Precipitation. What is precipitation? Definition and types of sediments formed
PRECIPITATION
PRECIPITATION, in meteorology, all forms of water, liquid or solid, falling from the atmosphere to the earth. Precipitation differs from CLOUDS, FOG, DEW and FROST in that it falls and reaches the ground. Includes rain, drizzle, SNOW and HAIL. They are measured by the thickness of the layer of fallen water and expressed in millimeters. Precipitation occurs due to the CONDENSATION of water vapor from clouds into small water particles, which merge into large drops with a diameter of about 7 mm. Precipitation also forms from melting ice crystals in clouds. Drizzle consists of very small drops, and snow is made of ice crystals, mainly in the form of hexagonal plates and six-rayed stars. Groats is formed when raindrops freeze into small ice pellets, and hail is formed when concentric layers of ice in cumulonimbus clouds freeze into fairly large, rounded chunks irregular shape, from 0.5 to 10 cm in diameter.
Precipitation. Thin clouds and clouds in the tropics do not reach freezing altitude, so ice crystals do not form in them (A). Instead, a larger-than-usual water particle in the cloud may combine with several million other water particles, resulting in the size of a raindrop. Electrical charges can cause water particles to stick together if they have opposite charges. Some drops break into pieces, forming water particles large enough to flow chain reaction, generating a stream of raindrops. Most rain in mid-latitudes, however, is the result of falling snow flakes that melt before they reach the ground (B). Many millions shallow waters The ice particles and ice crystals must combine to form a single drop or snowflake that is heavy enough to fall from the cloud to the ground. However, a snowflake can grow from ice crystals in just 20 minutes. In order for large hailstones to form, strong air currents (C) are required (hailstones with a diameter of 30 mm are formed at an air flow speed of 100 km/h). Eddy air currents during a thunderstorm transform frozen water particles into initial hailstones. Abundant supercooled moist water particles easily freeze to its surface. The hailstone is thrown from side to side by air currents, as a result of which numerous dense layers of ice are concentrated on it, which can be transparent or white. The opaque layer forms when air bubbles, and sometimes ice crystals, become trapped in a hailstone during rapid freezing in the cold upper layers of the cloud. Clear layers form in the warmer lower layers of the cloud, where water freezes much more slowly. A hailstone can have up to 25 or more layers (D), with the last - the clear layer of ice, often the thickest - forming when a hailstone falls through the moist and warm lower edge of the cloud. The largest hailstone was recorded on September 3, 1970 in Coffeyville, Kansas. Its diameter was 190 mm and its weight was 766 g.
Scientific and technical encyclopedic Dictionary .
Synonyms:See what "PRECIPITATION" is in other dictionaries:
Modern encyclopedia
Atmospheric water in a liquid or solid state (rain, snow, graupel, ground hydrometeors, etc.), falling from clouds or deposited from the air on earth's surface and on objects. Precipitation is measured by the thickness of the layer of fallen water in mm. IN… … Big Encyclopedic Dictionary
Groats, snow, drizzle, hydrometeor, lotions, rain Dictionary of Russian synonyms. precipitation noun, number of synonyms: 8 hydrometeor (6) ... Synonym dictionary
Precipitation- atmospheric, see Hydrometeors. Ecological encyclopedic dictionary. Chisinau: Main editorial office of the Moldavian Soviet Encyclopedia. I.I. Dedu. 1989. Precipitation, water coming from the atmosphere to the surface of the earth (in liquid or solid... Ecological dictionary
Precipitation- atmospheric, water in a liquid or solid state falling from clouds (rain, snow, pellets, hail) or deposited on the earth's surface and objects (dew, frost, hoarfrost) as a result of condensation of water vapor in the air. Precipitation is measured... ... Illustrated Encyclopedic Dictionary
In geology, loose formations deposited in a suitable environment as a result of physical, chemical and biological processes... Geological terms
PRECIPITATION, ov. Atmospheric moisture falling to the ground in the form of rain or snow. Abundant, weak o. Today there will be no precipitation (no rain, no snow). | adj. sedimentary, oh, oh. Ozhegov's explanatory dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary
- (meteor.). This name is usually used to denote the moisture that falls on the surface of the earth, being separated from the air or from the soil in dropwise liquid or solid form. This release of moisture occurs every time water vapor constantly... ... Encyclopedia of Brockhaus and Efron
1) atmospheric water in a liquid or solid state, falling from clouds or depositing from the air on the surface of the earth and on objects. O. falls from clouds in the form of rain, drizzle, snow, sleet, snow and ice pellets, snow grains,... ... Dictionary of emergency situations
PRECIPITATION- meteorological, liquid and solid bodies released from the air onto the surface of the soil and solid objects due to the thickening of water vapor contained in the atmosphere. If O. falls from a certain height, then the result is hail and snow; if they… … Great Medical Encyclopedia
Books
- Precipitation and thunderstorms from December 1870 to November 1871, A. Voeikov. Reproduced in the original author's spelling of the 1875 edition (St. Petersburg publishing house). IN…
Precipitation called drops of water and ice crystals falling from clouds or settling from the air onto the earth's surface. Precipitation from clouds provides more than 99% of the total amount of water coming from the atmosphere to the earth's surface; less than 1% comes from precipitation deposited from the air.
Precipitation x is characterized by quantity and intensity. Precipitation measured by the thickness (expressed in mm or cm) of the layer of water that they would form on the surface of the earth in the absence of infiltration, runoff and evaporation. Intensity − this is the amount of precipitation falling per unit of time (per minute or hour).
A necessary condition for the formation of precipitation is the enlargement of cloud elements to such sizes at which the rate of fall of these elements becomes greater than the rate of ascending flows. The consolidation process occurs mainly for the following reasons:
a) due to the recondensation of water from steam from water droplets onto ice crystals or from
small drops to large ones. This happens because the saturation elasticity over ice crystals is less than over water droplets, and over large drops it is less than over small ones.
b) due to the merging (coagulation) of water drops upon their collision as a result of turbulent air movements and different falling speeds of large and small drops. These collisions lead to the absorption of small droplets by large ones.
The growth of droplets due to condensation predominates until the radius of the droplet becomes equal to 20...60 μm, after which the main process of enlargement of cloud elements becomes coagulation.
Clouds that are homogeneous in structure, i.e. consisting only of identical
the size of droplets or only from ice crystals, precipitation is not given. Such clouds include cumulus and altocumulus, consisting of small water droplets, as well as cirrus, cirrocumulus and cirrostratus, consisting of ice crystals.
In clouds consisting of droplets of different sizes, larger droplets slowly grow at the expense of smaller ones. However, as a result of this process, only small drops of rain are formed. This process occurs in stratus and sometimes in stratocumulus clouds, from which precipitation can fall in the form of drizzle.
c) the main types of precipitation fall from mixed clouds, in which the enlargement of cloud elements occurs due to the freezing of supercooled drops on ice crystals. The consolidation of cloud elements proceeds rapidly and is accompanied by rain or snow. These clouds include cumulonimbus, nimbostratus and altostratus.
