Fog tends to form in those places. Why?

Fog mostly forms in wet valleys, depressions and basins; Land-sea junction (such as the coastal areas of East China and South China in China); Cold and warm air interchange; Sea surface (especially coastal waters).

Details are as follows:

Definition of fog: It is called "fog" when the horizontal visibility distance drops below 1 km due to the attachment of a large number of water droplets or ice crystal particles suspended in the ground air, and it is called "light fog" when the visibility distance is between 1 ~ 5 km. Generally, fog with a thickness of tens to hundreds of meters, a thickness of more than 1 km and a thickness of less than two meters is called shallow fog. Formation mechanism of fog: the formation mechanism of fog is that the air near the ground reaches saturation due to cooling or increasing water vapor content, and water vapor condenses or condenses to form fog. (indicator T-TD)

The main factors affecting the visibility in fog are the concentration and size of fog droplets.

Classification of fog:

① According to the different formation processes, fog can be divided into radiation fog, advection fog fog, uphill fog and evaporation fog.

② Fog can be divided into ice fog and water-ice mixed fog according to the material state, which are composed of water droplets, ice, crystals and water droplets with ice crystals respectively.

③ The synoptic classification of fog divides fog into air mass fog and frontal fog. In addition,

People usually call the fog that occurs at sea "sea fog".

Cooling fog:

Mainly due to air cooling, it can be divided into radiation fog, advection fog fog and uphill fog.

1. radiation fog: the fog formed by the condensation of water vapor in the surface gas layer due to the radiation cooling of the surface is called radiation fog. Generally, it appears in sunny and breezy night or early morning when the air layer near the ground is relatively humid, and it is easy to appear in autumn and winter (the night is long in autumn and winter, there are many sunny days, and the radiation refrigeration capacity is large).

1. Characteristics of radiation fog:

① There are obvious seasonal and daily changes: mostly in autumn and winter; Mostly from midnight to early morning, it is the strongest around sunrise, and the radiation temperature gradually increases during the day;

② It is closely related to geographical environment: wet valleys, depressions and basins;

③ Vertical and horizontal dimensions of radiation fog: the thickness varies from tens of meters to hundreds of meters, with an average of about1.50m.. The horizontal range is not large, the distribution is uneven, and it is often scattered, and it can also be connected together on the plain.

2. Formation conditions of radiation fog:

① Cooling conditions: It is partly cloudy at night or early in the morning (SKC, NSC), and the ground dissipates heat quickly, which makes the air layer near the ground cooler and is beneficial to water vapor condensation. When there is radiation inversion at low altitude, it is beneficial for a large number of fog droplets in the near-surface layer to accumulate under the inversion layer and form radiation fog.

② Water vapor condition: When the water vapor in the near-surface layer is sufficient, the temperature drops slightly and the water vapor will condense. The greater the humidity, the thicker the wet layer, which is more conducive to the formation of fog. When the air is humidified by rain and wet ground, it is especially beneficial to form such fog.

(3) Stratification condition: When the near-surface gas layer is relatively stable or has inversion temperature, it is beneficial to the accumulation of water vapor and dust impurities. If there is radiation cooling, water vapor is easy to condense and form fog. When the gas layer is unstable, it is beneficial to the heat exchange and water vapor diffusion between the upper and lower layers, but not to the formation of fog.

(4) Wind conditions: still wind is conducive to the formation of dew, frost or shallow fog, but not conducive to the formation of fog; Breeze (1-3m/s) is the most favorable for fog formation. In order to form a radiation fog with a certain intensity and thickness, it is not enough to simply cool by radiation, and a thicker cooling layer must be formed by combining with moderate vertical mixing. When the air is still, the vertical mixing is too weak, which is not conducive to the formation of radiation fog, while when the wind speed is too high (> 3 m/s) and the temperature stratification is unstable, the vertical mixing is too strong, which is not conducive to the formation of radiation fog.

3. Forecast method of radiation fog;

① Considering the weather scale: the pressure field is weak or uniform (such as saddle field), with high humidity and less cloud cover;

② Local topography and other factors: mostly occurring in wet valleys, depressions and basins;

③ Considering the climate background;

4. Dissipation analysis of radiation fog;

① Analyze the existence and thickness of inversion layer (it can be analyzed from sounding curves or weather maps at all levels).

