The Weather of Fog

“Fair is foul, and foul is fair, hover through fog and filthy air.” - William Shakespeare

The iconic image of the sun, gently shining through a slowly lifting fog, wafting over fields one cold and crisp spring morning has been a source of inspiration for artists around the world. Fog is one of the most dramatic, yet serene weather phenomena as it normally only exists when there is a lack of wind. 

Fog is a cloud that touches the ground, and appears when water vapor condenses, forming small droplets of liquid water, or sometimes ice crystals that can hang in the air. Fog is visible due to the water droplets, and water vapor, being a gas, is invisible.

For fog to form, there needs to be some element of air pollution, or airborne microscopic particles for the water vapor to condense around. Sea fog is formed around areas of salty water, when the water vapor condenses around pieces of salt.

Fog is normally dependent on the relative humidity, often forming when it approaches 100% and when the ambient air temperature falls - although these conditions do not always guarantee a fog of Shakespearian quality. Certain conditions cause fog to appear and disappear very quickly, known as a flash fog.

Grand Banks, an area of the Atlantic Ocean just off the coast of Newfoundland is thought to be the foggiest area of the world. This area forms the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. This mixture of currents regularly causes the formation of fog, with over 200 days of fog every year.

London’s “Pea Soupers”

The “Great Smog of London” in 1952, caused by air pollution, created severe travel disruption, and is reported to have even entered indoor areas. This ‘pea souper’ had a serious effect on people’s health, and is estimated to have caused over 100,000 people being made ill. 

On 4 December 1952, an anticyclone, coupled with extremely low wind speeds in London, caused a temperature inversion with relatively cool, stagnant air trapped under a layer of warmer air. This created a fog that mixed with domestic, industrial smoke and car exhaust fumes creating a smog that blanketed the city the following day.

These conditions eventually resulted in the UK government passing legislation to replace polluting coal fires with more environmentally friendly alternatives.

Fog Catchers

There has been a tradition in some ancient cultures to obtain water from fog by placing large vessels below plants to collect water. Today, water is harvested from fog in certain arid areas using large screens that work in a similar way to the plants. These fog catchers can collect over a hundred gallons of water as the fog droplets are caught in the fabric of the screen. Some villages in Peru that have limited access to water from other sources use these fog catchers to supply water for irrigation and drinking.

Types of Fog

Radiation Fog - forms when the heat absorbed by the ground during the day, radiates into the air at night, causing water droplets to form. This fog normally only remains until the following morning, when temperatures rise, and the fog is said to burn off.

Advection Fog - forms when warm air passes over a colder surface. This can often be seen when warm, tropical air meets cooler ocean water, such as the Pacific coast of the United States, from Washington to California.

Valley Fog - forms in mountain valleys when the geography prevents warm air from escaping, normally during the winter months. 

Freezing fog - happens when the liquid fog droplets freeze to solid surfaces. Often, the tops of mountains that are covered by clouds are often also covered in freezing fog. After the freezing fog has lifted, the ground and vegetation are often blanketed by a layer of frost. This type of fog is especially common in areas that are cold and damp, such as Scandinavia. 

Fog and Wine

In Italy's largest wine-growing region, Piedmont, the major growing hills of Langhe and Monferrato are frequently shrouded in thick blankets of fog. So notorious is the fog that one of its major wines is named after the Italian word for fog, 'nebbia'. Nebbiolo grapes ripen later in the year and are generally not harvested until late October, by which time fog is a frequent visitor creating spectacular vineyard vistas and a world-class wine.

Fog is also reputed to be of benefit to grapes such as Pinot Noir in California, where the foggy climate encourages the retention of a slight acidity in the grape, adding to the overall vibrancy of the final wine. 

Impact of Fog on Transport

It is estimated that fog costs UK industry approximately £226million per year. With the main disruption affecting transportation and logistics. 

Over the years, fog has caused some of the most severe road incidents, including the 200 vehicle pile-up near Luton in 1974, and a 130 vehicle crash on the Sheppey crossing in Kent in 2013. Even with modern technology, fog is a notorious weather phenomena that normal car camera safety systems have difficulty in seeing through. The Lidar systems that the new self-driving car technologies often rely on have difficulty in seeing through fog.  

Understanding Fog

The OpenWeather One Call API 3.0 provides nuanced yet easy to use access to essential global weather data, short-term and long-term forecasts and aggregated historical weather data. This includes the incredible daily aggregated weather for over 40 years historically, and up to 1.5 years forecast. This product also includes a visibility return parameter (along with a number of other useful values), that can be vital in understanding fog conditions for any global location.

So no matter where in the world you are, OpenWeather can help you understand where fog might appear.

About OpenWeather

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OpenWeather cooperates with global meteorological agencies such as MetOffice and NOAA, and enhances its model with data from radars, weather stations and satellites. The company provides great availability of service at 99.9% for enterprise-level products. 

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