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July-August 2015

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Weather Queries

We have just experienced two mornings with fog. How does it form and why?

Howard Andrews

Marianna, Florida

Fog consists of water droplets or ice crystals suspended in the air close to the ground that reduce visibility to less than 1 kilometer (0.62 miles). Fog is essentially a cloud at the ground. It forms when either the air cools to the dew point temperature, or enough moisture is added to the air to bring it to saturation (humidity is 100%).

There are several types of fog. Ice fog is most often seen at temperatures below −22°F under calm conditions. It consists of tiny crystals, ranging in size from a few micrometers (üm) to 30 üm. (The average width of a human hair is about 100 üm.) The smallest crystals, less than 10 üm in size, are usually droxtals (shaped like a tiny sphere but having facets); the larger crystals are flat hexagonal plates. The fall speed of these crystals is two centimeters per second or less.

Water-droplet fog comes in several categories. First, we'll consider fogs that form when enough water vapor is added to the air to cause saturation. Precipitation fog often occurs on the poleward side of a warm font. If light rain or drizzle forms in clouds above an inversion (a layer in which temperature increases with altitude) and then falls into colder air below, droplets partially evaporate into the cold air and can bring it to saturation. Fog then forms and will persist unless the precipitation captures the fog droplets and washes them out. Thus, a continuous slight drizzle favors this kind of fog. Light wind or calm usually accompany precipitation fog.

Fog drifts over downtown Chicago, from Lake Michigan on July 2, 2011.

Steam fog forms when water evaporates from a warmer surface into colder air. I have seen steam fog rising from plowed fields when a thin layer of slushy snow melted in the sun with no wind. The sun warmed the dark soil, which was moistened by melting snow. The temperature of the wet ground was 32°F, but in the air immediately above, it was 25°F. This resulted in a vapor gradient between saturated ground and unsaturated air immediately above. Vapor passed from ground to air and was rapidly cooled to the dew point temperature. A very shallow layer of fog formed, only a few feet deep. You can see similar effects when snow has almost melted from dark pavement on a cold, windless, sunny morning or when the sun heats a dark wood fence, soaked by overnight rains. Steam fog rises from these surfaces.

The most dramatic example of steam fog occurs when cold air passes over much warmer water. The most pleasant way to experience this is to soak in a hot springs pool on a windless winter day. Wisps of steam rise off the surface of the pool, sometimes in small, rotating columns (vortices) a few inches across. Steam fog also occurs when Arctic air passes off the New England and mid-Atlantic coasts. Here, it is called sea smoke. Under extreme conditions, when the water surface is at least 27–36°F warmer than the air, steam is constantly rising from the surface straight into the clouds above, and large vortices dot the sea surface like columns in a cathedral.

The most common types of fog occur when air is cooled to the dew point temperature. Upslope fog occurs when the wind is blowing uphill. As unsaturated air ascends, forced by the terrain, it cools at the rate of about 5.4°F per 1,000 feet of lift. With sufficient lift, the air will cool to its dew point temperature, and condensation occurs. Usually saturation first occurs in an elevated layer, and a cloud deck forms. Sometimes, however, especially in winter, at night, and over the gently sloping western Great Plains, saturation occurs at the ground and the air turns foggy. In steep terrain, upslope flow quite often leads to fog on the hillsides but not in the valleys.

Advection fog occurs when relatively warm, moist air travels over a colder surface. The air in contact with the surface cools to its dew point temperature, condensation occurs, and fog forms. A breeze accompanies advection fog. The surface may be chilly ocean water. For example, summer fog is frequent off the coasts of New England and Maritime Canada when a warm and humid air mass arrives from the southwest and passes over the water. Figure 1 is an example of advection fog invading downtown Chicago, from Lake Michigan. The surface can also be wet, cold ground. Snow cover is particularly effective in generating fog when maritime tropical air moves over it. Radiation fog results from nocturnal cooling at the ground when the sky is clear and the wind is very light or calm. Because this kind of fog is so common, I will describe it in more detail.

