1. Fogs, their classification and origin.
2. Precipitation, its classification.
3. Ground hydrometeors.
1. Fogs, their classification and origin.
Fog – 1) accumulation of condensation and sublimation products of water near earth's surface and associated severe clouding of the air;
2) view atmospheric precipitation(along with rain, snowfall, dew, etc., one of the stages of the water cycle in nature (Meteorological Dictionary, 1974).
The fog can consist of water droplets, ice crystals, or be mixed (depending on the air temperature). We speak of fog when the horizontal visibility (once the fog reaches its greatest density) is less than 1 km (Figure 41). Otherwise they talk about haze. With heavy fog, the visibility range is no more than 50 m, with moderate fog - from 50 to 500 m, with light fog - from 500 to 1000 m. Haze is called moderate (if the visibility range is 1–2 km) and weak (2–10 km).
Figure 41 – Fog ( http://www.photoline.ru/category/z?now=240)
The water content of fog depends on their type and ranges from 0.02–1.5 g/m 3 . The number of drops is 50–100/cm 3 in light fogs and 500–600 per cm 3 in thick (dense) fogs. The average size of fog droplets is 5 microns, at prolonged temperatures 5–15 microns, at negative temperatures - 2.5 microns. When droplets coalesce, they may fall out as drizzle (drizzle fog).
Fogs are divided into intramass and frontal fogs. The first ones arise within air masses regardless of the fronts.
Intra-mass:
a) cooling mists:
advective,
radiation (ground, high),
advective-radiative;
b) evaporation fogs;
2. Frontal and prefrontal.
Intramass fogs Due to the approach of air to a state of saturation, they are divided into cooling fogs and evaporation fogs. Cooling fogs are predominant and can occur under different conditions:
When moving warm air to a colder underlying surface. This is how advective fogs arise. They are observed when air masses move from low to high latitudes, from warm sea to cold land in winter and from warm land to cold sea in summer, as well as from warm sea surface to cold ones. On land they are most often observed in autumn and winter, at sea in spring and summer.
With radiation cooling of the underlying surface, radiation fogs arise. They come in two types: ground and high. Terrestrials appear only over land on clear, quiet nights. They are associated with radiation cooling of the soil, their distribution is local. They disappear after sunrise. High radiation fogs can be observed on land and at sea in stable anticyclones during the cold season. Formed as a result of prolonged cooling of air. Can persist for weeks over large areas
Mists of evaporation occur most often in cold air over warmer, open water. Appear in the evening or at night. May occur over moist soil. The vapor mist usually swirls and dissipates quickly.
Frontal fogs occur at points of contact between two air masses with different properties. Such fronts occur quite often, but they are not always accompanied by the formation of fog. Frontal fog occurs when warm rain evaporates through layers of cold air.
In the daily cycle, fogs on the plain have a maximum intensity and frequency in the morning. In the mountains, fogs are distributed more or less evenly throughout the day or have a weak maximum in the afternoon hours.
The geographical distribution of fog frequency is presented in map.
In the Republic of Belarus annual course The number of days with fogs is as follows: maximum in November-December, minimum in summer. The duration of fogs ranges from 140 (Verkhnedvinsk) to 850 (Novogrudok) hours per year. At higher elevations, the number of days with fogs and their duration are less than in lowlands. In Belarus, the average number of days with fog ranges from 35 to 100, increasing from lowlands to highlands. The maximum was recorded in 1964 - 140 days against the norm of 100 in Novogrudok.
Fog is an accumulation of water droplets or ice crystals suspended in the ground layer of air, impairing horizontal visibility to 1000 m or less. In essence, it is a cloud lying on the surface of the earth or water.
Based on the main physical reasons, causing the formation of fogs, they can be divided into two main classes: cooling fogs and evaporation fogs. The first of these classes absolutely predominates.
Cooling fogs are formed by condensation or water vapor when air cools from the earth's or water's surface. They are divided into two types: radiative and advective.
Radiation fogs appear in conditions of stagnant air due to its nighttime cooling from the cooling underlying surface. Observed on clear, quiet nights and in the morning before sunrise. As soon as Sun rays They begin to warm up, these fogs quickly disappear.