Precipitation falling from clouds can be liquid, solid or mixed.
Main forms of precipitation are:
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Wet snow– precipitation consisting of melting snow at a temperature of – 0°…+5°С.
Snow pellets– soft milky-white opaque grains of round shape with a diameter of 2...5 mm.
Ice grains – transparent grains with a dense white core in the center. The diameter of the grains is less than 5 mm. It is formed when raindrops or partially melted snowflakes freeze when falling through the lower layer of air with a negative temperature.hail– precipitation in the form of ice pieces of various sizes. Hailstones have an irregular or spherical (close to spherical) shape, their size ranges from 5 mm to 10 cm or more. Therefore, the weight of hailstones can be very large. In the center of the hailstones there is a whitish translucent grain, surrounded by several layers of transparent and opaque ice.
freezing rain– small transparent spherical particles with a diameter of 1...3 mm. They are formed when raindrops freeze, falling through the lower layer of air with a negative temperature (rain at a temperature of 0°... 5°C).
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By the nature of the loss, depending on physical educational conditions,
duration and intensity, precipitation is divided into three types:
1. Cover precipitation − these are long-lasting, medium-intensity precipitation in the form of raindrops or snow flakes, which are observed simultaneously over a large area. This precipitation falls from a system of frontal nimbostratus and altostratus clouds.
2. Rainfall − these are short-term, high intensity precipitation in the form of large drops, large snow flakes, sometimes ice pellets or hail, which are usually observed over small areas. They fall out of cumulonimbus, and sometimes powerful cumulus (in the tropics) clouds. They usually begin suddenly and do not last long, but in some cases they can recur several times. Rainfall is often accompanied by thunderstorms and squalls.
3. Drizzle - very small drops, tiny snowflakes or snow grains settling from the clouds to the ground almost imperceptibly to the eye. Observed simultaneously over a large area, their intensity is very low and is usually determined not by the amount of precipitation, but by the degree of deterioration in horizontal visibility. They fall from stratus and stratocumulus clouds.
To precipitation released directly from the air, include: dew, frost, frost, liquid or solid deposits on the windward side of vertically located objects.
Dew- this is liquid precipitation in the form of small droplets of water formed in summer nights and in the morning on objects located near the surface of the earth, plant leaves, etc. Dew is formed upon contact humid air with cooled objects, as a result of which water vapor condenses.
Frost- this is a white, fine-crystalline deposit formed as a result of sublimation of water vapor in cases where the temperature of the surface air and underlying surface is below 0°C;
High moisture content, low clouds and low winds contribute to the formation of dew and frost. A layer of air with a thickness of
200...300 m and more. Frost that forms on the surface of an aircraft on the ground must be carefully removed before departure, as this can lead to serious consequences due to the fact that the aerodynamic qualities of the aircraft deteriorate.
frost- This is white, loose, snow-like ice. It forms in foggy frosty weather with very light winds on branches of trees and bushes, wires and other objects. The formation of frost is mainly associated with the freezing of tiny supercooled droplets colliding with various objects. Snowy bang Roma frost can be of the most bizarre shapes. It easily crumbles when shaken, but when the temperature rises and a new cold snap occurs, it can freeze and freeze.
Liquid and solid plaque is formed on the windward part of vertically located objects cooled to a temperature below the ambient air temperature. In warm weather, a liquid coating forms, and at surface temperatures below 0°C, white translucent ice crystals form. This type of precipitation can form at any time of the day during sudden warming in the cold season.
Blowing snow is a special form of sediment transport. There are three types of blizzards:
drifting snow, blowing snow and general snowstorm.
Snowy drift And blowing snow are formed during the transfer of dry snow over the surface of the earth. Drifting snow is formed when the wind is 4...6 m/s, the snow rises to heights of up to 2 m above the ground. A blowing snow is formed when the wind is 6 m/s or more, the snow rises to heights of more than 2 m above the ground surface. At general snowstorm (does not have its own icon) snow is falling from the clouds, wind is 10 m/s or more, previously fallen snow is rising from the ground and visibility is less than 1000 m.
All types of precipitation complicate flight operations. The effect of precipitation on flights depends on its type, the nature of the precipitation and the air temperature.
1. During precipitation, visibility deteriorates and the cloud base decreases. In moderate rain, when flying at low speed, horizontal visibility deteriorates to 4...2 km, and at high flight speed - to 2...1 km. A significant deterioration in horizontal visibility is observed when flying in a snowfall zone. In light snow, visibility usually does not exceed 1...2 km, and in moderate and heavy snow it deteriorates to several hundred meters. In heavy rainfall, visibility sharply decreases to several tens of meters. The lower boundary of clouds in the precipitation zone, especially on atmospheric fronts, decreases to 50...100 m and can be located below the decision altitude.
2. Precipitation in the form of hail causes mechanical damage to aircraft. At high speed and flight, even small hailstones can make significant dents and destroy the cockpit glazing. Hail sometimes occurs at significant altitudes: small hail is observed at an altitude of about 13 km, and large hail is observed at an altitude of 9.5 km. The destruction of glazing at high altitudes can lead to depressurization, which is very dangerous.
3. When flying in an area of freezing rain, intense icing occurs
aircraft.
4. Long-term continuous precipitation in the warm season causes waterlogging of the soil and disables unpaved airfields for some time or another, disrupting the regularity of the departure and reception of aircraft.
5. Heavy rainfall worsens the aerodynamic qualities of the aircraft, which can lead to stalling. In connection with this, landing in heavy rainfall with visibility less than 1000 m prohibited .
6. When flying along VFR in a snowfall zone over a snow-covered surface, the contrast of all objects on the earth's surface is significantly reduced and, therefore, orientation is greatly deteriorated.
7. When landing on a wet or snow-covered runway, the aircraft's flight distance increases. Slip on a runway covered with snow is 2 times greater than on a concrete runway.
8. When an aircraft takes off from a runway covered with slush, hydroplaning may occur. The wheels of the aircraft throw off powerful jets of water and slush, causing strong braking and increasing the take-off run. Conditions may arise that the aircraft will not achieve take-off speed and a dangerous situation may arise.
9. Drop-down in winter time Snow requires additional work to remove and compact it on runways, taxiways and parking lots where aircraft and other machines and mechanisms are serviced.
Water that falls on the surface of the Earth in the form of rain, snow, hail, or is deposited on objects in the form of condensation as frost or dew is called precipitation. Precipitation may be heavy, associated with warm fronts, or showers associated with cold fronts.
