Fog forms when air becomes supersaturated with moisture, and can be advection or radiation fog. Fog can obscure visibility and affect the senses. London fog was actually smog, a form of pollution.
Fog is a cloud in contact with the ground. Just like clouds in the sky, it forms when the air becomes supersaturated, meaning it can no longer hold moisture in the form of vapor. As a result, water rushes out of the air, forming a fine mist of water droplets. If the air becomes saturated enough, the droplets will turn into drizzle or rain. The foggiest place on Earth happens to be Grand Banks, an area off the coast of Newfoundland.
The hotter the air, the more moisture it can hold in the form of vapor. Fog and clouds, therefore, form when the air cools rapidly and the humidity in the air adheres to particles such as dust or sea salt. Clouds form because moist air cools as it rises in the Earth’s atmosphere. Fog can form in a number of ways, although most are broken down into advection or radiation fog. Either way, cloudy air is often noticeably cooler than nearby clear air and can feel damp from all the moisture droplets.
When a warm front of moist air moves over colder ground, it can form advection fog. The cold earth causes the air in the warm front to cool, forcing it to become supersaturated. Many people who live near the ocean are familiar with this phenomenon. Radiation fog forms when the Earth’s surface cools, typically in the evening. When the earth cools, it causes the surrounding air to cool as well and fog appears. Fog can also settle around valleys and other depressions when warm air above a column of cold air forces the air to stay close to the ground. This type typically occurs in the morning, before the sun warms the cooler air, allowing it to rise so that the fog dissipates.
The fog can get quite thick, potentially obscuring visibility very seriously. In reference to this quality, many people use the term metaphorically to talk about blurred vision. Many image editing programs also offer a “fog” option that creates a slightly darkened image. Fog can also play tricks on your other senses, changing the way sound and light move and making it difficult to navigate.
The classic 19th century “London fog” was actually smog, a form of pollution. Smog forms when particulate matter in the air becomes so concentrated that it obscures visibility and can pose a serious health risk. A related concept, vog, is natural smog caused by volcanoes. The pollutants expelled by the volcano as it erupts mix with the air to form a dense cloud, which can often be very harmful due to the high concentrations of sulfur around volcanoes.
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