Recipe for Radiation Fog

  • 1. Start with a clear sky. As the sun sets, the heat balance at the earth's surface reverses direction. As the incoming solar radiation drops toward zero, the outgoing infrared radiation dominates. The surface begins to cool. Because the sky is clear, most of the outgoing radiation can escape to space. Atmospheric water vapor, carbon dioxide, and ozone absorb some of the outgoing radiation, but not much. (Clouds, especially low clouds, would absorb most of it and greatly retard surface cooling.) Trees or thick vegetation will intercept some of the radiation emitted at the ground, so an open, grassy area is best for maximizing surface cooling.

  • 2. Cooling alone can cause condensation, but add a source of surface moisture like wet ground, a river, or a lake. Or add snow cover. Any addition of vapor to the air close to the ground will reduce the amount of cooling required for fog.

  • 3. For the most intense cooling at the ground, the air should be still or very nearly so. Wind causes mixing of the air close to the ground. Once a surface inversion forms (the surface temperature can be many degrees lower than the air just a few feet above), any breeze can mix warmer air down to the surface and weaken the inversion. That impedes fog formation.

  • 4. Pooling of cold air. As the near-surface air cools rapidly in the evening, its density necessarily increases. If the terrain slopes, this dense air will drift toward lower elevations and pool in hollows and river valleys and by lakes. Over flat ground, the radiative cooling may be sufficient to generate fog, but the cooling is more pronounced if radiatively chilled air drains to low spots and pools there. Figure 2 is a satellite image of valley fog in the Appalachian mountains not long after sunrise on September 20, 1994.

Formation and Dissipation of Radiation Fog

As soon as the surface temperature falls to the dewpoint, dew will form on the grass. Because the dew comes from vapor in the air, a vapor gradient forms just above the surface. If the cooling of the air just inches above the surface is intense enough, removal of the vapor by dew formation will not be sufficient to prevent condensation in the air. Just the slightest air motion may be enough to saturate the air, and fog droplets form. Once fog becomes a few meters thick, its radiative properties become important. Like clouds, fog absorbs and emits infrared radiation very efficiently. After a while, the top of the fog supplants the ground as the important radiating surface. The ground cools much more slowly because its infrared radiation is absorbed by the fog, but the top of the fog now becomes the focus of radiative cooling. In this way, the fog may deepen with time.

Visible image of widespread valley fog in the Appalachians from the GOES-8 satellite, 1145 UTC (Universal Time Coordinated), shortly after sunrise, September 20, 1994. The coldest air settles in the valleys overnight, and rivers and streams contribute vapor for the formation of fog, which marks the drainage patterns in exquisite detail.

Over land, most fog droplets range from 3 to 15 üm in size. The fall speed of droplets in this size range is one or two centimeters per second. For a fog to persist, the loss of fog droplets by settling out must be balanced by the condensation of droplets near the top of the fog layer, where cooling is maximized.

Radiation fog usually dissipates when the sun comes up. As the sun rises, more and more solar radiation penetrates the fog layer and heats the ground. Until the dew evaporates, the air close to the ground will remain saturated, but then the ground will begin to heat, the inversion will weaken, starting at the bottom and working up, and the near-surface air will begin to stir. This often lifts the fog from the surface and leads to very low stratus clouds for a short time. The fog or low stratus clouds dissipate from the edges inward, as temperature gradients at the boundary between clear air and fog excite mini-circulations that mix drier, outside air with the fog and evaporate it. See, and look for the image “Fog dissipation over Pennsylvania.” This is a nice video animation of valley fog dissipating after sunrise. Higher clouds moving over a fog layer early in the day will slow surface heating and the dissipation of fog.

Weatherwise Contributing Editor THOMAS W. SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at, or by mail in care of Weatherwise, Taylor & Francis, 530 Walnut Street, Suite 850, Philadelphia, PA 19106.       

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