The appearance of rain is caused by the merging of small droplets of water in a cloud into larger ones, which, overcoming the force of gravity, fall to the Earth. If the cloud contains small particles solids(dust particles), the condensation process proceeds faster, since they act as condensation nuclei. At subzero temperatures, condensation of water vapor in the cloud leads to snowfall. If snowflakes from the upper layers of the cloud fall into the lower layers with a higher temperature, where a large number of cold drops of water are contained, then the snowflakes combine with water, losing their shape and turning into snowballs with a diameter of up to 3 mm.
Precipitation formation
Hail is formed in clouds of vertical development, characteristic features which is the presence of positive temperatures in the lower layer and negative temperatures in the upper layer. IN in this case spherical snowballs with rising air currents rise to the upper parts of the cloud with lower temperatures and freeze to form spherical ice floes - hailstones. Then, under the influence of gravity, the hailstones fall to the Earth. They usually vary in size and can range in diameter from a pea to a chicken egg.
Types of precipitation
Such types of precipitation as dew, frost, frost, ice, fog are formed in the surface layers of the atmosphere due to the condensation of water vapor on objects. Dew appears when more high temperatures ah, frost and frost - when negative. When there is an excessive concentration of water vapor in the surface atmospheric layer, fog appears. If the fog mixes with dust and dirt in industrial cities, it's called smog. 
Precipitation is measured by the thickness of the water layer in millimeters. On average, our planet receives approximately 1000 mm of precipitation per year. To measure the amount of precipitation, a device such as a rain gauge is used. For many years, observations have been made of the amount of precipitation in different regions planets, thanks to which general patterns of their distribution over the earth’s surface were established.
Maximum precipitation is observed in equatorial belt(up to 2000 mm per year), minimum - in the tropics and polar regions (200-250 mm per year). In the temperate zone, the average annual precipitation is 500-600 mm per year.
In every climatic zone There is also unevenness in precipitation. This is explained by the terrain features of a certain area and the prevailing wind direction. For example, on the western outskirts of the Scandinavian mountain range 1000 mm falls per year, and on the eastern edges it falls more than half as much. Areas of land have been identified where there is almost no precipitation. These are the Atacama Desert, the central regions of the Sahara. In these regions, the average annual precipitation is less than 50 mm. Huge amounts of precipitation are observed in the southern regions of the Himalayas and Central Africa (up to 10,000 mm per year).
Thus, the defining features of the climate of a given area are the average monthly, seasonal, and average annual precipitation, its distribution over the Earth’s surface, and intensity. These climate features have a significant impact on many sectors of the human economy, including agriculture.
Related materials:
Atmosphere
Atmosphere pressure
The meaning of atmosphere
Types of precipitation
There are different classifications for precipitation.
Atmospheric precipitation and its chemical composition
A distinction is made between blanket precipitation, which is associated with warm fronts, and rainfall, which is associated with cold fronts.
Precipitation is measured in millimeters - the thickness of the layer of fallen water. On average, in high latitudes and deserts about 250 mm falls per year, and in general globe about 1000 mm of precipitation per year.
Measuring precipitation is essential for any geographical research. After all, precipitation is one of the most important links in moisture circulation on the globe.
The defining characteristics for a particular climate are considered to be the average monthly, annual, seasonal and long-term amount of precipitation, its daily and annual cycle, its frequency and intensity.
These indicators are extremely important for most sectors of the national (agricultural) economy.
Rain is liquid precipitation - in the form of drops from 0.4 to 5-6 mm. Raindrops can leave a mark in the form of a wet spot on a dry object, or on the surface of water - in the form of a diverging circle.
There are different types of rain: freezing rain, freezing rain and sleet. Both freezing rain and ice rain fall at subzero air temperatures.
Supercooled rain is characterized by liquid precipitation, the diameter of which reaches 5 mm; After this type of rain, ice may form.
A freezing rain represented by sediments in a solid state - these are ice balls with frozen water inside. Snow is precipitation that falls in the form of flakes and snow crystals.
Horizontal visibility depends on the intensity of snowfall. A distinction is made between sleet and sleet.
The concept of weather and its features
The state of the atmosphere in a particular place in exact time called weather. Weather is the most variable phenomenon in environment. It will start to rain, then the wind will start, and after a few hours the sun will shine and the wind will subside.
But even the variability of weather has its own patterns, despite the fact that the formation of weather is influenced by a huge number of factors.
The main elements characterizing the weather include the following meteorological indicators: solar radiation, Atmosphere pressure, air humidity and temperature, precipitation and wind direction, wind strength and cloud cover.
If we talk about weather variability, then most often it changes in temperate latitudes - in regions with continental climate. And the most stable weather occurs in polar and equatorial latitudes.
Weather changes are associated with the change of season, that is, changes are periodic, and over time weather are repeated.
Every day we observe the daily change in weather - night follows day, and for this reason weather conditions change.
Climate concept
The long-term weather pattern is called climate. Climate is determined in a specific area - thus, the weather pattern must be stable for a certain geographical location.
In other words, climate can be called the average value of weather over a long period of time. Often this period is over several decades.
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Cover precipitation
Long-term (from several hours to a day or more) precipitation in the form of rain (covering rain) or snow (covering snow), falling over a large area with a fairly uniform intensity from nimbostratus and high stratus clouds on a warm front. Continuous precipitation moisturizes the soil well.
Rain- liquid precipitation in the form of droplets with a diameter of 0.5 to 5 mm. Individual drops of rain leave a mark on the surface of water in the form of a diverging circle, and on the surface of dry objects - in the form of a wet spot.
Freezing rain- liquid precipitation in the form of drops with a diameter of 0.5 to 5 mm, falling at negative air temperatures (most often 0...-10°, sometimes up to -15°) - falling on objects, the drops freeze and ice forms. Freezing rain forms when falling snowflakes hit a layer of warm air deep enough for the snowflakes to completely melt and become raindrops. As these droplets continue to fall, they pass through a thin layer of cold air above the earth's surface and their temperature drops below freezing. However, the droplets themselves do not freeze, so this phenomenon is called supercooling (or the formation of “supercooled droplets”).
freezing rain- solid precipitation that falls at negative air temperatures (most often 0...-10°, sometimes up to -15°) in the form of solid transparent ice balls with a diameter of 1-3 mm. They are formed when raindrops freeze as they fall through the lower layer of air with a negative temperature. There is unfrozen water inside the balls - when falling on objects, the balls break into shells, the water flows out and ice forms.
Snow- solid precipitation that falls (most often at negative air temperatures) in the form of snow crystals (snowflakes) or flakes. With light snow, horizontal visibility (if there are no other phenomena - haze, fog, etc.) is 4-10 km, with moderate snow 1-3 km, with heavy snow - less than 1000 m (the snowfall increases gradually, so Visibility values of 1-2 km or less are observed no earlier than an hour after the start of snowfall). In frosty weather (air temperature below -10...-15°), light snow may fall from a partly cloudy sky. Separately, the phenomenon of wet snow is noted - mixed precipitation that falls at positive air temperatures in the form of flakes of melting snow.
