Moisture circulation
The initial source of atmospheric moisture is the World Ocean, from the surface of which water evaporates. Some of it condenses in the clouds and falls as precipitation right there on the ocean, completing the small moisture cycle. Another part of the evaporated moisture in the form of water vapor is transferred to land, where it also condenses in the clouds and falls in the form of liquid or solid precipitation, seeps into the ground, flows in rivers into the ocean and is consumed by plants and animals. This link in the moisture cycle is not closed, since most of the water vapor of plants is decomposed into hydrogen and oxygen during photosynthesis, and the smaller part is bound, irreversibly excluding it from water exchange. Moisture circulation is characterized quantitatively water balance.
Water balance - ϶ᴛᴏ the algebraic sum of all forms of inflow and outflow of moisture in the atmosphere, in a selected territory or at sea, on the continent or ocean and on the earth’s surface as a whole.
Precipitation (P) that falls on the territory partially evaporates (E) into the atmosphere, partially flows (R): into the ocean
P = E + R,
that is, precipitation is equal to evaporation plus runoff. This is the water balance. The above equation was proposed by A.I. Voeikov in 1884.
In 1932 ᴦ. G.N. Vysotsky proposed an equation in which evaporation and runoff are divided into their component parts. Evapotranspiration E consists of direct evaporation E n and transpiration T:
E = En + T.
Full drain R was dissected into superficial S and underground U :
K = S + U.
The supply or shortage of groundwater in past years also takes part in the water balance of the territory. ±W.
Today the water balance formula looks like this:
P = En + T + S + U ±W
Complete equation The water balance of a limited area includes (in addition to the components already listed) moisture condensation on the surface, surface inflow, underground inflow, changes in water reserves in the snow cover, the same in swamps, water intake, transfer to other systems and return of water from household needs. With the help of a few components, it reflects the diverse relationships between water, atmospheric air, soil and vegetation.
Evaporation consists in the transition of water from the liquid or solid phase to the gaseous phase and the entry of water vapor into the atmosphere.
Evaporation is primarily an energy process. It depends on the amount of thermal energy that can be expended on a given surface per unit time, and is therefore determined by the equation heat balance on the earth's surface. On the oceans, up to 90% of the energy of solar radiation is spent on evaporation.
The second meteorological condition that determines the amount of evaporation is the moisture capacity of the air, the degree of dryness or humidity. Quantitatively, it is characterized by a moisture deficit, which in turn depends on air temperature and, to a lesser extent, on wind. Of course, evaporation can only occur in the presence of water. On land, this condition is not present everywhere and not always: arid zones are characterized by a moisture deficit, while in humid zones there may be a lack of moisture in certain periods. In this regard, meteorology has developed the concept of volatility (Ec).
Volatility - ϶ᴛᴏ the maximum possible evaporation under given meteorological conditions, not limited by moisture reserves. The same applies to the term “potential evaporation”.
Evaporation is one of the most important processes of the geographical envelope. It consumes most of the solar heat . The latent heat of vaporization, released during moisture condensation, heats the atmosphere, and this is the main heat source for the atmosphere. Evaporated moisture enters the continents and provides them with precipitation. During phase transitions of water, heat is absorbed or released, and during atmospheric circulation it is redistributed. One of the types of evaporation, transpiration, takes part in biological processes and the formation of biological mass.
The climatic and, especially, biophysical significance of evaporation is essentially that it shows the drying ability of air: the more it can evaporate with limited moisture reserves in the soil, the more pronounced is the aridity. In some places this leads to the appearance of deserts, in others it causes temporary droughts, and thirdly, where evaporation is negligible, waterlogging conditions are created.
IN Northern Europe evaporation is close to its upper limit - evaporation - about 100 mm per year. In the dry steppe zone of South-East Europe, as well as in the arid regions of the Mediterranean subtropics, evaporation reaches 1200 - 1300 mm, but actual evaporation due to lack of moisture is only 300 mm. Moisture deficiency - the difference between precipitation and evaporation in arid zones is approximately 600-800 mm.
