On a recent morning, when visibility across the lake at the family cabin did not reach much beyond the end of our dock, my wife innocently asked “Just how does fog form?” For a phenomenon that I’ve seen regularly for more decades than I care to admit, I tripped over my tongue in delivering an answer. The answer should be simple and straightforward, but it isn’t always, if for no other reason than that there are different kinds of fog, caused by different weather and environmental conditions.
What basic fog is isn’t a mystery. It’s the change of water molecules in the air from a gas state to a liquid state, the visible droplets being so tiny and light they can hang suspended in the air. We don’t often think of water as a gas, but that’s what high humidity in the air is: a high concentration of water molecules in their gaseous state. And when conditions are right, those invisible gas molecules condense into visible fog.
Fog isn’t a seasonal thing here, though it tends to be common in the fall of the year. That may be because the conditions that cause one particular kind of fog are common then, too. Radiation fog can form when the air cools more rapidly than earth’s surfaces, not only solid ground, but lakes and wetlands, too. Unlike the height of summer when nighttime temperatures often remain elevated, the fall of the year tends to see more rapidly declining temperatures after nightfall. Heat escaping from these warmer surfaces causes the invisible water vapor in the air to condense into those visible mini-droplets of fog. Typically, this fog dissipates or “burns off” with the arrival of morning sunshine.
We don’t see this often in winter hereabouts because most surfaces at earth level—whether solid ground or water—are frozen. But in places where there is open water during the depths of winter, such as along the Great Lakes, fog can occur with very cold air temperatures. Warm water discharges from power plants, such as near Monticello, Minnesota on the Mississippi, can also cause the same phenomenon. Perhaps you’ve seen photographs of trumpeter swans gathered there in icy January, the fog rolling off the warmer discharge water into the frigid air above these majestic birds.
There are other kinds of fog that differ from the most common we find here. The opposite of radiation fog is “advection” fog. It happens when warm moist air meets and flows over cooler water, and occurs regularly along the U.S. Pacific Coast, from Washington to California. The California Current is a Pacific Ocean current that brings cold water from the North Pacific down along our western coastline. When warm, moist coastal air encounters this cold ocean current, fog can blanket coastal towns and travel corridors.
Minnesota has its own version of advection fog along the North Shore of Lake Superior. Superior is the coldest of all the Great Lakes, and the North Shore is the contact point where Superior’s frigid waters can meet a warmer air mass onshore. Fog is the result, and over a century and a half of commerce contributed to more than a few of the lake’s many shipwrecks. Duluth, the westernmost major Great Lakes port, averages more than 50 days of heavy fog per year.
One of the most spectacularly beautiful manifestations of fog occurs occasionally in winter, when trees and other surfaces both natural and man-made become coated with a layer of ice crystals we call rime ice. It’s natural to think that any moisture-related event in winter will result in snowfall. But while that is most common, it is not always the case. Air masses containing moisture—sometimes supercooled gaseous moisture—move from region to region in winter just as they do in summer. Under the right conditions, generally including relatively still air, this moisture condenses to form fog and freezes on contact with sub-freezing surfaces. The result is a photographer’s dream of crystalline coatings anywhere and everywhere.
Unlike the intermittent clouds of forest fire smoke that have been drifting across much of Minnesota during the last month and more, fog’s danger lies solely in its ability to obscure our environment. The suspended water droplets in fog can make it difficult to see the road ahead when we’re driving, to know when a boat on the water may be on a collision course with our own, or can confuse a pilot searching for an airport runway during an approach for a landing. These are not dangers to be dismissed. But fortunately fog does not pose the threat to our health that smoke particles—sometimes laced with the chemicals of combustible materials other than just trees—can pose to our lungs.
Fog also has a dimension of mystery. Fog has been a prop—often man-made—that has figured in countless horror and thriller movies that date all the way back to silent films, generating suspenseful accompaniment to the Wolf Man, Count Dracula and even Sherlock Holmes walking the streets of 19th-century London.
The mystery that fog provides you and me is far more tame. Nothing more ominous than letting our imagination fill in the details of what we can’t see, and leave us pondering when the fog will clear so we can safely proceed!