Rain with snow- mixed precipitation that falls (most often at positive air temperatures) in the form of a mixture of drops and snowflakes.
Precipitation
If rain and snow fall at subzero air temperatures, precipitation particles freeze onto objects and ice forms.
Drizzle
Drizzle- liquid precipitation in the form of very small drops (less than 0.5 mm in diameter), as if floating in the air. A dry surface becomes wet slowly and evenly. When deposited on the surface of the water, it does not form diverging circles on it.
Freezing drizzle- liquid precipitation in the form of very small drops (with a diameter of less than 0.5 mm), as if floating in the air, falling at negative air temperatures (most often 0 ... -10 °, sometimes up to -15 °) - settling on objects, the drops freeze and form ice
Snow grains- solid precipitation in the form of small opaque white particles (sticks, grains, grains) with a diameter of less than 2 mm, falling at negative air temperatures.
Fog- an accumulation of condensation products (droplets or crystals, or both) suspended in the air directly above the surface of the earth. Cloudiness of the air caused by such accumulation. Usually these two meanings of the word fog are not distinguished. In fog, horizontal visibility is less than 1 km. Otherwise, the cloudiness is called haze.
Rainfall
Shower- short-term precipitation, usually in the form of rain (sometimes wet snow, cereals), characterized by high intensity (up to 100 mm/h). Occurs in unstable air masses on a cold front or as a result of convection. Typically, torrential rain covers a relatively small area.
Rain shower- torrential rain.
Shower snow- shower snow. It is characterized by sharp fluctuations in horizontal visibility from 6-10 km to 2-4 km (and sometimes up to 500-1000 m, in some cases even 100-200 m) over a period of time from several minutes to half an hour (snow “charges”).
Shower rain with snow- mixed rainfall precipitation, falling (most often at positive air temperatures) in the form of a mixture of drops and snowflakes. If rain shower falls with snow at negative air temperatures, precipitation particles freeze onto objects and ice forms.
Snow pellets- solid precipitation of a storm nature, falling at an air temperature of about zero degrees and having the appearance of opaque white grains with a diameter of 2-5 mm; The grains are fragile and easily crushed by fingers. Often falls before or simultaneously with heavy snow.
Ice grains- solid rainfall precipitation, falling at air temperatures from +5 to +10° in the form of transparent (or translucent) ice grains with a diameter of 1-3 mm; in the center of the grains there is an opaque core. The grains are quite hard (they can be crushed with your fingers with some effort), and when they fall on a hard surface they bounce off. In some cases, the grains may be covered with a film of water (or fall out along with droplets of water), and if the air temperature is below zero, then falling on objects, the grains freeze and ice forms.
hail- solid precipitation that falls in the warm season (at air temperatures above +10°) in the form of pieces of ice various shapes and size: usually the diameter of hailstones is 2-5 mm, but in some cases individual hailstones reach the size of a pigeon or even a chicken egg (then the hail causes significant damage to vegetation, car surfaces, breaks window glass, etc.). The duration of hail is usually short - from 1-2 to 10-20 minutes. In most cases, hail is accompanied by rain showers and thunderstorms.
Ice needles- solid precipitation in the form of tiny ice crystals floating in the air, formed in frosty weather (air temperature below -10...-15°). During the day they sparkle in the light of the sun's rays, at night - in the rays of the moon or in the light of lanterns. Quite often, ice needles form beautiful glowing “pillars” at night, extending from the lanterns upward into the sky. They are most often observed in clear or partly cloudy skies, sometimes falling from cirrostratus or cirrus clouds.
Many factors determine how much rain or snow will fall on the earth's surface. These are temperature, altitude, location of mountain ranges, etc.
Probably the rainiest place in the world is Mount Waialeale in Hawaii, on the island of Kauai. The average annual rainfall here is 1197 cm. Cherrapunjee in India has perhaps the second highest rainfall with an average annual rainfall ranging from 1079 to 1143 cm. Once 381 cm of rain fell in Cherrapunjee in 5 days. And in 1861 the amount of precipitation reached 2300 cm!
To make it more clear, let's compare the amount of rainfall in some cities around the world, London receives 61 cm of rain per year, Edinburgh receives about 68 cm, and Cardiff receives about 76 cm. New York receives about 101 cm of rain. Ottawa in Canada gets 86cm, Madrid about 43cm and Paris 55cm. So you see how Cherrapunji contrasts.
The driest place in the world is probably Arica in Chile. Here the precipitation level is 0.05 cm per year. The driest place in the US is Greenland Ranch in Death Valley. There, the average annual precipitation is less than 3.75 cm.
In some vast regions of the Earth heavy showers there are all year round. For example, almost every point along the equator receives 152 cm or more of precipitation every year. The equator is the junction point of two large air currents. Everywhere along the equator, air moving down from the north meets air moving up from the south.
There is a basic upward movement of hot air mixed with water vapor. As air rises to colder heights, large amounts of water vapor condense and fall as rain.
Most of the rain falls on the windward side of the mountains. The other side, called the leeward side, receives much less rainfall. An example is the Cascade Mountains in California. Western winds carrying water vapor move with Pacific Ocean. Having reached the coast, the air rises along the western slopes of the mountains, cooling.
Precipitation. Pattern and types of precipitation
Cooling causes water vapor to condense, which falls as rain or snow.
Depending on the nature of cloudiness and precipitation regime, two types are distinguished: diurnal cycle: continental and maritime. The continental type is characterized by two maxima: the main one - in the afternoon from convective cumulonimbus, and at the equator and from cumulus clouds and a minor one - in the early morning from stratus clouds, between them there are minima: at night and before noon.
What is precipitation? What types of precipitation do you know?
In the marine (coastal) type, there is one maximum of precipitation at night (due to unstable air stratification and convection) and one minimum during the day. These types of daily precipitation patterns are observed throughout the year in the hot zone, and in temperate zones possible only in summer.
The annual course of precipitation, i.e., its change by month during the year, is very different in different places on the Earth. This depends on many factors: radiation regime, general atmospheric circulation, specific physical-geographical situation, etc. Several main types of annual precipitation can be outlined and expressed in the form of bar charts (Fig. 47).
Rice. 47. Types of annual precipitation using the example of the northern hemisphere
Equatorial type – heavy rainfall fall fairly evenly throughout the year, there are no dry months, two small maximums are noted - in April and October, after the days of the equinoxes, and two small minimums in July and January, after the days of the solstices.
Monsoon type - maximum precipitation in summer, minimum in winter. It is characteristic of subequatorial latitudes, where the annual variation of precipitation due to the dryness of winter is expressed very sharply, and also east coasts continents in subtropical and temperate latitudes. However annual amplitude The precipitation here is somewhat smoothed out, especially in the subtropics, where frontal rains also fall in winter. The annual amount of precipitation gradually decreases from the subequatorial to the temperate zone.