Maximum evaporation, naturally, occurs in deserts, especially in the Sahara. In its central parts it exceeds 4500 mm. Evaporation, limited by an insignificant amount of precipitation, does not exceed 100 mm per year. Here, not only precipitation is consumed for evaporation, but also underground water flowing from the Atlas Mountains and from the basin Central Africa. The difference between potential (4500 mm) and actual (about 100 mm) evaporation expresses the degree of dryness of the Sahara.
The greatest evaporation (about 1,200 mm) occurs in swampy lowlands Central Africa-in basin of Lake Chad and Upper Nile. Plants provided here with warmth and moisture provide the greatest increase in plant mass on Earth. In equatorial Africa, a layer of water of 1000 mm evaporates per year.
Evaporation and evaporation reflect both precipitation and heat patterns. The ratio of the inflow and outflow of atmospheric moisture is usually called atmospheric humidification.
Water in the atmosphere. Properties of water
Water is everywhere on earth. Oceans, seas, rivers, lakes and other bodies of water occupy 71% of the earth's surface. Water, which is contained in the atmosphere, is the only substance that can be there in all three phase states (solid, liquid and gaseous) at the same time.
The most important physical properties of water for meteorology are presented in Table 6.
Table 6 – physical characteristics water (Rusin, 2008)
Properties of water important for climate formation:
· water is an absorber of radiant energy;
· has one of the highest values of specific heat capacity among other substances on earth (this affects the difference in heating of land and sea, the penetration of radiation and heat deep into the soil and water bodies);
· ideal (almost) solvent;
· the dipole (bipolar) structure of water molecules provides a high boiling point (without hydrogen bonds, the boiling point would be -80°C).
Expands when frozen, unlike other substances that contract. (the maximum density of water is observed at a temperature of +4°C; the density of ice is less than the density of water: distilled by 1/9, sea by 1/7; lighter ice floats on the surface of the water).
Thanks to the processes of evaporation and condensation, the water cycle continuously occurs in the atmosphere, in which a significant mass of it participates. On average, the long-term water cycle is characterized by the following data (Table 1):
Table 1 - Characteristics of the water cycle on Earth (Matveev, 1976)
| Precipitation, mm/year | Evaporation, mm/year | Runoff, mm/year | |
| Continents | |||
| World Ocean | |||
| Earth |
A layer of water 1127 mm thick (or 4.07 10 17 kg of water) evaporates from the surface of the oceans (361 million km 2) during the year, and 446 mm (or 0.66 10 17 kg of water) from the surface of the continents. The thickness of the layer of precipitation falling per year on the oceans is 1024 mm (or 3.69 10 17 kg of water), on the continents - 700 mm (or 1.04 10 17 kg of water). The amount of precipitation on the continents significantly exceeds evaporation (by 254 mm, or 0.38·10 17 kg of water). This means that a significant amount of water vapor reaches the continents from the oceans. On the other hand, water that has not evaporated on the continents (254 mm) flows into rivers and further into the ocean. On the oceans, evaporation exceeds (by 103 mm) the amount of precipitation. The difference is replenished by water runoff from the oceans.
Evaporation and volatility
Water enters the atmosphere as a result of evaporation from the Earth’s surface (reservoirs, soil); it is secreted by living organisms in the process of life (respiration, metabolism, transpiration in plants); it is a by-product of volcanic activity, industrial production and the oxidation of various substances.
Evaporation(usually water) - the entry of water vapor into the atmosphere due to the separation of the fastest moving molecules from the surface of water, snow, ice, wet soil, drops and crystals in the atmosphere.
Evaporation from the surface of the earth is called physical evaporation. Physical evaporation and transpiration together - evapotranspiration.
The essence of the evaporation process is the separation of individual water molecules from the water surface or from moist soil and the transfer of air as water vapor molecules. The steam contained in the atmosphere condenses when the air cools. Condensation of water vapor can also occur through sublimation (the process of direct transition of a substance from gaseous to solid, bypassing liquid). Water is removed from the atmosphere by precipitation.