(2) Analysis of heating conditions: fog layer thickness and sky conditions.

③ Analysis of wind conditions: dry and cold air flow in the north, warm and humid air flow in the south, and wind force (> 3 MPa).

④ System weather analysis: frontal transit, precipitation, etc.

Advection fog: advection fog is formed by warm and humid air flowing through the cold ground and gradually cooling. In the coastal areas of China, when the warm and humid air on the ocean moves to the colder sea surface or ground, advection fog often forms. This kind of fog may appear at any time of the day, and it is still mostly at night and early morning.

1. Formation conditions of advection fog:

The temperature difference between warm and humid air and cold underlying surface is large: advection and inversion (limiting the development of vertical mixing and accumulating water vapor)

Warm and humid air with high humidity: water vapor condition

Medium wind speed (2 ~ 7m/s), wind direction

Stratified stability: inversion layer.

2. Forecasting methods in advection fog;

Analyze the weather situation: the ocean denaturalization high in the west, the Pacific high in the west and the cyclone and trough in the east;

Analyze the temperature, humidity and turbulence conditions in the near-surface layer: temperature and humidity conditions-condensation, turbulence conditions-fog or low clouds;

Compare the meteorological elements of this station and the upstream station.

3. Produce weather conditions in advection fog:

(1) the sea cold high pressure in the west of advection fog.

Whether advection fog can be formed in the west after cold high pressure enters the sea mainly depends on the thickness of the system and the time that the system stays at sea. Generally speaking, the thicker and bigger the system is, the more favorable it is for the formation of advection fog in the western part of the system. This kind of advection fog is more common in spring and generally appears in the land-sea border area.

(2) advection fog in the western part of the Pacific High.

After summer, the Pacific ridge of high pressure extends to the northwest. If the western edge of this ridge just extends to China's coastal areas, it will be beneficial to the emergence of advection fog in coastal areas. Because the Pacific ridge is a warm and deep system with a long Witt time, the advection fog affected is wide and thick. And the duration is long, from 1-2 days to 5-6 days or even longer.

③ Cyclone and advection fog in the east of the trough.

Advection fog caused by Jianghuai cyclone mainly appears in the Yellow Sea coast, and advection fog caused by Yellow River cyclone; Mainly along the Bohai Sea; The southwest trough of low pressure causes advection fog and advection fog to appear mainly in the eastern and southern coastal areas of the Pearl River Estuary.

When the above three kinds of advection fog appeared, there were warm advection at the heights of 850 mbar and 70 mbar in coastal areas. If the thickness of warm advection is too thin, advection fog is less likely.

Advection fog before the front: The southerly wind flow field before the stationary front, and the cold front or trough of low pressure is beneficial to the transportation of warm and humid air, and advection fog often appears.

4. Matters needing attention in advection fog Zhongben Station:

(1) Wind direction and speed

(2) The difference between the dew point in the upstream area and the temperature of this station.

(3) Changes of temperature, dew point difference and relative humidity at the station.

(4) The weather around this station and so on.

5. Dissipation analysis of advection fog;

① Analysis of advection and inversion conditions: daily variation of temperature.

② Wind condition analysis: wind direction and speed.

(3) System influence: system movement and frontal cross-cutting destroy advection conditions.

6. Features of advection fog:

The diurnal variation of (1) is not obvious, but the annual variation is obvious (more in spring and summer, less in autumn and winter), and the temperature difference between land and sea and the seasonal variation of cold and warm ocean currents are obvious.

(2) advection fog at sea lasts for a long time, sometimes for several days. Land is often advection radiation fog *, that is, warm and wet advection first and then radiation cooling.

(3) The vertical thickness of advection fog can reach tens of meters to two kilometers, and the horizontal range can reach hundreds of kilometers. The intensity of advection fog is also greater than the radiation fog.

(4) Weather conditions: advection fog is often accompanied by stratiform clouds, broken rain clouds, Mao Mao rain and other weather phenomena, and the weather is generally stable.

7. Mountain fog

Air rises along the hillside and forms fog due to adiabatic expansion and cooling. When the fog is formed on the uphill slope, the gas layer must be convective stable stratification, and the fog appears on the windward slope.

Third, evaporation fog:

Evaporation fog is cold air flowing over warm water. Due to the evaporation of warm water surface, the water vapor in cold air increases, resulting in saturated condensation and fog formation.