Advective fogs are associated with the cooling of warm air as it moves to a cold surface (the same process is associated with the formation of low stratus clouds, which can turn into fog as they descend). These are the most powerful and lasting fogs. They are observed at any time of the day and can be transported over long distances.
Evaporation fogs (vapor) are formed due to the evaporation of water vapor from an underlying surface warmer than the air. Such fogs are most intense in winter over non-freezing bays and polynyas; over rivers and lakes they appear more often in autumn, and on land - after heavy rains in the evening and at night in the summer - autumn period.
Most common feature distribution of fogs over the earth's surface is an increase in their frequency high latitudes. Depending on the conditions of formation, fogs in Russia have different continuous durations. In continental areas, short-term fogs prevail, lasting no more than 2–4 hours, and on the coasts of the northern seas they can not stop for several days.
In the European part of Russia greatest number days with fogs are observed on the sea coast, in high mountain areas, on the slopes of hills facing moisture-carrying streams. Particular attention should be paid to (), where the frequency of fogs is extremely high throughout the year. In some places, the average annual number of days with fog can reach 230–280, and their duration per year is 2050 hours. One fog lasts on average 9 hours.
Under monsoon circulation conditions in the Far East a large number of fogs are observed in the warm period of the year. On and on individual years the number of days with fogs can exceed 160–180 per year with a duration of 1000–1400 hours.
The annual cycle depends on geographical conditions: over continents, fogs most often form in autumn, over seas and oceans - in spring, when the water surface is coldest.
Fogs with visibility of 500–200 meters are already seriously impeding traffic. Fogs with visibility less than 50 m and duration of 12 hours or more are classified as special fogs. hazardous phenomena weather conditions and can completely paralyze the work of air and sea ports and ground transport. The average continuous duration of fog with visibility of 500 m or less is usually 2–4 hours, but in some cases they can persist for more than a day.
In large industrial centers, due to the large number of active condensation nuclei emitted by industrial enterprises, fogs can form even in unsaturated air and are observed 1.5–2 times more often than in the surrounding area. In Yakutsk, for example, the duration of fogs in the city center is 1300 hours, and on the outskirts (near the airport) - 475 hours.
IN major cities In the North-West, the number of days with visibility less than 500 m can range from 6 to 65. The longest fogs with such visibility are observed here in the autumn months. Their continuous duration is on average 3 hours. The maximum duration of fogs per year in some cities is close to 200–300 hours.
In the cities of Western Siberia, fogs with visibility less than 500 m are observed in 50–70% of cases from total number, and in the Cis-Baikal region - about 10%.
Large populated areas of the Republic of Sakha (Yakutia) are characterized by frosty fogs that form at temperatures of –42°C and below, in which visibility during the day decreases to 40–50 meters. The most dangerous fogs form in December – January.
In coastal cities Far East, Primorye, Kamchatka Peninsula, Sakhalin Island, summer fogs are the most intense.
Fog is an accumulation of small water droplets or ice crystals, or both, in the surface layer of the atmosphere, up to a height of several hundred meters, reducing horizontal visibility to 1 km or less.
Fog is formed as a result of condensation or sublimation of water vapor on aerosol (liquid or solid) particles contained in the air. Fog of water droplets is observed at air temperatures above -20 °C, but can occur even at temperatures below -40 °C. At temperatures below -20 °C, ice fogs predominate.
Visibility in fog depends on the size of the particles that form the fog and on its water content (the amount of condensed water per unit volume.). The radius of fog droplets ranges from 1 to 60 microns. Most drops have a radius of 5-15 microns at positive air temperatures and 2-5 microns at negative temperatures. The water content of fog usually does not exceed 0.05-0.1 g/m3, but in some dense fogs it can reach 1-1.5 g/m3. The number of drops per 1 cm3 ranges from 50-100 in light fogs to 500-600 in dense fogs. In very dense fogs, visibility can be reduced to a few meters.