The molecules of a liquid are always in motion, and some of them can break through the surface of the liquid and escape into the air. Those molecules come off whose speed is higher than the speed of movement of molecules at a given temperature and is sufficient to overcome the forces of adhesion (molecular attraction). As the temperature rises, the number of molecules coming off increases. Vapor molecules can return from air to liquid. When the temperature of a liquid increases, the number of molecules leaving it becomes greater than the number returning, i.e. liquid evaporates. Lowering the temperature slows down the transition of liquid molecules into air and causes steam to condense. If water vapor enters the air, it, like all other gases, creates a certain pressure. As water molecules move into the air, the vapor pressure in the air increases. When a state of mobile equilibrium is reached (the number of molecules leaving the liquid is equal to the number of molecules returning), evaporation stops. This condition is called saturation , water vapor in this state – saturating , and the air rich . The pressure of water vapor at saturation is called saturated water vapor pressure (E), or saturation elasticity, or maximum elasticity.
Until the saturation state is reached, the process of water evaporation occurs, and the elasticity of water vapor (e) above the liquid is less than the maximum elasticity: e<Е.
If the number of returning water molecules is greater than the number of escaping ones, then the process of condensation or sublimation takes place (above the ice): e>E.
The pressure of saturated water vapor depends on
· air temperature,
on the nature of the surface (liquid, ice),
on the shape of this surface,
water salinity.
Most of the water vapor enters the atmosphere from the surface of the seas and oceans. This especially applies to humid, tropical regions of the Earth. In the tropics, evaporation exceeds precipitation. At high latitudes the opposite relationship occurs. In general, throughout the globe, the amount of precipitation is approximately equal to evaporation.
Evaporation is regulated by some physical properties terrain, in particular the temperature of the surface of the water and large bodies of water, the prevailing wind speeds there. When the wind blows over the surface of the water, it carries the moistened air aside and replaces it with fresh, drier air (i.e., advection and turbulent diffusion are added to molecular diffusion). The stronger the wind, the faster the air changes and the more intense the evaporation.
Evaporation can be characterized by the speed of the process. Evaporation rate (V) is expressed in millimeters of water layer evaporated per unit time from a unit surface. It depends on the saturation deficit, atmospheric pressure and wind speed.
Evaporation is difficult to measure in real conditions. To measure evaporation, evaporators of various designs or evaporation pools (with an area cross section 20 m2 or 100 m2 and 2 m deep). But the values obtained from evaporators cannot be equated to evaporation from a real physical surface. Therefore, they resort to calculation methods: evaporation from the land surface is calculated based on data on precipitation, runoff and soil moisture content, which are easier to obtain by measurements. Evaporation from the sea surface can be calculated using formulas close to the overall equation.
A distinction is made between actual evaporation and evaporation.
Volatility– potential evaporation in a given area under the existing atmospheric conditions.
This means either evaporation from the surface of water in the evaporator; evaporation from the open water surface of a large body of water (natural freshwater); evaporation from the surface of excessively moist soil. Evaporation is expressed in millimeters of the layer of evaporated water per unit of time.
In polar regions, evaporation is low: about 80 mm/year. This is due to the fact that there are low temperatures evaporating surface, and the pressure of saturated water vapor E S and the actual pressure of water vapor are small and close to each other, therefore the difference (E S – e) is small.
In temperate latitudes, evaporation changes over a wide range and tends to increase when moving from the northwest to the southeast of the continent, which is explained by an increase in the saturation deficit in the same direction. Lowest values in this belt of Eurasia are observed in the north-west of the continent: 400–450 mm, the largest (up to 1300–1800 mm) in Central Asia.
In the tropics evaporation is low on the coasts and increases sharply in inland parts to 2500–3000 mm.
At the equator evaporation is relatively low: does not exceed 100 mm due to the small value of the saturation deficit.
