Context - Delhi witnessed this winter’s densest fog cover on Monday with visibility dropping to 0 metres at the Indira Gandhi International Airport and affecting flight operations.
Fog is formed when air at or near the earth's surface becomes saturated by any of the three processes - cooling, addition of moisture, or mixing with another air parcel. Generally, fog forms in a stable air mass environment. Fog does not generally form with an unstable atmosphere because vertical mixing results in convective or layered cloudiness.
Types of fog:
- Radiation Fog: This fog forms when all solar energy exits the earth and allows the temperature to meet up with the dew point. The best condition to have radiation fog is when it had rained the previous night. This help to moisten up the soil and create higher dew points. This makes it easier for the air to become saturated and form fog. However, the winds must be light less than 15 mph to prevent moist and dry from mixing.
- Precipitation Fog: This is fog that forms when rain is falling through cold air. This is common with a warm fronts but it can occur with cold fronts as well only if it's not moving too fast. Cold air, dry at the surface while rain is falling through it evaporates and causes the dew point to rise. This saturation forms fog.
- Advection Fog: This type of fog forms from surface contact of horizontal winds. This fog can occur with windy conditions. Warm air, moist air blows in from the south and if there is snow or cool moisture on the ground it will come in contact with the warm, moist winds. This contact between the air and ground will cause the air blowing in to become cool. Then dew point rises and creates high humidity and forms fog.
- Steam Fog: This type of fog is commonly seen in the Great Lakes but can be seen on any lake. This forms during the fall season. As summer ends, water temperatures don't cool right away but air temperature does. As a mass of dry, cold air moves over a warmer lake the warm lake conducts warm, moist air into the air mass above. This transport between the lake and air evens out. This corresponds to the second law of thermodynamics and this law state "any two bodies that come into contact, the system will become equilibrium state." Steam fog does not become very deep but enough to block some of the sunlight.
- Upslope Fog: This fog forms adiabatically. Adiabatically is the process that causes sinking air to warm and rising air to cool. As moist winds blow toward a mountain, it up glides and this causes the air to rise and cool. The cooling of the air from rising causes to meet up with the dew point temperature. Fog forms on top of the mountains.
- Valley Fog: Valley fog forms in the valley when the soil is moist from previous rainfall. As the skies clear solar energy exits earth and allow the temperature to cool near or at the dew point. This form deep fog, so dense it's sometimes called tule fog.
- Freezing Fog: Freezing fog occurs when the temperature falls at 32°F (0°C) or below. This fog produces drizzle and these tiny droplets freeze when they come into contact with an object. But at the same time there is sublimation going on.
- Ice Fog: This type of fog is only seen in the polar and artic regions. Temperatures at 14 F (-10°C) is too cold for the air to contain super-cooled water droplets so it forms small tiny ice crystals.
Related basic concepts: The Difference Between Mist, Fog, Smog, Haze And Vog:
- Mist and fog are caused by water droplets in the air, and the only difference is how far you can see. Fog is denser than mist. By international agreements Fog is when you can see less than 1,000 metres away, and if you can see further than 1,000 metres, call it mist. However, in forecasts for the public, fog generally refers to a situation when the visibility is less than 180 metres. On the other hand, the term ‘mist’ is used when the droplets do not reduce horizontal visibility to less than 1 kilometre. Because of this reason, the mist is considered synonymous with ‘light fog’. Mist and fog occur when water droplets hang in the air.
- Haze is the reflection of sunlight off air pollution. Haze happens when the particles in the air are dry pollutants. Most of the time, haze occurs in areas far from the original source of the pollutants, which are carried by wind currents to where they ultimately gather. Haze forms when light reflects off airborne pollution particles and interferes with visibility.
- Smog is the stuff that will make you cough and burn your eyes—that’s because it’s majorly made up of ozone. When certain pollutants enter the air— like nitrogen oxides—they react with the sunlight to form ozone. Most of the smog that we see nowadays is called photochemical smog. When sunlight reacts with nitrogen oxides and at least one volatile organic compound (VOC) in the atmosphere, smog occurs. Nitrogen oxides originate from car exhaust, coal power plants, and factory emissions, whereas, gasoline, paints, and many cleaning solvents release the VOC. When sunlight hits these chemicals, they form airborne particles and ground-level ozone, or smog.
- Vog is a specific type of air pollution that comes only from volcanic activity. When a volcano erupts—or begins to erupt—it releases sulfur dioxide which then reacts with the other gases that are 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.