A seemingly romantic killer - thick "fog" ahead

Produced by: Science Popularization China

Author: Meteorological Science Team

Producer: China Science Expo

In the early morning, fog comes and the mist covers the mountains and forests, making it look like a fairyland. It sounds romantic, but in fact, fog is one of the weather conditions that has the greatest impact on human traffic activities. Due to the uneven density of fog, it will cause visual errors. The driver's judgment of distance and speed will be greatly different from the actual situation, the sight distance will be shortened, and it is easy to collide with the vehicle in front. Especially on the highway, the speed is high. Once a traffic accident occurs, it often causes a chain reaction and eventually leads to a serious traffic accident.

The bridge shrouded in fog (Photo source: veer photo gallery)

In addition, on foggy days, pollutants combined with water vapor in the air will become difficult to diffuse and settle, which makes most of the pollutants gather at the height where people often move. If people stay in this environment for a long time, the human body will inhale harmful substances, consume nutrients, cause damage to the body, and easily induce or aggravate diseases.

So, how is fog formed?

Fog formation

Both fog and clouds are condensed water vapor composed of small water droplets or ice crystals floating in the air, but fog is formed in the near-ground layer of the atmosphere, while clouds are formed in the higher layers of the atmosphere. Since fog is condensed water vapor, its cause should be found in the conditions that cause condensation of water vapor.

There are two reasons why the water vapor in the atmosphere reaches saturation: one is due to evaporation, which increases the water vapor in the atmosphere; the other is due to the cooling of the air itself. Cooling is more important for fog. When there are condensation nuclei in the air, if the saturated air continues to increase in water vapor or continues to cool, condensation will occur. When the condensed water droplets reduce the horizontal visibility to less than 1 km, fog is formed.

In addition, excessive wind speed and strong disturbance are not conducive to the formation of fog.

Therefore, fog is most likely to form in areas that are conducive to low-level cooling of the air, if there is sufficient water vapor, mild wind, stable atmospheric stratification, and a large number of condensation nuclei. Generally, fog is more likely to form in industrial areas and urban centers because there are abundant condensation nuclei there.

Five types of fog, each with its own unique characteristics

Normally when we see fog, we may not recognize what kind of fog it is. “Isn’t fog just fog?” But this is not the case. Depending on the specific conditions under which the air reaches supersaturation, fog is usually divided into the following five types.

The most common type of fog on land is radiation fog:

This type of fog is formed when the air reaches supersaturation due to radiation cooling. It mainly occurs at night or in the morning when the weather is clear, breezy, near 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 out 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.

Wind speed also has a certain influence 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 exchange of air between the upper and lower layers will be very fast, and the flow will be large. It will be difficult for the temperature to drop a lot, and it will be difficult to reach a supersaturated state. Only when there is a breeze of 1 to 3 meters per second, there is an exchange of appropriate intensity, which can not only extend the cooling effect to a certain height, but also not affect the sufficient cooling of the lower layer of air, which is most conducive to the formation of radiation fog.

Radiation fog appears in clear, cloudless nights or mornings. As the sun rises, the ground temperature rises, the air returns to an unsaturated state, and the fog droplets evaporate and dissipate immediately. Therefore, the appearance of radiation fog in the morning often indicates that the weather will be good that day. "If the ground is covered with fog in the morning, you can still dry the rice" and "Nine out of ten fogs will be sunny" refer to this kind of radiation fog.

Early morning fog (Photo source: veer photo gallery)

The second type of fog is 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 an 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.

The third type of fog is steam fog:

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 currents from the south enter the polar regions, the cold polar air covers the warm water surface and forms steam fog. For example, there is a strong warm ocean current in the North Atlantic called the Gulf Stream, which often rushes into the Arctic Ocean, causing large-scale steam fog on the Arctic Ocean surface. Sometimes, the cold Arctic air stays on the ice surface, and 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 region, so people often call it "Arctic smog."

In addition to polar regions, cold air covering warm water often occurs in inland lakeside areas. The lake surface is warmer than the land surface at night. When the land breeze blows to the warm lake surface at night, a relatively shallow layer of steam fog will form on the lake surface. In autumn and winter, when cold air moves south, on clear and windless mornings, when the warm water surface has not had time to cool down, this kind of steam fog will spread.

The fourth type of fog is uphill fog:

This is the fog caused by moist air rising along the hillside, and the air becomes supersaturated due to adiabatic expansion and cooling. This moist air must be stable and the slope must be small, otherwise convection will form and fog will be difficult to form.

The fifth type of fog is frontal fog:

It often occurs near the front where cold and warm air meet. It can occur both before and after the front, but it is more common near the warm front. Frontal fog is formed when raindrops from the warm air clouds above the front fall into the cold air on the ground, evaporate, and the air becomes supersaturated and condenses; while post-frontal fog is formed when warm and humid air moves to the area originally occupied by the cold air before the warm front, and becomes supersaturated after cooling. Because fog near the front often moves with the front, the military often uses this frontal fog to cover troops and launch surprise attacks on the enemy.

Conclusion

With the development of modern industry, many new types of fog have been added. For example, photochemical smog formed by industrial exhaust gas, black smoke emitted by boilers, kilns and small coal stoves in daily life, etc. All of the above belong to other fogs. But whether it is naturally formed fog or artificially created other fogs, they will also affect traffic conditions and may cause harm to people's bodies. Therefore, you need to be cautious when encountering fog and don't take it lightly.

Pipeline emitting smoke (Photo source: Veer Gallery)