If fog closes in around your boat, start by slowing down: every vessel is required to hold a safe speed for the conditions of restricted visibility, not just the cruising speed you had set a moment ago. Alongside that: get everyone into a PFD, switch your navigation lights on (required in restricted visibility at any time of day, not only after dark), post extra lookouts who watch AND listen, and know that restricted visibility also requires its own sound signal. If you can still fix your position, do it now, then head for open water clear of any shipping channel — a vessel following one may be navigating by radar alone. Fog itself forms when warm, moist air moves over colder water and cools to saturation; checking a forecast's visibility outlook before you leave the dock is what keeps most encounters from happening at all.
A clear day can close down to almost nothing once fog rolls in. The shoreline you were steering by simply isn't there anymore, and neither is much of anything else past a small circle of grey water around your boat. Fog isn't rare or exotic — it has a routine physical cause, and once you understand that cause, checking for it becomes part of planning a passage rather than an afterthought. The harder problem is what to do if it catches you out anyway, with the shore gone, other traffic somewhere out there, and nothing certain except the deck under your feet. This article walks through why fog forms in the first place, the drill for getting caught in it, why a radar reflector helps but doesn't solve the problem, and what's left to steer by once every landmark has disappeared.
How Fog Actually Forms
Fog at sea almost always comes down to one mechanism: warm, moist air moving over water that's cold enough to cool it down to saturation. Two independent national meteorological services describe it the same way. The UK Met Office explains that fog happens when moist, warm air passes over a colder surface and is cooled until it can no longer hold its moisture, forcing it to condense into the tiny water droplets you see as fog. Canada's own aviation meteorology reference describes the identical process — warm, moist air moving over a colder surface, cooling to full humidity — and notes that coastal areas see more of it than inland ones for exactly this reason. This is advection fog, the kind a sailor runs into most, and it's not unpredictable: it shows up in a marine forecast's visibility outlook before it shows up on the water. Checking that field before you leave the dock, alongside the wind and sea-state fields you already check, is the single move that keeps most fog encounters from happening in the first place.
Caught in Fog: The Response Drill
If fog does close in around you, there's a clear place to start: slow down. Every vessel is required to hold a safe speed for the conditions of restricted visibility, not just the cruising speed you had the throttle set to a moment ago — it's the one step here with a clear claim to coming first, since it buys you time for everything else. The rest don't have to happen in any particular order, but they all need to happen: get everyone into a PFD; switch your navigation lights on, since they're required in restricted visibility at any time of day, not only after dark; post extra lookouts who watch AND listen, not just look; and know that restricted visibility requires its own sound signal too. If you can still fix your position, do it while you still have a reference to fix it from, then head for open water clear of any shipping channel — a vessel following one may be navigating by radar alone in these conditions, so standing off matters more, not less, once you can't see who else is out there.
Why a Radar Reflector Isn't a Guarantee
A radar reflector genuinely helps a ship's radar pick a small boat out of the clutter — and on a boat with little metal in its hull, that help matters. Most small boats, especially fibreglass ones, return a very weak echo to radar at best, which is exactly the gap a reflector is meant to close. But fitting one isn't the same as being seen. Even a well-fitted reflector can leave a boat "not readily visible" on a ship's radar in some circumstances, and reflectors carry their own blind angles — nulls — where they return almost nothing at all. How well one performs in practice also depends on how it's mounted and how far the boat happens to be heeled at the moment a ship's radar sweeps past. Carry a reflector. Mount it well. Then navigate as if a ship might still not see you, because it might not.
Steering by Compass When the Shore Disappears
Once fog has closed in and the shoreline is gone, there's a real question: what do you actually steer by? Every other reference sailors normally use — a landmark ashore, a transit between two fixed points — depends on being able to see something solid, and in fog, you can't. What's left is the compass card and lubber line: the same instrument you've likely used only as an occasional check while steering to a landmark, now doing the whole job on its own. The lubber line marks your boat's own heading against the card beneath it; hold that relationship steady and you hold your course, with nothing else left to sight. This is exactly why the habit of trusting your compass has to already be built before the fog arrives, not learned for the first time in the middle of it — a sailor who's only ever steered by landmark can find the compass feels like a backup gauge, right up until it's the only reference left.
None of this is complicated once the pieces are in place — fog isn't a mystery, and the drill for getting caught in it is short enough to run through in your head before you ever need it. What takes real preparation is having the habit already built: checking the forecast's visibility outlook before you leave the dock, trusting your compass as more than an occasional glance, and knowing what to work through if the horizon disappears anyway. SailStarter's lesson on fog and poor visibility walks through the same response drill as an interactive diagram, tied back to the forecast-reading and compass-steering skills you'd actually lean on the moment visibility drops. If you sail anywhere fog can form — which, given how ordinary its cause is, is nearly everywhere on the water — this is worth having down before conditions make you find out the hard way.