Mediterranean type - maximum precipitation in winter due to active frontal activity, minimum in summer. It is observed in subtropical latitudes on the western coasts and inland.
In temperate latitudes, there are two main types of annual precipitation: continental and marine. The continental (inland) type is distinguished by the fact that in summer there is two to three times more precipitation than in winter, due to frontal and convective precipitation.
Marine type - precipitation is distributed evenly throughout the year with a slight maximum in autumn-winter. Their number is greater than in the previous type.
The Mediterranean and temperate continental types are characterized by a decrease in the total amount of precipitation as one moves inland.
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Atmospheric precipitation is one of the meteorological elements that strongly depend on a number of local landscape features.
Let us, however, try to trace what conditions influence their distribution.
First of all, it is necessary to note the air temperature. Temperature decreases from the equator to the poles; Consequently, both the intensity of evaporation and the moisture capacity of the air decrease in the same direction. In cold areas, evaporation is small, and cold air is not able to dissolve much water vapor; therefore, during condensation, a large amount of precipitation cannot be released from it. In warm areas, strong evaporation and high moisture capacity of the air lead to condensation of water vapor copious discharge precipitation. Thus, a pattern must inevitably appear on Earth, namely that in warm regions there is especially a lot of precipitation, while in cold regions there is little of it. This pattern actually manifests itself, but, like other phenomena in nature, it is complicated, and in some places completely obscured, by a number of other influences, and above all by atmospheric circulation, the nature of the distribution of land and sea, relief, altitude and sea currents.
Knowing the conditions necessary for the condensation of water vapor, it is possible to predict how atmospheric circulation affects the distribution of precipitation. Since air is a carrier of moisture, and its movement covers vast spaces on Earth, this inevitably leads to smoothing out differences in the amount of precipitation caused by temperature distribution in areas where the air experiences rises (above the equator, in cyclones, on the windward slopes of mountain ranges), an environment favorable for precipitation is created, and all other factors become subordinate. In those places where downward air movements predominate (in subtropical highs, in anticyclones in general, in the area of trade winds, on the leeward slopes of mountains, etc.), there is much less precipitation.
It is generally accepted that the amount of precipitation in a given area in high degree depends on its proximity to the sea or distance from the sea. In fact, there are many examples where very dry areas of the Earth are located on the ocean coasts and, conversely, far from the sea, inland (as, for example, on the eastern slope of the Andes in the upper reaches of the Amazon), huge amounts of precipitation fall. The point here is not so much the distance from the sea, but the nature of atmospheric circulation and surface structure, i.e., the absence or presence of mountain ranges that interfere with the movement of air masses carrying moisture. During the southwest monsoon in India air masses pass over the Thar Desert without irrigating it with rain, since the flat terrain does not interfere with the movement of air, and the heated desert has a rather drying effect on the air masses.
Types of precipitation.
But the same monsoon on the windward slope of the Western Ghats, not to mention the southern slopes of the Himalayas, leaves a huge amount of moisture.
The need to distinguish orographic sediments into a special type indicates exclusively big role arrangement of the earth's surface in the distribution of precipitation. True, in this case, as in all others, the relief matters not only in itself, as a mechanical obstacle, but in combination with absolute height and atmospheric circulation.
Penetration of warm sea currents in high latitudes contributes to the formation of precipitation due to the fact that warm currents associated cyclonic circulation of the atmosphere. Cold currents have the opposite effect, since high pressure spurs usually develop above them.
Of course, none of these factors affects the distribution of precipitation independently of the others. In each case, the loss of atmospheric moisture is regulated by a complex and sometimes contradictory interaction of both general and local agents. However, if we ignore the details, the main conditions that determine the placement of precipitation in the landscape envelope still include temperature, general atmospheric circulation and relief.
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What is water vapor? What properties does it have?
Water vapor is the gaseous state of water. It has no color, taste or smell. Contained in the troposphere. Formed by water molecules during its evaporation. When water vapor cools, it turns into water droplets.
In what seasons of the year does it rain in your area? When does it snow?
Rain falls in summer, autumn, and spring. Snowfalls - winter, end of autumn, beginning of spring.
Using Figure 119, compare the average annual precipitation in Algeria and Vladivostok. Is precipitation distributed equally across months?
The annual precipitation in Algeria and Vladivostok is almost the same - 712 and 685 mm, respectively. However, their distribution throughout the year differs. In Algeria, maximum precipitation occurs in late autumn and winter. Minimum - for the summer months. In Vladivostok, most of the precipitation falls in summer and early autumn, with the minimum falling in winter.
Look at the picture and tell us about the alternation of belts with different annual precipitation amounts.
The distribution of precipitation generally shows changes in the direction from the equator to the poles. They fall in a wide strip along the equator greatest number- over 2000 mm per year. In tropical latitudes there is very little precipitation - an average of 250-300 mm, and in temperate latitudes it again becomes more. With further approach to the poles, the amount of precipitation again decreases to 250 mm per year or less.
Questions and tasks
1. How are precipitation formed?
Atmospheric precipitation is water that falls to the ground from clouds (rain, snow, hail) or directly from the air (dew, frost, frost). Clouds are made up of tiny droplets of water and ice crystals. They are so small that they are held by air currents and do not fall to the ground. But droplets and snowflakes can merge with each other. Then they increase in size, become heavy and fall to the ground in the form of precipitation.
2. Name the types of precipitation.
Precipitation can be liquid (rain), solid (snow, hail, pellets) and mixed (snow and rain)
3. Why does the collision of warm and cold air lead to precipitation?
When faced with cold air warm air, displaced by the heavy cold, rises up and begins to cool. Water vapor in warm air condenses. This leads to the formation of clouds and precipitation.
4. Why in cloudy weather Doesn't it always rain?
Precipitation only occurs if the air is oversaturated with moisture.
5. How can you explain that there is a lot of precipitation near the equator, but very little near the poles?
Large amounts of precipitation fall near the equator, as high temperatures cause large amounts of moisture to evaporate. The air quickly becomes saturated and precipitation occurs. At the poles, low air temperatures prevent evaporation.
6. How much precipitation falls per year in your area?
In the European part of Russia, an average of about 500 mm will fall per year.
Classification of precipitation. By type, precipitation is divided into liquid, solid and ground.
Liquid precipitation includes:
rain – precipitation in the form of drops of various sizes with a diameter of 0.5–7 mm;
drizzle - small droplets with a diameter of 0.05–0.5 mm, seemingly in suspension.
Solid sediments include:
snow – ice crystals that form various kinds of snowflakes (plates, needles, stars, columns) 4–5 mm in size. Sometimes snowflakes are combined into snow flakes, the size of which can reach 5 cm or more;
snow pellets - precipitation in the form of opaque spherical grains of white or matte white (milky) color with a diameter of 2 to 5 mm;
ice pellets are solid particles that are transparent on the surface and have an opaque, matte core in the center. The diameter of the grains is from 2 to 5 mm;
hail – more or less large pieces of ice (hailstones), having a spherical or irregular shape and a complex internal structure. The diameter of hailstones varies within a very wide range: from 5 mm to 5–8 cm. There are cases when hailstones weighing 500 g or more fell.
