On December 4, 1952, a continuous dense fog shrouded London, England for a week. A large amount of smoke and gas emitted from factories and residential areas accumulated in the low altitude, making the city gray. During the fog cover period, more than 4700 people died of respiratory diseases. After the fog dispersed, more than 8000 people died of similar diseases, resulting in the "fog capital disaster" that shocked the world.
Fog is a phenomenon that a large amount of abundant water vapor is cooled by cold air. After reaching the saturation state, it condenses into countless tiny water droplets and is suspended in the air near the ground. These water vapor can be evaporated from the vast ocean, rivers and lakes on land, wet land surface and lush vegetation; It can also be brought from the warm air flow elsewhere. The smallest diameter of these small water droplets is only a few thousandths of a millimeter, and the largest is only 1 / 10mm.
There are many ways of condensation of small water droplets: evaporation and transpiration will greatly increase the water vapor in the local air, and condensation will occur when it reaches saturation; When the air flowing from other places meets the local original air, it will saturate the air that is close to saturation and produce condensation; The ground surface will radiate heat outward, and the surface air temperature after heat radiation will decrease rapidly, resulting in the water vapor of the original air tending to saturate and condense. The large amount of condensation of small water droplets forms a fog. If the temperature on the underlying surface of the fog rises, the fog droplets are evaporated; Or when the wind speed increases, the fog will be dispersed or lifted into clouds, and the fog will dissipate.
Due to the dense fog and extremely low visibility, it is easy to cause damage to aircraft, vehicles and ships in the air, water, highways or other traffic lines, resulting in casualties. The fog will also block the sunlight and hinder the normal growth of crops. In addition, the wet fog will rot the mature fruits and vegetables and cause huge economic losses to agricultural production. So people call fog "invisible killer".
According to the size of horizontal visibility, the intensity of fog can be divided into four levels. Those with horizontal visibility less than 50m are called strong dense fog, which is the main "killer" of disaster.