Science Behind Fog & Flash Fog

Fog, at its simplest, is a cloud that forms at ground level. It occurs when air near the surface cools enough for water vapour to condense into tiny droplets suspended in the air. Fog can be thin or thick, meaning people have difficulty seeing through it. In some conditions, fog can be so thick that it makes it hard to drive safely because it obscures the road and other cars. Even monuments like London Bridge, in London, England, or the Golden Gate Bridge, in San Francisco, California, are almost impossible to see in thick fog. In winter across north India, this usually happens slowly overnight as temperatures drop, a process known as radiation fog.
Fog shows up when water vapour, or water in its gaseous form, condenses. During condensation, molecules of water vapour combine to make tiny liquid water droplets that hang in the air. You can see fog because of these tiny water droplets. Water vapour, a gas, is invisible. Fog happens when it’s very, very humid. There has to be a lot of water vapour in the air for fog to form. In order for fog to form, dust or some kind of air pollution needs to be in the air. Water vapour condenses around these microscopic solid particles. Sea fog, which shows up near bodies of salty water, is formed as water vapour condenses around bits of salt.
Depending on the humidity and temperature, fog can form very suddenly and then disappear just as quickly. This is called flash fog. Fog is not the same thing as mist. Fog is denser than mist. This means fog is more massive and thicker than mist. There are more water molecules in the same amount of space in a fog. Fog cuts visibility down to one kilometre, meaning it will prevent you from seeing further away than one kilometre from where you’re standing. Mist can reduce visibility to between one and two kilometres.
There are several different types of fog, including radiation fog, advection fog, valley fog, and freezing fog. Radiation fog forms in the evening when heat absorbed by the Earth’s surface during the day is radiated into the air. As heat is transferred from the ground to the air, water droplets form. Sometimes people use the term “ground fog” to refer to radiation fog. Ground fog does not reach as high as any of the clouds overhead. It usually forms at night. Fog that is said to “burn off” in the morning sun is radiation fog. Advection fog forms when warm, moist air passes over a cool surface. This process is called advection, a scientific name describing the movement of fluid. In the atmosphere, the fluid is wind. When the moist, warm air makes contact with the cooler surface air, water vapour condenses to create fog. Advection fog shows up mostly in places where warm, tropical air meets cooler ocean water. The Pacific coast of the United States, from Washington to California, is often covered in advection fog. The cold California Current, which runs along the western coast of North America, is much cooler than the warm air along the coast. Valley fog forms in mountain valleys, usually during winter. Valley fog develops when mountains prevent the dense air from escaping. The fog is trapped in the bowl of the valley. In 1930, vapour condensed around particles of air pollution in the Meuse Valley, Belgium. More than 60 people died as a result of this deadly valley fog. Freezing fog happens when the liquid fog droplets freeze to solid surfaces. Mountaintops that are covered by clouds are often covered in freezing fog. As the freezing fog lifts, the ground, the trees, and even objects like spider webs, are blanketed by a layer of frost. The white landscapes of freezing fog are common in places with cold, moist climates, such as Scandinavia or Antarctica.
Winter fog over the Indo-Gangetic Plain is infamous for bringing daily life to a standstill. Every year, dense fog disrupts flights, delays trains, slows road traffic and causes widespread economic losses across northern India. While fog itself is not new, scientists have long struggled to explain why some fog events become exceptionally thick and persistent, lasting for days. A new study by researchers at the Indian Institute of Technology, Madras offers a clear answer: air pollution is actively making fog worse. By combining 15 years of satellite observations with advanced computer simulations, the team shows that high levels of atmospheric pollution directly lead to deeper, denser fog layers over northern India. The research, published in Science Advances, draws on data from NASA’s CALIPSO satellite, which uses LIDAR, a laser-based technique, to probe the vertical structure of clouds and fog from space.
The satellite data revealed a striking pattern. When pollution levels are high, fog layers grow 15–20% thicker compared to fog forming in cleaner air. Polluted fog also contains larger water droplets near its top, making it more opaque and harder to dissipate. Crucially, these relationships remained even after accounting for changes in temperature, humidity and winds, pointing to a direct role played by aerosols, tiny pollution particles suspended in the air. To understand how this happens, the researchers ran high-resolution weather simulations that explicitly include aerosols. They found that pollution particles act as seeds on which water vapour condenses, creating a much larger number of fog droplets. As these droplets form, they release heat, which causes gentle upward motion within the fog. This internal mixing helps the fog grow vertically. At the same time, fog with more water droplets loses heat more efficiently from its top, especially at night. This cooling keeps the air saturated and encourages even more condensation. Together, these processes form a positive feedback loop that thickens and strengthens the fog, a process the scientists describe as “fog invigoration.”
The effect is strongest during night-time, when cool, humid conditions allow pollution particles to activate most efficiently, and sunlight is absent to break the fog apart. The findings are significant because they extend the idea of pollution-driven “invigoration,” previously associated mainly with storm clouds, to one of the atmosphere’s calmest phenomena. They also carry major policy implications. By directly linking particulate pollution to thicker and longer-lasting fog, the study suggests that improving air quality could reduce not only health risks but also severe winter disruptions across one of the world’s most densely populated regions. Quick fog events are particularly dangerous because they catch commuters, drivers and pilots off guard. Unlike forecast overnight fog, flash fog leaves little time for warnings, making sudden visibility loss a serious safety risk. As winter continues and cold wave conditions persist, meteorologists warn that such rapid fog episodes may recur—short, sharp and disruptive, just like this morning’s surprise act.
As the sun began to rise, instead of steadily warming the surface, a brief influx of colder air, often due to light winds shifting direction, caused temperatures near the ground to dip or stagnate suddenly. This rapid cooling pushed the air to its dew point, triggering immediate condensation. Think of it like breathing on a cold mirror: fog appears instantly. High humidity is the key villain. When moisture levels are already high, even a tiny temperature change can produce a dramatic result. In urban areas like Delhi-NCR, pollution particles act as condensation nuclei, giving water vapour more surfaces to cling to. The result: thicker fog with visibility dropping to just a few metres in minutes. Low wind speeds also played a role. Without enough airflow to disperse the fog, it pooled rapidly over roads, residential areas and open spaces.
Once sunlight strengthened, surface temperatures rose enough to break the delicate balance that created the fog. Slight warming, coupled with marginally increased air movement, allowed the droplets to evaporate back into water. The fog didn’t “move away,” it simply ceased to exist. Quick fog events are particularly dangerous because they catch commuters, drivers and pilots off guard. Unlike forecast overnight fog, flash fog leaves little time for warnings, making sudden visibility loss a serious safety risk. As winter continues and cold wave conditions persist, meteorologists warn that such rapid fog episodes may recur—short, sharp and disruptive, just like morning’s surprise act at Delhi NCR on 14 January 2026. Pollution particles further intensified the fog by acting as condensation nuclei, causing visibility to collapse within minutes. Unlike routine winter fog that forms overnight, flash fog develops and dissipates rapidly, making it particularly dangerous for road traffic, aviation, and morning commuters.
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