If precipitation does not fall from clouds, but is deposited from atmospheric air on the surface of the earth or on objects, then such precipitation is called ground precipitation. These include:
dew - tiny drops of water that condense on the horizontal surfaces of objects (decks, boat covers, etc.) due to radiation cooling them on clear cloudless nights. A slight wind (0.5–10 m/s) promotes the formation of dew. If the temperature of horizontal surfaces is below zero, then under similar conditions water vapor sublimates on them and frost forms - a thin layer of ice crystals;
liquid deposit – tiny drops of water or a continuous film of water, formed in cloudy and windy weather on the windward predominantly vertical surfaces of cold objects (walls of superstructures, protective devices of winches, cranes, etc.).
glaze is an ice crust that forms when the temperature of these surfaces is below 0 °C. In addition, a hard coating may form on the surfaces of the vessel - a layer of crystals densely or densely sitting on the surface or a thin continuous layer of smooth transparent ice.
In foggy frosty weather with low winds, granular or crystalline frost may form on the vessel's equipment, ledges, cornices, wires, etc. Unlike frost, rime does not form on horizontal surfaces. The loose structure of frost distinguishes it from solid plaque. Granular rime is formed at air temperatures from -2 to -7 ° C due to freezing on the subject of supercooled fog droplets, and crystalline rime, which is a white precipitate of crystals of a fine structure, is formed at night with a cloudless sky or thin clouds from particles of fog or haze at a temperature from –11 to –2 °C and above.
According to the nature of precipitation, precipitation is divided into shower, heavy and drizzling.
Rainfall falls from cumulonimbus (thunderstorm) clouds. In summer it is large drops of rain (sometimes with hail), and in winter it is heavy snowfall with frequent changes in the shape of snowflakes, snow or ice grains. Precipitation occurs from nimbostratus (summer) and altostratus (winter) clouds. They are characterized by small fluctuations in intensity and long duration of fallout.
Drizzle falls from stratus and stratocumulus clouds in the form of small drops with a diameter of no more than 0.5 mm, descending at very low speeds.
Based on intensity, precipitation is divided into strong, moderate and light.
Clouds and precipitation.
Upper level clouds.
Cirrus (Ci)- Russian name feathery, individual tall, thin, fibrous, white, often silky clouds. Their fibrous and feathery appearance is due to the fact that they are composed of ice crystals.
Cirrus appear in the form of isolated bunches; long, thin lines; feathers like smoke torches, curved stripes. Cirrus clouds can appear in parallel bands that cross the sky and appear to converge on a single point on the horizon. This will be the direction to the area low pressure. Because of their height, they become illuminated earlier than other clouds in the morning and remain illuminated after the Sun has set. Cirrus are generally associated with clear weather, but if they are followed by lower and denser clouds, then there may be rain or snow in the future.
Cirrocumulus (Cc) , the Russian name for cirrocumulus, are tall clouds made up of small white flakes. Usually they do not reduce illumination. They are placed in the sky in separate groups of parallel lines, often like ripples, similar to sand on the coast or waves on the sea. Cirrocumulus are composed of ice crystals and are associated with clear weather.
Cirrostratus (Cs), The Russian name is cirrostratus - thin, white, high clouds, sometimes covering the sky completely and giving it a milky tint, more or less distinct, reminiscent of a thin tangled network. The ice crystals they are made of refract light to form a halo with the Sun or Moon at the center. If the clouds subsequently thicken and lower, precipitation can be expected in about 24 hours. These are the clouds of a warm front system.
Upper level clouds do not produce precipitation.
Mid-level clouds. Precipitation.
Altocumulus (Ac), Russian name altocumulus,- middle-tier clouds, consisting of a layer of large individual spherical masses. Altocumulus (Ac) are similar to the upper level clouds of sirrocumulus. Since they lie lower, their density, water content and the size of individual structural elements are greater than those of sirrocumulus. Altocumulus (Ac) can vary in thickness. They can range from blinding white if they are illuminated by the Sun to dark gray if they cover the entire sky. They are often mistaken for stratocumulus. Sometimes individual structural elements merge and form a series of large swells, like ocean waves, with stripes of blue sky between them. These parallel stripes differ from cirrocumulus in that they appear on the palate in large dense masses. Sometimes altocumulus appear before a thunderstorm. As a rule, they do not produce precipitation.
Altostratus (As) , Russian name altostratified, - middle-tier clouds that look like a gray fibrous layer. The sun or moon, if visible, shines through, as if through frosted glass, often with crowns around the luminary. Halos do not form in these clouds. If these clouds thicken, lower, or turn into low ragged Nimbostratus, then precipitation begins to fall from them. Then you should expect prolonged rain or snow (for several hours). In the warm season, drops from altostratus, evaporating, do not reach the surface of the earth. In winter they can produce significant snowfalls.
Low level clouds. Precipitation.
Stratocumulus (Sc) Russian name stratocumulus– low clouds that look like soft, gray masses, like waves. They can be formed into long, parallel shafts similar to altocumulus. Sometimes precipitation falls from them.
Stratus (St), The Russian name is stratified - low, homogeneous clouds resembling fog. Often their lower boundary is at an altitude of no more than 300 m. The curtain of dense stratus gives the sky a hazy appearance. They can lie on the very surface of the earth and are then called fog. Stratus can be dense and transmit sunlight so poorly that the Sun is not visible at all. They cover the Earth like a blanket. If you look from above (having broken through the thickness of the clouds on an airplane), they are dazzlingly white illuminated by the sun. Strong wind sometimes tears the stratus into shreds, called stratus fractus.
Lungs can fall out of these clouds in winter ice needles, and in the summer - drizzle– very small droplets suspended in the air and gradually settling. Drizzle comes from continuous low stratus or from those lying on the surface of the Earth, that is, from fog. Fog is very dangerous in navigation. Freezing drizzle can cause icing on the boat.
Nimbostratus (Ns) , Russian name for stratostratus, - low, dark. Stratus, shapeless clouds, almost uniform, but sometimes with damp patches at the base. Nimbostratus usually cover vast territories measured in hundreds of kilometers. Throughout this vast territory there is simultaneously snow or rain. Precipitation falls for long hours (up to 10 hours or more), drops or snowflakes are small in size, the intensity is low, but during this time a significant amount of precipitation can fall. They are called cover. Similar precipitation may also fall from Altostratus, and sometimes from Stratocumulus.
Clouds of vertical development. Precipitation.