Actual evaporation on the oceans coincides with evaporation. On land it is significantly less, mainly depending on the moisture regime. Difference between evaporation and precipitation can be used to calculate air humidification deficit.
The most important component of water balance is evaporation. The problem of obtaining climate-reliable information on evaporation is much more acute than for precipitation. The overwhelming majority of known data is based on calculation methods. Calculations are more or less reliable over water surface, where evaporation can be taken as evaporation and this value can be calculated. Over land, such an approach is impossible, therefore, direct measurements of evaporation are made on a sparse network, but spatial climatic generalization of these data is difficult (Kislov A.V., 2011).
In Fig. 3.5 and in table. Table 3.3 shows the calculated annual amounts of evaporation from the underlying surface, from which it follows that evaporation from the oceans significantly exceeds evaporation from land. Over most of the World Ocean in middle and low latitudes, evaporation varies from 600 to 2500 mm, and maximums reach 3000 mm. In polar waters, in the presence of ice, evaporation is relatively small. On land, annual evaporation amounts range from 100–200 mm in polar and desert regions (even less in Antarctica) to 800–1000 mm in humid tropical and subtropical regions (southern Asia, the Congo River basin, southeastern USA, east coast of Australia , islands of Indonesia, Madagascar). The maximum values on land are slightly more than 1000 mm (Khromov S.P., Petrosyants M.A., 2001).
Rice. 3.5. Distribution of average annual values (mm/year) of evaporation from the underlying surface (Atlas of Heat Balance of the Globe, 1963)
Table 3.3. Annual evaporation values (mm) for different zones of the Northern Hemisphere (according to M.I. Budyko, 1980)
Thus, on average over latitudinal zones in the Northern Hemisphere, the highest annual evaporation values are observed in the tropics. As you move from the tropics to the poles, evaporation decreases. In the equatorial zone and at high latitudes, the average annual evaporation values over land and sea are approximately the same, but in the tropics and temperate latitudes, evaporation from the sea surface is greater than from the land surface. The distribution of evaporation is similar in the Southern Hemisphere, but in the hemisphere as a whole, evaporation is higher and amounts to approximately 1250 mm, so the area occupied by the ocean is larger in that hemisphere (for the Northern Hemisphere, the average annual evaporation value is about 770 mm) (Climatology, 1989).
To obtain physically substantiated ideas about the features of the spatial pattern of evaporation, one can take into account the fact that the turbulent flow of water vapor is determined by the vertical moisture gradient in the surface layer and the development of the turbulent regime, which can be parametrically characterized by the magnitude of the wind speed vector and the stability criterion of atmospheric stratification. From this point of view, it becomes clear, for example, why along the rods warm currents(Gulf Stream, Kuroshio, Brazilian, East Australian) evaporation is high. It especially increases in winter time, when dry cold air, formed in extratropical continental high pressure centers, enters the sea (due to the predominance of westerly transport). At the same time, the specific humidity gradient increases and turbulence sharply increases due to the emerging unstable temperature stratification.
The considered provisions make it possible to explain the existence of large precipitation in the ITC from the point of view of the balance of precipitation (r) and the amount of evaporation (E)(Fig. 3.6). Over vast parts of the oceans, trade wind air masses accumulate moisture (here E– r> 0) and “pour” this water into the VZK (where E– r< 0). Cloud systems of polar frontal cyclones form in tropical moist air, so that the water vapor they transport to high latitudes and continents (where E– r< 0) was also collected from tropical and subtropical waters of the World Ocean.
The balance of moisture “evaporation minus precipitation” allows us to understand the main geographical patterns of river flow formation - the deepest rivers are those whose basins are located in territories where E -r< 0. Typical examples are the rivers Amazon, Congo, Ganges, Brahmaputra, etc. Moreover, not only the named great rivers, stretching for thousands of kilometers, are full-flowing, but also relatively small rivers of large islands, for example Indonesia, are fed year-round by abundant precipitation, the amount of which is significant exceeds evaporation.
