A River of Fog

In an earlier blog, I shared some of my photographs of the night sky. I ended that discussion by introducing a new direction that I took in my astrophotography, making time-lapse videos of the night sky. I showed two of my videos featuring the apparent rotation of stars due to the earth’s rotation, one view from the Northern Hemisphere, and the other view from the Southern Hemisphere. In a subsequent blog, I shared a dozen other time-lapse videos, including two that I captured during the day in which clouds developed and diminished. I finished that second blog with a comment that I had attempted a time-lapse video of fog in Red River Gorge, and I promised to take the subject up again in a future blog. Here is that video.

I made this video from nearly 600 personal photographs over 2 hours and 40 minutes on the morning of September 27, 2021, captured with my Nikon D5600 camera with a f/2.8 14mm lens. All exposure times were three seconds at ISO = 6400. The illumination was from the waning gibbous moon. The video ends at dawn, when the increasing light of approaching sunrise began to overexpose the images. The view is to the west-northwest from Chimney Top Rock. The valley below is that of the north fork of Red River, with the direction of flow from right to left. Chimney Top Rock is 400 feet above the river. I was quite taken aback by the motion of the fog in the same direction, following the meanders of the river. I’ll explain that presently.

I was so encouraged by the results that I made a second video from 1,969 photographs spanning nearly nine hours that I took on the night of October 18–19, 2021. This time, the illumination was from a full moon (astronomical full moon was 28 hours after the last photograph was taken). Since the moon was brighter this night than on the morning of September 27, the exposure time of each photograph was only one second rather than three seconds. Otherwise, all camera settings were the same. Since this sequence began so early, fog did not form until about one third of the way into the video. The occasional flashes of light in the trees are from vehicles traveling along KY highway 715 on the other side of Red River. Late in the video, the moon enters from the upper left. It is greatly overexposed, so it looks like a blob of light. About this time, dew that had formed on the camera lens slightly blurred the images as well. As the moon approached setting, its light diminished, making the fog less visible. The video ends shortly after moonset, about an hour before sunrise.

As before, the fog flowed downstream, but there were a couple episodes of partial clearing and some brief back flow. Why did the fog behave this way? To answer that question, I need to discuss what fog is and how fog forms and dissipates.

Some Weather Basics

Water is the word we use for H₂O in the liquid state. What do we call H₂O in the other two states of matter? Solid H₂O normally is called ice, while gaseous H₂O may be called steam or water vapor. When water is exposed to air, some of the water evaporates to form water vapor in the air. We are so familiar with this phenomenon that we often forget that evaporation requires energy (think of applying heat to water to cause it to boil). Where does the energy of evaporation come from? The energy required from evaporation comes from heat within the water. Therefore, as water evaporates, heat is extracted from the water. Evaporation slightly cools the remaining water. Or, if the water is thinly distributed on a surface, the evaporation can cool the surface. This is how perspiration cools our skin. Our body temperature regulation system is designed to take advantage of this phenomenon.

There is only so much water vapor that can be dissolved in air. Once the maximum amount of water vapor that air can hold is reached, we say that the air is saturated. The percentage of how much water is dissolved in air compared to the maximum amount of water that could be dissolved in the air is called the relative humidity. When air is nearly saturated (high relative humidity), water is not inclined to evaporate. This is why we feel uncomfortable in warm air that has high relative humidity—as perspiration forms on our skin, it fails to evaporate, leaving us both warm and sticky. On the other hand, dry, warm air has the capacity to absorb more water vapor, and evaporation of perspiration happens quickly, leaving us cooler and dry.

The maximum amount of water that air can hold depends upon its temperature. Warmer air can hold more water than cooler air. The dew point is the temperature at which the current amount of water in the air would result in the air being saturated. As air is cooled, the relative humidity increases. Once the dew point is reached (100% humidity), the air cannot hold any more water, and water will condense on any surface present. Since condensation is the opposite of evaporation, and evaporation requires energy, then condensation liberates energy. Therefore, condensation is a heating mechanism. We often see condensation happen with containers of cold drinks, such as a glass of ice water. If the ice cools the glass to the dew point, then water begins to form on the outside of the container. The liberation of energy in this process slightly warms the container, preventing the container’s temperature from falling any lower than the dew point.

Much weather is driven by the processes of evaporation and condensation. Another important factor in weather is heating and cooling. Most heating comes from sunlight striking the ground, while most cooling is caused by the radiative loss of heat from the ground when the sky is clear. On a clear day, sunlight warms the ground. As the ground heats, conduction transfers some of its heat to the air that is in contact with the ground. As air in contact with ground warms, it expands, making it less dense than the air around it. Being less dense, parcels of warm air become buoyant and rise. With increasing elevation, air temperature generally decreases. As the rising air cools, its relatively humidity rises. Eventually, the rising air reaches the dew point, and tiny droplets of water condense upon microscopic solid bits suspended in the air (condensation nuclei). When enough droplets form, the air becomes opaque, and we say a cloud has formed.