Cumulus (Cu) . Russian name cumulus, - dense clouds formed in vertically rising air. As the air rises, it cools adiabatically. When its temperature reaches the dew point, condensation begins and a cloud appears. Cumulus have a horizontal base, convex top and side surfaces. Cumulus appear as separate flakes, and never cover the palate. When the vertical development is small, the clouds look like tufts of cotton wool or cauliflower. Cumulus are called "fair weather" clouds. They usually appear by midday and disappear by evening. However, Cu can merge with altocumulus, or grow and turn into thunderous cumulonimbus. Cumulus are distinguished by high contrast: the white, illuminated by the Sun, and the shadow side.
Cumulonimbus (Cb), Russian name cumulonimbus, - massive clouds of vertical development, rising in huge columns on greater height. These clouds begin in the lowest tier and extend to the tropopause, and sometimes extend into the lower stratosphere. They are taller than the highest mountains on Earth. Their vertical thickness is especially great in equatorial and tropical latitudes. The upper part of Cumulonimbus is composed of ice crystals, often stretched by the wind in an anvil shape. At sea, the top of the cumulonimbus can be visible at a great distance, when the base of the cloud is still below the horizon.
Cumulus and cumulonimbus are called clouds of vertical development. They are formed as a result of thermal and dynamic convection. On cold fronts, cumulonimbus arise as a result of dynamic convection.
These clouds can appear in the cold air at the rear of the cyclone and at the front of the anticyclone. Here they are formed as a result of thermal convection and give, accordingly, intramass, local rainfall. Cumulonimbus and associated showers over the oceans occur more often at night, when the air above the water surface is thermally unstable.
Particularly powerful cumulonimbus develop in the intertropical convergence zone (near the equator) and in tropical cyclones. Associated with cumulonimbus are: atmospheric phenomena like rain showers, snow showers, snow pellets, thunderstorms, hail, rainbows. It is with cumulonimbus that tornadoes (tornadoes), the most intense and most often observed in tropical latitudes, are associated.
Shower rain (snow) characterized by large drops (snow flakes), sudden onset, sudden end, significant intensity and short duration (from 1-2 minutes to 2 hours). Rain showers in summer are often accompanied by thunderstorms.
Ice grains It is a hard, opaque piece of ice up to 3 mm in size, moist on top. Ice pellets fall with heavy rain in spring and autumn.
Snow pellets has the appearance of opaque soft grains of white branches from 2 to 5 mm in diameter. Snow pellets are observed when the wind is squally. Snow pellets are often observed simultaneously with heavy snow.
hail falls only in the warm season exclusively during showers and thunderstorms of their most powerful cumulonimbus and usually lasts no more than 5-10 minutes. These are pieces of ice with a layered structure, about the size of a pea, but there are also many larger sizes.
Other precipitation.
Precipitation in the form of drops, crystals or ice on the surface of the Earth or objects is often observed, not falling from clouds, but precipitating from the air under a cloudless sky. This is dew, frost, frost.
Dew drops that appear on the deck at night in summer. At negative temperatures it forms frost. Frost - ice crystals on wires, ship equipment, racks, yards, masts. Frost forms at night, more often when there is fog or haze, at air temperatures below -11°C.
Ice an extremely dangerous phenomenon. It is an ice crust that results from the freezing of supercooled fog, drizzle, raindrops or droplets on supercooled objects, especially on windward surfaces. A similar phenomenon occurs from splashing or flooding of the deck. sea water at negative air temperatures.
Determining cloud height.
At sea, cloud heights are often determined approximately. This is a difficult task, especially at night. The height of the lower base of clouds of vertical development (any variety of cumulus), if they were formed as a result of thermal convection, can be determined from psychrometer readings. The height to which the air must rise before condensation begins is proportional to the difference between the air temperature t and the dew point td. At sea, this difference is multiplied by 126.3 to obtain the height of the lower boundary of the cumulus clouds N in meters. This empirical formula looks like:
H = 126.3 ( t – t d ). (4)
The height of the base of lower layer stratus clouds ( St, Sc, Ns) can be determined using empirical formulas:
H = 215 (t – t d ) (5)
H = 25 (102 - f); (6)
Where f – relative humidity.
Visibility. Fogs.
Visibility This is the maximum horizontal distance at which an object can be clearly visible and recognized in daylight. In the absence of any impurities in the air, it is up to 50 km (27 nautical miles).
Visibility is reduced due to the presence of liquid and solid particles in the air. Visibility is impaired by smoke, dust, sand, and volcanic ash. This occurs when there is fog, smog, haze, or precipitation. The visibility range decreases due to splashes in the sea in stormy weather with a wind force of 9 or more (40 knots, about 20 m/s). Visibility becomes worse during low, continuous clouds and at dusk.
Haze
Haze is a clouding of the atmosphere due to solid particles suspended in it, such as dust, as well as smoke, burning, etc. With severe haze, visibility decreases to hundreds, and sometimes to tens of meters, as in thick fog. Haze is usually a consequence of dust (sand) storms. Even relatively large particles are lifted into the air by strong winds. This is a typical phenomenon of deserts and plowed steppes. Large particles spread in the lowest layer and settle near their source. Small particles are carried over long distances by air currents, and due to air turbulence they penetrate upward to a considerable height. Fine dust remains in the air for a long time, often in the complete absence of wind. The color of the Sun becomes brownish. The relative humidity during these events is low.
Dust can be transported over long distances. It was celebrated in the Greater and Lesser Antilles. Dust from the Arabian deserts is carried by air currents into the Red Sea and the Persian Gulf.
However, during haze, visibility is never as bad as during fog.
Fogs. General characteristics.
Fogs pose one of the greatest dangers to navigation. They are responsible for many accidents, human lives, and sunken ships.
Fog is said to occur when horizontal visibility, due to the presence of droplets or water crystals in the air, becomes less than 1 km. If visibility is more than 1 km, but not more than 10 km, then such a decrease in visibility is called haze. Relative humidity during fog is usually more than 90%. Water vapor itself does not reduce visibility. Visibility is reduced by water droplets and crystals, i.e. water vapor condensation products.
Condensation occurs when the air is oversaturated with water vapor and the presence of condensation nuclei. Above the sea it is mainly small particles of sea salt. Supersaturation of air with water vapor occurs when the air is cooled or in cases of additional supply of water vapor, and sometimes as a result of mixing of two air masses. In accordance with this, fogs are distinguished cooling, evaporation and mixing.
Based on intensity (based on the visual range D n), fogs are divided into:
strong D n 50 m;
moderate 50 m<Д n <500 м;
weak 500 m<Д n < 1000 м;
heavy haze 1000 m<Д n <2000 м;
light haze 2000 m<Д n <10 000 м.
Based on their state of aggregation, fogs are divided into droplet-liquid, icy (crystalline) and mixed. Visibility conditions are worst in icy fogs.
Cooling mists
Water vapor condenses as the air cools to its dew point. This is how cooling fogs are formed - the largest group of fogs. They can be radiative, advective and orographic.
