Causes of fog formation

Fog is a very common natural phenomenon. If there is fog in the air, it will have a certain impact on people's normal travel. If it is haze, it will even affect human health. There are many types of fog, such as radiation fog, advection fog, steam fog, top fog and frontal fog. Different fogs have different causes. Below, we will introduce in detail the reasons for the formation of these types of fog.

This type of fog is formed when the air reaches supersaturation due to radiation cooling, and mainly occurs at night or in the morning when the weather is clear, breezy, close to the ground, and when there is abundant water vapor. At this time, there are no clouds in the sky, the heat from the ground radiates outwards quickly, and the air temperature near the ground drops rapidly. If there is too much water vapor in the air, it will quickly reach supersaturation and condense into fog.

In addition, wind speed also has a certain impact on the formation of radiation fog. If there is no wind, there will be no exchange of air between the upper and lower layers, and the radiation cooling effect will only occur in the air layer close to the ground, and only a thin layer of shallow fog will be generated. If the wind is too strong, the air exchange between the upper and lower layers is fast and the flow is large, the temperature is not easy to drop much, and it is difficult to reach an oversaturated state. Only when there is a breeze of 1 to 3 m/s and an exchange of appropriate intensity can the cooling effect be extended to a certain height without affecting the sufficient cooling of the lower air, which is most conducive to the formation of radiation fog.

2. Advection fog

When warm and humid air flows over cold sea or land surfaces, the lower layers of air become supersaturated due to contact cooling and condense into fog, which is advection fog.

As long as there is appropriate wind direction and wind speed, once fog is formed, it will often last for a long time. If there is no wind, or the wind direction changes and the source of warm and humid air is cut off, the fog will dissipate immediately.

If the water surface is warm and the air is cold, when the temperature difference between them is large, water vapor will continuously evaporate from the water surface, break into the cold air, and then condense from the cold air to become steam mist.

Generally, when the warm ocean current from the south enters the polar region, the cold polar air covers the warm water surface and forms steam fog. For example, there is a powerful warm ocean current called the Gulf Stream in the North Atlantic, which often breaks into the Arctic Ocean, causing large-scale steam fog on the Arctic Ocean surface. Sometimes, the cold air from the Arctic stays on the ice surface. Where the ice surface cracks, the warmer water under the ice is exposed, forming local steam fog. Steam fog mostly appears in the high-latitude Arctic regions, so people often call it "Arctic smog."

4. Uphill fog

This is fog caused by moist air rising up the hillside and adiabatic cooling causing the air to become supersaturated. This humid air must be stable and the slope of the mountain must be small, otherwise convection will form and fog will be difficult to form.

It often occurs near the front where cold and warm air meet. It can be found both in front and behind the front, but is mostly found near the warm front. Pre-frontal fog is formed when raindrops in the warm air clouds above the front fall into the cold air on the ground, evaporating and causing the air to become supersaturated and condense; while post-frontal fog is formed when warm and humid air moves to the area originally occupied by the cold air in front of the warm front, and becomes supersaturated after cooling. Because the fog near the front often moves along with the front, the military often uses this frontal fog to cover troops and launch surprise attacks on the enemy.