Why Does Fog Keep Forming Next to Cold Ocean Water?

How Can One Country Have Both a Very Foggy City and an Almost Fog-Free One?

The Japan Meteorological Agency’s Kushiro office publishes 30-year averages (1991 to 2020) for summer fog. Kushiro has 47.3 foggy days between June and August, Obihiro (also in Hokkaido, but inland) has 20.8, Sapporo has 0.7, and Tokyo has 0.2. The agency marks the fog figures as reference values [1].

Its explanation is short: warm, moist air from the south is cooled by the cold Kuril Current, which Japanese speakers call the Oyashio, and that makes fog that drifts into the Kushiro area. Fog becomes especially likely when the Pacific high-pressure system strengthens in summer [1].

Here is a way to picture it. A glass of iced lemonade gets wet on the outside because the air touching the cold glass is chilled and its water vapor turns into droplets. Cold sea water does the same thing to the air above it, except the droplets stay floating in the air instead of running down a glass. Meteorologists call this kind of fog advection fog, because “advection” means air being carried sideways, here over a cold surface. NASA uses the same name for the fog off California [2].

That answers the title, but it raises a bigger question. Why is the water in these places cold in the first place?

Why Is the Sea Next to Some Coasts So Cold?

There are two ingredients, and NASA’s Earth Observatory describes both for the California coast. The first is a cold current. The California Current flows south along the Pacific coast from British Columbia, so it carries water that started in cooler northern seas [2].

The second is upwelling. In spring and summer, a high-pressure system over the Pacific strengthens and moves toward the coast, which changes the wind pattern. Winds push surface water away from the shore, and deeper, colder water rises to replace it [2]. You can picture it as sliding a layer of warm water aside so the cold layer underneath shows up at the top.

The result is an ocean surface that is chilly even in summer. Then sea breezes carry air across that cold water, and the air condenses into advection fog [2]. The Japanese case fits the same pattern at the top level, because a cold current meets warm, moist air. I did not check whether upwelling also matters off Hokkaido, so I make no claim about it.

What Pulls the Fog Onto Land in San Francisco?

San Francisco’s summer fog does not just sit offshore. NASA explains that warm air rises off California’s inland Central Valley, and westerly breezes push cold marine air inland to replace it [2]. The fog rides in on that cool air.

So a hot valley and a cold sea work as a pair: warm air rises inland, and cool air from the sea moves in to replace it. The NASA page I read does not say how the inland temperature changes the strength of that flow. That may be why a person can be sweating in one part of California and wearing a jacket in the fog an hour’s drive away, though how far the fog reaches depends on hills and weather, and the NASA page I read gives no distances.

Does it count as fog, or just mist? By international convention, the droplets must cut visibility to 1,000 meters (about 3,000 feet) or less to be called fog [2].

How Can a Desert With Almost No Rain Still Get Fog?

The Namib Desert, along the southwest coast of Africa, gets about 30 mm (roughly 1 inch) of rain in an average year, and some years none at all. The European Geosciences Union’s blog explains that the cold Benguela Current, which brings water from the depths of the Atlantic to the surface, cools the air above it. That makes it harder for the air to carry moisture, which is one reason the land is dry. The same cold upwelling also chills the moisture that is already there, so it condenses into fog that often drifts in from the coast [3].

So a cold sea can be bad news and good news at once. It keeps rain away, yet it sends in fog. Plants and animals there have adapted to use the fog as a water supply [3].

Scientists have also tried to put numbers on this link. A 2026 study in the journal Atmospheric Chemistry and Physics used satellite data from 2004 to 2019 to follow fog and low clouds in two Namib hotspots. In the northern one, the Angolan Namib, years when warm tropical water spread south along the coast (an event called a Benguela Niño) had less fog. The paper reports a correlation of −0.68 and says these events explain nearly half (46 percent) of the year-to-year changes in fog and low clouds there. In the Central Namib, an index of coastal upwelling explained 22 percent [4]. In plain words, years with warmer tropical water moving south tended to have less fog. That fits the idea that cold water is what makes the fog, though a correlation alone does not prove the cause.

The same paper also found more fog when the air above the coast was more stable, meaning a warm layer sits over the cool air near the surface and keeps it from mixing upward [4].

Will Warmer Seas Mean Less Coastal Fog?

The Namib study reached a result that surprised me. Over the long run, it found almost no trend in fog since 1982. It explains this as two opposite effects canceling out: warming of the sea surface reduces fog, while a more stable atmosphere increases it. The authors note that the trends are not statistically significant [4].

That is one region and one statistical model, so I would not stretch it to other coasts. What the paper does show is that fog depends on more than one thing. What will happen to coastal fog as the climate changes still looks like an open question from what I read.

How Can You Check This Yourself?

• Find Kushiro, Obihiro, Sapporo, and Tokyo on a map. Which are next to the cold Pacific current, and which are inland or on a different coast? Does that line up with the fog numbers?
• Next time you have a cold drink on a warm day, watch the outside of the glass. You are seeing the same cooling-and-condensing step that makes advection fog.
• If you live near a coast, look up its sea-surface temperature in summer. Is the water cold, and is fog common? What is the wind doing on foggy mornings?

Sources

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