Radiation mists. The Earth's surface emits long-wave radiation. During the day, energy losses are offset by the arrival of solar radiation. At night, radiation causes the Earth's surface temperature to drop. On clear nights, the cooling of the underlying surface occurs more intensely than in cloudy weather. The air adjacent to the surface also cools. If the cooling is to the dew point and below, then dew will form in calm weather. A weak wind is required for fog to form. In this case, as a result of turbulent mixing, a certain volume (layer) of air is cooled and condensation forms in this layer, i.e. fog. Strong wind leads to mixing of large volumes of air, dispersion of condensate and its evaporation, i.e. to the disappearance of the fog.
Radiation fog can extend up to a height of 150 m. It reaches its maximum intensity before or shortly after sunrise, when the minimum air temperature occurs. Conditions necessary for the formation of radiation fog:
High air humidity in the lower layers of the atmosphere;
Stable stratification of the atmosphere;
Partly cloudy or clear weather;
Light wind.
The fog disappears as the earth's surface warms up after sunrise. The air temperature rises and the droplets evaporate.
Radiation mists above the water surface are not formed. Daily fluctuations in the temperature of the water surface, and therefore the air, are very small. The temperature at night is almost the same as during the day. Radiative cooling does not occur, and there is no condensation of water vapor. However, radiation fogs can cause problems in navigation. In coastal areas, fog, as a single whole, flows with cold, and therefore heavy, air onto the water surface. This can also be amplified by night breezes from land. Even clouds formed at night over elevated coasts can be carried by the night breeze to the surface of the water, as is observed on many coasts of temperate latitudes. The cloud cap from the hill often flows down, covering the approaches to the shore. More than once this led to a collision between ships (port of Gibraltar).
Advection fogs. Advective fogs result from the advection (horizontal transfer) of warm, moist air onto a cold underlying surface.
Advective fogs can simultaneously cover vast horizontal spaces (many hundreds of kilometers), and vertically extend up to 2 kilometers. They do not have a daily cycle and can exist for a long time. Over land at night they intensify due to radiation factors. In this case, they are called advective-radiative. Advective fogs also occur with significant winds, provided that the air stratification is stable.
These fogs are observed over land in the cold season when relatively warm and humid air enters it from the water surface. This phenomenon occurs in Foggy Albion, Western Europe, and coastal areas. In the latter case, if fogs cover relatively small areas, they are called coastal.
Advective fogs are the most common fogs in the ocean, occurring near the coasts and in the depths of the oceans. They always stand above cold currents. In the open sea, they can also be found in warm sectors of cyclones, in which air is transported from warmer areas of the ocean.
They can be found off the coast at any time of the year. In winter, they form over land and can partially slide to the water surface. In summer, advective fogs occur off the coast in cases where warm, moist air from the continent, in the process of circulation, passes to a relatively cold water surface.
Signs of the imminent disappearance of advective fog:
- change in wind direction;
- disappearance of the warm sector of the cyclone;
- it started to rain.
Orographic fogs. Orographic fogs or slope fogs are formed in mountainous areas with a low-gradient baric field. They are associated with the valley wind and are observed only during the day. The air rises up the slope with the valley wind and is cooled adiabatically. Once the temperature reaches the dew point, condensation begins and a cloud forms. For residents of the slope it will be fog. Sailors can encounter such fogs off the mountainous coasts of islands and continents. Fogs can obscure important landmarks on the slopes.
Mists of evaporation
Condensation of water vapor can occur not only as a result of cooling, but also when the air is oversaturated with water vapor due to evaporation of water. The evaporating water should be warm and the air cold, the temperature difference should be at least 10 °C. Cold air stratification is stable. In this case, an unstable stratification is established in the lowest driving layer. This causes a large amount of water vapor to flow into the atmosphere. It will immediately condense in the cold air. A fog of evaporation appears. Often it is small vertically, but its density is very high and, accordingly, visibility is very poor. Sometimes only the masts of the ship stick out from the fog. Such fogs are observed over warm currents. They are characteristic of the Newfoundland region, at the junction of the warm Gulf Stream and the cold Labrador Current. This is an area of heavy shipping.
In the Gulf of St. Lawrence, fog sometimes extends vertically up to 1500m. At the same time, the air temperature can be below 9°C below zero and the wind is almost gale force. The fog in such conditions consists of ice crystals and is dense with very poor visibility. Such dense sea fogs are called frost smoke or arctic frost smoke and pose a serious danger.
At the same time, with unstable air stratification, there is a slight local hovering of the sea, which does not pose a danger to navigation. The water seems to be boiling, streams of “steam” rise above it and immediately dissipate. Such phenomena occur in the Mediterranean Sea, off Hong Kong, in the Gulf of Mexico (with the relatively cold north wind “Norther”) and in other places.
Mixing mists
Fog can also form when two air masses mix, each of which has high relative humidity. The reservoir may be oversaturated with water vapor. For example, if cold air meets warm and humid air, the latter will cool at the mixing boundary and fog may appear there. Fog ahead of a warm front or occluded front is common in temperate and high latitudes. This mixing fog is known as frontal fog. However, it can also be considered as evaporation fog, since it occurs when warm droplets evaporate in cold air.
Mixing fogs form at the edge of ice and above cold currents. An iceberg in the ocean can be surrounded by fog if there is enough water vapor in the air.
Geography of fogs
The type and shape of clouds depend on the nature of the prevailing processes in the atmosphere, the season of the year and the time of day. Therefore, much attention is paid to observations of the development of clouds over the sea when sailing.
There are no fogs in the equatorial and tropical regions of the oceans. It’s warm there, there are no differences in temperature and air humidity day and night, i.e. There is almost no daily variation of these meteorological quantities.
There are a few exceptions. These are vast areas off the coast of Peru (South America), Namibia (South Africa) and off Cape Guardafui in Somalia. In all these places it is observed upwelling(rising of cold deep waters). Warm, moist air from the tropics flows over cold water and forms advective fog.
Fogs in the tropics can occur near continents. Thus, the port of Gibraltar has already been mentioned; fog is possible in the port of Singapore (8 days a year); in Abidjan there are up to 48 days of fog. Their greatest number is in the Bay of Rio de Janeiro - 164 days a year.
In temperate latitudes, fogs are a very common phenomenon. Here they are observed off the coast and in the depths of the oceans. They occupy vast territories and occur in all seasons of the year, but are especially frequent in winter.
They are also typical for polar regions near the boundaries of ice fields. In the North Atlantic and the Arctic Ocean, where the warm waters of the Gulf Stream penetrate, there is constant fog during the cold season. They are often found at the ice edge in summer.
Fogs most often occur at the junction of warm and cold currents and in places where deep water rises. The frequency of fogs is also high along the coasts. In winter, they occur when warm, moist air advects from the ocean onto land, or when cold continental air flows down onto relatively warm water. In the summer, air from the continent hitting the relatively cold water surface also produces fog.