Cumulus clouds are some of the most interesting types of clouds to watch. Most of the clouds in the two daytime videos in my previous blog were cumulus clouds. Those clouds seem to magically form, but as they continued to develop, they appeared to dissipate too. What causes this dissipation? Some portions of the clouds reach drier air, and the droplets that formed earlier evaporate. If the condensation rate exceeds the evaporation rate, then a cloud grows, but if the evaporation rate exceeds the condensation rate, then a cloud shrinks. Often there is a steady state between the rates of condensation and evaporation. It is this roiling action of cumulus clouds that I find fascinating to watch.

How and Why Fog Forms

Fog is merely a ground-level cloud, so much of the physics is the same as with clouds, though there are some differences in how the physics plays out. For instance, unlike clouds, fog generally does not rise due to a fundamental difference in the conditions in which fog forms. Fog usually forms when cool air passes over a warmer body of water. This situation often arises on clear nights in autumn. The water is still warm from the warm days of summer. On a clear autumn day, the ground and the air in contact with the ground are still heated, but not as efficiently as it was on summer days. Some of the warm water evaporates into the warm air in contact with the water. Late in the day and after sunset, the ground is no longer heated by the sun, but the ground radiates heat upward, effectively cooling the ground. The cooler ground in turn chills the air that is contact with the ground. Cooler air contracts, making it denser than the air above it. This causes the air to sink rather than rise as during the day. The lowest lying areas occur over streams and ponds. As the cooler air arrives over the water, it mixes with the warmer air already there, cooling the air above the water. As the air cools, it eventually reaches the dew point, and fog forms. The air above is warmer, so there is no buoyant force on the lower lying cooler air, so, unlike clouds, fog remains in the lower areas.

Fog can form any time of the year when the conditions are right. But the conditions for fog formation commonly exist on clear autumn nights, which is why fog is so common on clear nights in autumn. Here in Northern Kentucky/Cincinnati, the Ohio River is an entrenched meander; fog often forms along the river on clear autumn nights. When the fog is especially thick, the fog fills the valley and begins to encroach into the areas on the bluffs along the river, such as the Creation Museum’s location. I live a few miles away at a slightly higher elevation than the Museum. On clear autumn mornings, I often leave the house in no fog but encounter fog by the time I arrive at work. A few hours after sunrise, light from the sun warms the air to above the dew point, and the fog dissipates. With knowledge of how fog forms, I can anticipate on which nights fog is likely to form, which is how I knew to attempt the time lapse video on October 18–19. I was going to spend the next day in Red River anyway, so I drove down and camped out the night before.

Why There Was a River of Fog

As previously stated, I was initially surprised when I saw fog flow downstream in the first video. However, a little reflection (and a suggestion by a meteorologist I know) helped to explain that phenomenon. There is a gradient in elevation going downstream. It is this down-hill slope in the valley that causes the water to flow one direction. Being a liquid, water is a fluid, and fluids are free to flow. Gravity carries the fluid downstream. Air is a gas. Many people don’t realize that gases are fluids too, subject to gravity as much as any other matter, including liquids. Remember that it is the denser, cooler air descending the slopes of the valley at night that caused fog to form in the first place. But once the cooler air reached the bottom of the valley where the water was, there was still a slope going downstream, so the cooler air flowed down the valley, taking its suspended water droplets (fog) along with it.

What about the dissipation of fog that sometimes appears in these videos? Just as with clouds, water droplets in fog can evaporate when they encounter warmer and drier air. In the second video, a backwash appears from the left a few times, sometimes briefly producing an eddy. Off to the left is the valley of Chimney Top Creek, a small tributary of Red River. There is not enough water in the creek to produce much fog. Any fog there likely flowed up the creek from the river. However, a downward flow of cooler, drier air along the creek usually prevents much fog from creeping up the creek. I suppose that the downward flow of the cooler, drier air in the creek drainage is a bit stochastic, resulting in occasional light gusts that briefly interrupt the normal flow of fog down the river.

In the videos, you can occasionally see thin sheets of fog suspended above the main body. This is due to a more humid layer of air briefly finding itself above the main body of fog, with less humid air sandwiched in between. This is a bit unstable, so these sheets of fog don’t last very long.

Both liquids and gases are fluids, but there is one difference. Gases are compressible, while liquids generally are not. Compressibility means that when pressure is applied to a substance, the volume of the substance decreases. Both solids and liquids will compress very slightly when great pressure is applied to them. But gases readily decrease their volumes when pressure is applied. Consequently, volumes of gases can be greatly reduced by applying pressure. This makes for efficient storage of gases used in industry. Perhaps in a future blog I will further discuss fluids and the laws that describe their behavior.