What if fog never lifted?

Fog is like a cloud on the ground. It is made of tiny drops of water.

Most fog goes away by lunchtime. What if it never went away? Let's imagine it.

On a foggy morning, houses and trees seem to fade into a soft gray cloud. That is exactly what fog is: a cloud touching the ground, made of tiny water droplets floating in the air. Morning fog often disappears within a few hours after sunrise.

What if fog never lifted? In this thought experiment, you can follow three paths: how fog forms and why it usually goes away, what travel is like in fog, and how people in dry places catch water from fog with nets.

The National Weather Service defines fog as water droplets suspended in the air at Earth's surface, often hazardous when visibility drops to a quarter mile or less. It forms when air becomes saturated by cooling, by added moisture or by mixing with another air mass.

Most fog is temporary because sunshine and wind remove the conditions that made it. This thought experiment asks what a world of permanent fog might be like. Real places offer clues: coasts where fog lingers all day, forests that drink it, and deserts where people harvest it. The imagined world itself is reasoning, labeled as ideas.

On a clear, calm night, the ground cools down. The air touching it cools too.

The cool air makes tiny drops of water. That is fog! In the morning, the Sun warms the ground, and the fog fades away.

Many morning fogs form on clear, calm nights. The ground loses heat to the sky, the air touching it cools, and when that air cannot hold all its water vapor, tiny droplets form. In hilly places, cool air slides downhill and collects in valleys, so valley fog is often thickest around daybreak.

After sunrise, sunshine warms the ground, the air warms too, and the droplets evaporate. The National Weather Service says valley fog usually disappears within a few hours after sunrise.

The NWS glossary lists several types. Radiation fog forms when the ground radiates heat away and cools the air above it below its dew point. Advection fog forms when warm, moist air flows over a colder surface, as sea fog does over cold water. Upslope fog forms as moist air is pushed up a slope, and steam fog when very cold air drifts over much warmer water.

Radiation fog favors clear skies, light wind and damp ground; NWS forecasters note that with no wind at all, dew forms instead, while strong wind mixes in drier air. Fog clears when the air is warmed or dried: sunlight heating the ground, warmer air arriving, or mixing with drier air above.

Let's pretend the fog never goes away.

We might see only a little way ahead. Cars might drive slowly. Plants might get wet from the fog every day. This is only pretend.

Now imagine a town where the fog never lifted. You might only see a short way down the street. Cars, ships and planes might need to move slowly and carefully every single day.

The gray sky might keep things cool and damp, and less sunshine might reach gardens. But fog also carries water. In some foggy places, trees catch fog drips, and in dry deserts, people already catch drinking water from fog with nets. These are ideas, worked out from what fog does today.

Permanent fog might act like a lid of cloud at ground level. Visibility limits would make travel by road, sea and air slower and more cautious; NWS notes that fog already contributes to accidents and aviation delays. Less direct sunlight might keep the ground cooler, which could itself help the fog persist.

Fog would also become a steady water source. Redwood forests already take about 40 percent of their moisture from fog, and Chilean villages collect thousands of liters a day with nets. A world of endless fog might therefore suit fog-drinking ecosystems and fog collectors while challenging transport. All of this is reasoning, not prediction.

Fog is made of tiny, tiny drops. They are too small to fall as rain.

When the air warms up, the drops turn back into invisible water vapor. Then the fog is gone!

Fog droplets are tiny, smaller than 40 micrometers, too small to fall as rain. They stay floating as long as the air stays cool and damp.

When sunlight reaches the ground, the ground warms the air above it, and the droplets evaporate back into invisible water vapor. A breeze can also mix drier air into the fog. Fog lasts longer where the sunshine is weak or where cool, moist air keeps flowing in, such as along cold ocean coasts.

Fog droplets, typically under 40 micrometers, fall too slowly to reach the ground as rain. A cubic meter of desert fog may hold only 0.05 to 0.5 grams of liquid water, so at least 2,000 cubic meters of fog contain one liter.

NWS training material lists how fog dissipates: heat carried up from warm ground, warmer air moving in, drier air mixing in, or cloud spreading above that reduces cooling at the fog top. Solar heating is the usual cause after sunrise. Fog persists where these fail: where cold water keeps chilling incoming air, or where moist air keeps being supplied, as on summer coasts with cold currents offshore.

Some places have fog almost every day in summer.

Tall redwood trees drink water from the fog. In a dry desert, people catch fog water with big nets.

At Point Reyes in California, the National Park Service says the beaches and headlands often have heavy fog all day in July, August and September. Farther north, along California's coast, redwood trees get about 40 percent of their water from fog dripping off their branches.

In Chile's Atacama Desert, the driest desert outside the polar regions, fog rolls in from the cold Pacific. Villagers string nets in the mountains to catch it. In the village of Chungungo, nets supply about 10,000 liters of water a day.

Point Reyes National Seashore shows near-permanent fog in miniature: inland valleys clear by midday in summer, but the headlands, often 20 degrees cooler, can stay fogged in all day from July to September. In Redwood National and State Parks, fog accounts for about 40 percent of the trees' moisture intake.

In the Atacama, fog banks called camanchacas roll inland off cold Pacific currents, but their droplets are too small to rain. A professor began experimenting with nets there in the 1950s, and polypropylene mesh collectors now supply villages: about 10,000 liters a day in Chungungo, and about 100 liters per landowner per day in Peña Blanca from roughly 140 square meters of mesh. Similar projects now exist in many countries.