Sea Fog: I am not an ordinary fog!

Produced by: Science Popularization China

Author: Blue Ocean Team

Producer: China Science Expo

When spring comes and the weather turns from cold to warm along the coast of my country, misty and drizzly weather often occurs, and visibility is significantly reduced, even when objects are difficult to see even at a distance of several meters. This is the well-known sea fog.

(Photo source: veer photo gallery)

Sea fog is a weather phenomenon caused by the condensation of water vapor in the lower atmosphere above the sea. Because it can reflect light of various wavelengths, it often appears milky white. The formation of fog requires two different physical processes: the condensation of water vapor and the accumulation of condensed water droplets (or ice crystals) in the low altitude. In these two processes, two conditions must be met: one is that there must be a condensation nucleus, such as salt particles or dust, during condensation, otherwise it is very difficult for water vapor to condense; the other is that water droplets (or ice crystals) must be suspended in the near-sea surface layer, so that the horizontal visibility is less than 1 km.

For water vapor to condense in the atmosphere, there must be sufficient water vapor. Although a large amount of water vapor evaporates day and night on the vast ocean, condensation does not occur there every day. This is because once the atmosphere reaches saturation, evaporation will stop immediately, and the water vapor in the air will no longer increase, so it is difficult to reach a supersaturated state, causing the water vapor to condense. Only when the water surface temperature is much higher than the air temperature, the warm water surface can continuously evaporate water vapor and diffuse it into the cold air layer continuously, keeping it in a supersaturated state, and the condensation process can continue, resulting in evaporation-like fog, which is the so-called advection steam fog.

(Photo source: veer photo gallery)

Another way of condensation is to rely on lowering the temperature of water vapor to achieve the appearance of supersaturated water vapor. When warm and humid air flows over the cold sea surface, it transfers heat to the cold sea surface and lowers its own temperature. At this time, the saturated water vapor volume becomes supersaturated as the temperature decreases, and condensation occurs. This condensation phenomenon is common in sea fog areas and is usually called advection cooling fog. The sea fog that appears in my country's sea areas is mainly this kind of advection cooling fog. The sea fog that appears in many famous sea fog areas in the world is also mostly caused by advection cooling fog. There are many other types of sea fog that is formed by cooling down and causing the water vapor volume to reach a supersaturated state. For example, the sea surface covered with ice and snow at high latitudes can often form ice surface radiation fog due to radiation cooling on the ice and snow surface (especially at night); in addition, the oblique rise effect of the island terrain often lifts the warm air blowing from the sea surface on the windward side of the island, which may promote condensation into terrain fog due to upward temperature rise.

(Photo source: veer photo gallery)

Sea fog is one of the dangerous weather conditions on the ocean. It has a direct impact on maritime navigation and coastal activities. Marine meteorologists have conducted extensive research on the generation and dissipation mechanism of sea fog for a long time, providing a more substantial theoretical basis and practical technology for predicting sea fog. Currently, there are three commonly used methods for predicting sea fog.

The first is the meteorological method, which treats sea fog as a weather phenomenon and takes into account the role of various hydrological and meteorological elements and their interrelationships as much as possible. Generally speaking, the hydrological and meteorological elements related to sea fog mainly include wind direction, wind speed, precipitation, evaporation, air temperature, humidity, water temperature, ocean currents and stability. The wind blows from the sea to the land, often bringing warm and humid air from the sea to the coast, which is conducive to the growth of fog. The wind speed has different effects on sea fog of different natures. Radiation fog can only exist in a breeze, while advection fog is most suitable for winds of 4-5 levels, and the fog will be blown away if it exceeds 6 levels. Precipitation is a necessary condition for the production of mixed fog, and it dissipates radiation fog and advection fog. Evaporation is a necessary process for the production of steam fog. Temperature and humidity can be considered as two aspects of a factor. The fog in the spring and summer seasons near the coast of China will no longer appear when the sea surface temperature exceeds 24 degrees Celsius. The relationship between sea fog and ocean currents is closer. The boundary area between cold and warm ocean currents and the upwelling cold current area are both sea areas where sea fog often appears and concentrates. On the surface, this is a correlation between ocean currents and advection fog, but in fact it reflects that sea water temperature is one of the important conditions for the generation of various types of sea fog. Stability is also closely related to sea fog. Stable air stratification is conducive to the generation and persistence of fog. The meteorological method is to predict sea fog, that is, to seek the relationship between the above-mentioned factors and the formation and disappearance of sea fog, and to predict the changes of sea fog in combination with the development of weather conditions. Most of the hydrological and meteorological stations along the coast of my country still use this method to predict sea fog.

The second is the statistical method, which is to use historical data to conduct temporal and spatial distribution statistics on the relationship between hydrological and meteorological elements and sea fog, and find out the laws in various records. In the statistical process, the internal connection between hydrological and meteorological elements and weather phenomena and their physical significance should be fully considered, so that the statistical results have meteorological and climatological significance.

The third is the numerical method. Since there are many factors in the formation of sea fog, some are mainly based on air advection; some are mainly based on air (or underlying surface) radiation; some are through cooling; some rely on increasing the amount of water vapor to reach supersaturation and condensation; some are a combination of several methods. Therefore, it is difficult to use a numerical model to reflect the formation process of sea fog. At present, when the forecasting department makes a numerical forecast of sea fog, it considers radiation fog and advection fog separately.

In recent years, numerical forecasting and statistical forecasting have been combined into statistical dynamic forecasting, which has achieved relatively obvious results.