutory codification or regulatory treatment of the term under modern rules of the road or inland navigation regulations, which post-date these editions. Modern maritime researchers should consult applicable navigation rules (such as the International Regulations for Preventing Collisions at Sea, or their domestic equivalents) for the operative definitions in force.
Jurisdictional Note
The expansive maritime definition is a creature of federal admiralty jurisdiction in the United States and has been applied consistently in that context. The agricultural sense of "foggage" is primarily English and early American; it does not appear to have generated meaningful American legal doctrine.