Based on visibility range they distinguish the following types fogs:
1) Haze is a very thin fog, a continuous more or less uniform gray or bluish clouding of the atmosphere with a horizontal visibility range (at the eye level of an observer standing on the ground, i.e. about 2 m above the earth’s surface) from 1 to 9 km. It can be observed before or after fog, and more often as an independent phenomenon. Often observed during precipitation, especially liquid and mixed (rain, drizzle, rain with snow, etc.) due to humidification of air in the surface layer of the atmosphere due to partial evaporation of precipitation.
Haze should not be confused with deterioration in horizontal visibility due to dust, smoke, etc. In contrast to these phenomena, relative humidity air during haze exceeds 85-90%.
2) Ground fog - fog that spreads low over the earth's surface (or body of water) in a continuous thin layer or in the form of separate wisps, so that in the fog layer the horizontal visibility is less than 1000 m, and at a level of 2 m it exceeds 1000 m. It is observed how usually in the evening, night and morning hours.
3) Translucent fog - fog with horizontal visibility at a level of 2 m less than 1000 m (usually it is several hundred meters, and in some cases it drops even to several tens of meters), poorly developed vertically, so that it is possible to determine the state of the sky (amount and cloud shape). It is most often observed in the evening, at night and in the morning, but can also be observed during the day, especially in the cold half of the year when the air temperature rises.
4) Fog - continuous fog with horizontal visibility at a level of 2 m less than 1000 m (usually it is several hundred meters, and in some cases it drops even to several tens of meters), sufficiently developed vertically, so that it is impossible to determine the state of the sky (amount and cloud shape). It is most often observed in the evening, at night and in the morning, but can also be observed during the day, especially in the cold half of the year when the air temperature rises.
According to the method of occurrence, cooling fogs are divided into evaporation fogs. The first ones occur when the air is cooled below the dew point temperature, the water vapor contained in it reaches saturation and partially condenses; the second - with additional supply of water vapor from a warmer evaporating surface into cold air, as a result of which saturation is also achieved. Cooling fogs are the most common.
According to the synoptic conditions of formation, they are distinguished:
1) intra-mass fogs, formed in homogeneous air masses,
2) frontal fogs, the appearance of which is associated with atmospheric fronts.
Intramass fogs predominate, in most cases these are cooling fogs. Intramass fogs are divided into radiation and advection.
Radiation ones are formed over land when the temperature decreases due to radiative cooling of the earth's surface, and from it the air. They most often occur on clear nights with light winds, mainly in anticyclones. After sunrise, radiation fogs usually dissipate quickly. However, in the cold season, in stable anticyclones they can persist during the day, sometimes for many days in a row.
Advection fogs are formed due to the cooling of warm, moist air as it moves over a colder surface of land or water. The intensity of advective fogs depends on the temperature difference between the air and the underlying surface and on the moisture content of the air. They can develop both over land and over the sea and cover a huge area, sometimes on the order of several tens or even hundreds of thousands of km2. Advective fogs usually occur in cloudy weather and most often in the warm sectors of cyclones. Advection fogs are more persistent than radiation fogs and often do not dissipate during the day. Some advective fogs are evaporative fogs and occur when cold air is transferred to warm water. Fogs of this type are frequent, for example, in the Arctic, when air flows from the ice cover to the open surface of the sea.
Frontal fogs form near atmospheric fronts and move with them. Air saturation with water vapor occurs due to the evaporation of precipitation falling in the front zone. Fall plays some role in increasing fog ahead of fronts atmospheric pressure, which creates a slight adiabatic decrease in air temperature. Fogs occur more often in populated areas than far away from them. This is facilitated by the increased content of hygroscopic condensation nuclei (for example, combustion products) in the air.
Fog significantly affects visibility, which is one of the most important factors of safe navigation for a navigator. Visibility is the distance at which during the day the last signs of the observed object disappear (its outlines become indistinguishable), and at night an unfocused light source of a certain intensity becomes indistinguishable. Visibility is assessed in points, measured by eye for a number of objects located at different distances from the observer, according to the international visibility scale (Table 1):
Table 1. International visibility scale.
