If the sky are clear and the wind is light, fog is extremely likely. Wind mixing activity is required for fog to form; without wind, dew will form instead of fog. A modest wind will allow the layer of air at the surface to remain near saturation if the surface is near saturation. As this layer moves upward with rising heat it will begin to evaporate, causing condensation on nearby objects.
When there is fog no one thing can be said with certainty about its cause. It may be wind-driven moisture from clouds or raindrops, or it may be ground water vapor. Fog can also be caused by insects that fly into a cloud of insect spray and die. The body heat they release causes more moisture in the air above them to turn to fog.
Fog is most common in warm climates like California, but it can happen anywhere where it is hot enough for water to change state from liquid to gas. Fog is colorless and tasteless, and it does not burn. It only becomes dangerous when someone tries to drive through it without knowing what is ahead.
People have been killed by fog banks, which are low-lying areas of fast-moving dense water vapor that can occur during tropical storms or hurricanes. Fog banks can reach 30 feet high and stretch for many miles.
Fog signals are important warnings that enable drivers to avoid accidents.
High wind speeds mix the air at the surface and higher up in the atmosphere. This prevents fog from forming or dissipating too quickly.
Fog can also be generated by heat rising from below that condenses the water vapor in the air. This occurs most often in areas where there are large bodies of water and/or land masses that can retain heat long after it has disappeared elsewhere. An example is the fog that forms over large bodies of water like the Pacific Ocean or Indian Ocean during the summer months when they are warmest. The heat from the sun causes moisture in the air to turn to liquid water and then into fog as it approaches the earth's surface.
Finally, fog can be created by animals. When animals breathe out carbon dioxide (CO2), they use oxygen (O2) from the air to do so. If there is not enough oxygen, the body tissues start to shut down their activities one by one until they reach the brain first. Without breathing, there is no way to tell how far this process has progressed. However, since brains cannot function without blood flow, once the tissues around the brain stop receiving sufficient oxygen, an animal will usually die soon afterward.
A gentle breeze Wind mixing activity is required for fog to form; without wind, dew will form instead of fog. This removes the need for dew formation since the air is cooled sufficiently for water vapor to precipitate out as rain or snow.
The light from stars, planets, and galaxies is scattered by molecules in the atmosphere and can be seen from far away under clear skies or when clouds are present. The density of these molecules increases with altitude above ground level. At high levels they scatter blue light more than red light, which is why the sky is blue. Below about 5000 feet (1500 m) this effect becomes less significant because the scattering particles become too rarefied to do so.
At middle levels (between 5,000 and 10,000 feet or 1500 and 3000 m) aerosols such as dust particles or chemical compounds found in smoke or spray can also affect how much light reaches the earth's surface. By blocking out light they can cause a twilight condition during the day. At night time they allow enough light to reach the ground that no additional lighting is needed.
Above about 10,000 feet (3000 m) gases such as ozone block out light entirely. There is no light at all at this height.
As a result, the air temperature rises above the dew point temperature, causing the fog droplets to evaporate. As the hours lengthen in fall, we frequently see clear and cloudless sky at night. However, even though it may appear dark out, there are still stars visible during the day thanks to sunlight that reaches the earth's atmosphere. The sun's rays are refracted by the moisture in the air, creating clouds that block out much of its light.
The best time to go outside and view fog is early in the morning, when there is less likelihood that you will encounter other people who might not agree with your choice of activity!
Fog is one of those phenomena that causes many questions without really providing many answers. We know that it is water vapor in combination with air temperature that determines whether or not fog will form, but we cannot say with any certainty what causes it to begin with or why it disappears at sunset. Perhaps someone with more expertise than we can offer will be able to provide some insight into this curious aspect of nature.