Jan 9, 2015

Fog, Smog, Vog, Haze, and Mist

This time of year we see many of these weather conditions and some people can get a bit foggy about the definitions.

Mist and fog are caused by water droplets in the air, and the only difference is how far you can see. The airline industry’s definition of fog uses the guideline of not being able to see more than 1,000 meters (3,280 ft), although the civilian definition of fog is when visibility is less than 200 meters (650 ft). If you can see farther than that, it is considered mist. Different types of fog include Valley fog, Upslope fog, and coastal fog. Also, evaporation fog causes freezing fog.

Haze is the reflection of sunlight off air pollution. Some naturally occurring sources of haze include smoke particles from fires. Most haze is air pollution, carried by the wind often hundreds of miles from where it originated.

Smog was first used in London during the early 1900's to describe the combination of smoke and fog. It occurs when pollution causes low-lying ozone. When certain pollutants enter the air, such as nitrogen oxides, they react with the sunlight to form ozone. Major smog occurrences often are linked to heavy motor vehicle traffic, high temperatures, sunshine, and calm winds. During 1952, weather conditions led to massive smog descending on and gathering over London. Visibility was less than 30 centimeters (12 in), the air was black with coal and pollution, and the usually bustling city came to a standstill. By the time the smog cleared, 4,000 people died from exposure to the pollution, and another 8,000 died during the following weeks.

Vog only happens when a nearby volcano is releasing sulfur dioxide into the air to react with what is already there. When a volcano erupts, or begins to erupt, it releases sulfur dioxide, which then reacts with other gases already in the air. When lava reaches the sea, it also reacts with the water to produce other chemicals like hydrogen sulfide. The resulting “fog” is called vog and can mean anything from severely reduced visibility to adding a mild, blue-grey tint to the landscape.