1. Characteristics of evaporation fog:

Evaporation fog is generally not too thick, generally around 50 ~ 100 meters, which is roughly consistent with the lower bound of inversion layer.

Evaporation fog is neither stable nor uniform. It disappears with birth, sometimes thick and sometimes light.

2. Conditions for the formation of evaporation fog:

Evaporative fog can be divided into ocean fog and autumn fog on rivers and lakes. In winter, cold air flows from the mainland to the warm ocean, forming sea fog. This fog is particularly strong in the polar regions. This kind of fog often appears in unfrozen bays and winter ice caves. When the water surface in the lake and the lake is much warmer than the land surface, if the cold air flows to the water surface, it will form autumn fog on the river surface and the lake surface due to strong evaporation. This kind of fog is common in dry autumn mornings. This kind of valley fog often appears in autumn and winter in the valley of Hengduan mountain area in China.

Fourth, mixed fog.

The fog formed by two nearly saturated air masses mixing with each other in the horizontal direction to reach saturation condensation is called mixed fog.

Conditions conducive to the formation of mixed fog

① The temperature difference between the two air masses involved in mixing should be greater than 10℃, and the relative humidity should be greater than 95%, the greater the better. This kind of fog sometimes appears near the coast when the air temperature between land and sea is very different and the wind is weak.

(2) Precipitation is a necessary condition to produce mixed fog, which can disperse radiation fog and advection fog to some extent.

frontal fog

Fog often occurs at the junction of cold and warm air, which is called frontal fog. In frontal fog, it usually happens near the warm front, and it may also happen behind the front. The fog in front of the front is formed by the warm raindrops in the warm air above the front falling into the ground cold air and evaporating and condensing with the air. The fog behind the front is formed by warm and humid air moving to the area originally occupied by cold air before the warm front and cooling to supersaturation.

sea fog

When the lower atmosphere is in a stable state, the air near the sea surface is gradually saturated or supersaturated due to the increase of water vapor and the decrease of temperature. At this time, water vapor condenses with hygroscopic particles such as fine salt particles as the core to form fog.

Classification of sea fog

According to the formation characteristics of sea fog and the characteristics of marine environment, sea fog can be divided into four types: advection fog, mixed fog, radiation fog and topographic fog.

Sea fog-advection fog

Advection fog is a fog generated when air flows horizontally on the sea surface. When warm and humid air moves over the cold sea surface, the bottom layer cools and water vapor condenses to form advection cooling fog. This kind of fog has a large thickness, a wide range and a long duration, and is mostly produced in cold areas. In spring and summer in China, the sea fog in the East China Sea and the Yellow Sea mostly belongs to this category. The fog produced by cold air flowing through the warm sea surface is called advection evaporation fog, which mostly appears in the high latitude sea surface in cold season.

Sea fog-mixed fog

Mixed fog is the fog produced by mixing two kinds of moist air with large temperature difference in the ocean. Due to storm activity, air with humidity close to or reaching saturation is generated, which is mixed with cold air from high latitudes in cold season to form mixed fog in cold season and mixed with warm air from low latitudes in warm season to form mixed fog in warm season.

Sea fog-radiation fog, terrain fog

The fog produced by radiation cooling at night is called radiation fog. Appears around dawn and gradually dissipates after sunrise.

In the process of climbing to the island and coast, the fog formed by the cooling and condensation of warm and humid air on the sea surface is called topographic fog.

Characteristics of sea fog in China

Advection cooling fog is the most common in the offshore of China. In spring, the fog season in Zhixia is extended from south to north: the fog in the South China Sea mostly appears from February to April, mainly in the coastal areas of Guangdong, Guangxi and Hainan, with the most in the eastern part of Leizhou Peninsula; The sea fog in the East China Sea is mostly from March to July, especially from the Yangtze River estuary to Zhoushan Islands and the northern mouth of the Taiwan Province Strait. The foggy season in the Yellow Sea is from April to August, and the whole sea area is foggy. The sea area near Chengshantou is commonly known as "fog cave", with an average of nearly 83 days of fog every year. Sea fog is common in Bohai Sea from May to July, more in the east than in the west, and concentrated in Liaodong Peninsula and the northern coast of Shandong.