For the ocean, the atmospheric moisture balance of "evaporation minus precipitation" is the vertical flow of "fresh water". It determines in its main features the spatial heterogeneity of the water salinity field. IN Pacific Ocean precipitation exceeds evaporation, and in the Atlantic (and Indian Ocean) evaporation exceeds precipitation and higher surface salinity, and its spatial distribution follows the distribution of the “precipitation minus evaporation” balance. However, not all features of the salinity field are determined solely by this balance. Thus, water desalination increases locally near the mouths large rivers(Amazon, Congo, Ganges). In polar latitudes, in addition to the above factors, the salinity field plays an active role in the process of formation. fresh waters, formed when snow and ice cover melts (Kislov A.V., 2011).
Rice. 3.6. Atmospheric moisture balance “evaporation minus precipitation” over the oceans (cm/year): 1 – isolines >0 ; 2 – isolines <0 (Kislov A.V., 2011)
Evaporation and volatility. Geographic distribution of evaporation and volatility (analysis of evaporation and volatility maps)
EVAPORATION (Russian) - the transition of a substance from a liquid or solid state to a gaseous state - to steam. In nature, water vapor enters the atmosphere from the surface of water, soil, vegetation, ice, and snow. Evaporation depends on the temperature and humidity of the air, the evaporating surface and wind speed.
EVAPORATIZABILITY -- the maximum possible evaporation under given meteorological conditions from a sufficiently moistened underlying surface, that is, in conditions of an unlimited supply of moisture. Evaporation is expressed in millimeters of the layer of evaporated water and is very different from actual evaporation, especially in the desert, where evaporation is close to zero and evaporation is 2000 mm per year or more.
Heat is expended on evaporation, as a result of which the temperature of the evaporating surface decreases. This is of great importance for plants, especially in equatorial-tropical latitudes, where evaporation reduces their overheating. The southern oceanic hemisphere is colder than the northern hemisphere partly for the same reason.
The daily and annual course of evaporation is closely related to air temperature. The evaporation values in polar latitudes are about 60-80 mm with a maximum value of 100-120 mm due to low air temperatures and, as a consequence, close values of E1 (actual water vapor pressure) and e (maximum elasticity).
In polar regions, at low temperatures of the evaporating surface, both the saturation elasticity Es and the actual elasticity e are small and close to each other. Therefore, the difference (Es - e) is small, and along with it the evaporation rate is small. In Spitsbergen it is only 80 mm per year, in England about 400 mm, in Central Europe about 450 mm. In the European territory of Russia, evaporation increases from northwest to southeast along with an increase in humidity deficiency. In Leningrad it is 320 mm per year, in Moscow 420 mm, in Lugansk 740 mm. In Central Asia, with its high summer temperatures and a large deficit of humidity, evaporation is much higher: 1340 mm in Tashkent and 1800 mm in Nukus.
In the tropics, evaporation is relatively low on the coasts and increases sharply inland, especially in deserts. Thus, on the Atlantic coast of the Sahara, the annual evaporation rate is 600-700 mm, and at a distance of 500 km from the coast - 3000 mm. In the driest regions of Arabia and the deserts of Colorado it is above 3000 mm. Only in South America there are no areas with an annual evaporation rate of more than 2500 mm.
At the equator, where the humidity deficit is small, evaporation is relatively low: 700-1000 mm. In the coastal deserts of Peru, Chile and South Africa, annual evaporation is also no more than 600-800 mm.
The geographic distribution of actual evaporation by latitude is as follows:
At latitude 0-10°, evaporation on land is 112 cm, in the ocean - 110 cm.
At a latitude of 20-30°, evaporation on land is 37 cm, in the ocean - 130 cm.
At a latitude of 40-50°, evaporation on land is 37 cm, in the ocean - 70 cm.
At a latitude of 60-90°, evaporation on land is 8 cm, in the ocean - 15 cm.