Score Visibility range Score Visibility range
0
1
2
3
4 0-50 m
50-200 m
200-500 m
500-1000 m
1-2 km 5
6
7
8
9 2-4 km
4-10 km
10-20 km
20-50 km
50 km
Table 2. Designation of fog when plotting data on weather maps.
Fogs occur at any time of the year, but most often they occur in late summer or autumn, when the air cools faster than the ground cools. As a result, cool air falls to the ground or water, which still retains heat, condensation occurs, and many water droplets hang in the air. It looks like a huge cloud is hanging right above the ground or a body of water. In the place where the fog formed, the air humidity is 100%. Fogs have different structures. If the air temperature is not very cold, above 10 degrees below zero, then the foggy cloud consists of water droplets. At a temperature of 10-15 degrees below zero, the cloud consists of a mixture of water droplets with ice crystals. If the temperature drops below 15 degrees below zero, then ice fog forms when the entire cloud consists of ice crystals. In cities and towns, fogs are denser due to the fact that condensation mixes with exhaust gases and dust.
What kind of fogs are there?
Fogs are different. It depends on how good the visibility is in the foggy area.
Haze is the weakest type of fog.
Ground fog is fog that spreads over the ground or body of water in a thin layer. This fog does not have much effect on visibility.
Translucent fog, visibility in which ranges from several tens to several hundred meters. Through such fog the sun and clouds are visible.
Continuous fog, when a whitish cloud envelops the earth, through which it is impossible to see literally anything at a distance of several meters, and sometimes even at arm's length. With such fog, traffic becomes impossible. If a driver finds himself in a cloud of continuous fog, it is better for him to wait until the fog clears.
There are not only natural fogs, but also artificial ones. Artificial fogs arise from human industrial activities. Artificial fog consists of dust, smoke, exhaust gases, chemical substances, other combustion products. Otherwise it is called smog. Smog-one of the most important problems modern cities, as it causes irreparable harm to human health and pollutes the environment.
Fog is essentially a cloud that exists low above the surface. It appears when warm, moist air inevitably comes into contact with colder air.
The level of water vapor concentration in the air is determined by temperature: the colder the air, the less vapor it can contain. If the vapor exceeds the amount possible at a given temperature (a point called the saturated vapor level), it condenses into fog.
If the temperature is low enough, fog can form even in relatively dry air. Fog is most likely to be present in air that contains large amounts of dust or other particles to which water droplets can attach. In polar regions, where temperatures can drop below -15°C, frozen fog consisting of ice crystals is sometimes observed.
Condensation is everywhere
The same condensation process that creates fog accompanies some ordinary Everyday life phenomena. For example, cold air outside the window cools the warm air in the room (1, above). As the indoor air cools, the water vapor condenses, forming water particles that cause the window to fog up. When warm breath comes out of your mouth wet air(2), it cools quickly and the water vapor condenses, causing the exhaled air to resemble fog. Cold juice in a glass (3) radiates coldness around itself, causing water vapor in the air to condense and form drops on the glass. The water vapor escaping from the kettle (4) is cooled by the air and condenses into a fog-like cloud.
How is radiant fog formed?
At night, as the soil begins to release the heat it absorbed during the day, the air temperature above it begins to drop. When sufficiently cooled, the water vapor condenses into a radiant mist. This type of fog often appears in low-lying areas on clear and relatively windless nights. (In the picture at the top of the article)
Formation of advective fog
Advection fog, common in areas close to the oceans, occurs when a mass of moist, warm air suddenly rises above a cold surface. The lower layer of air cools, causing condensation and the formation of fog particles.
How is creeping fog formed?
As warm, moist air rises up the mountainside, it expands and cools, causing the water vapor in the air to condense into fog. Climbers often encounter this kind of fog, which can resemble uneven patches. If the air currents continue to rise, the spreading fog eventually turns into clouds.
How is steam mist formed?
Sometimes cold air circulates over places that retain heat at night - such as rivers or ponds. Steam condenses in cold air warm water, forming steam mist. The greater the temperature amplitude between water and air, the thicker the fog will be.