Total radiation this is the sum of a straight line (on a horizontal surface) and scattered radiation:
The composition of the total radiation, that is, the ratio between direct and scattered radiation, varies depending on the height of the sun, the transparency of the atmosphere and cloudiness.
1. Before sunrise, the total radiation consists entirely, and at low altitudes of the sun, mainly of scattered radiation.
2. The more transparent the atmosphere, the lower the proportion of scattered radiation in the total.
3. Depending on the shape, height and amount of clouds, the proportion of scattered radiation increases to varying degrees. When the sun is obscured by dense clouds, the total radiation consists only of scattered radiation. With such clouds, scattered radiation only partially compensates for the decrease in the straight line, therefore, an increase in the number and density of clouds, on average, is accompanied by a decrease in the total radiation. But with light or thin clouds, when the sun is completely open or not completely covered by clouds, the total radiation due to an increase in the scattered radiation may turn out to be greater than with a clear sky,
Reflection of solar radiation from the earth's surface
The total radiation arriving at any surface is partially absorbed by it and partially reflected. The ratio of the amount of solar radiation reflected by a given surface to the incoming total radiation is called reflectivity or albedo: A = R K / Q
where Rk - reflected radiation flux. The albedo is usually expressed as a fraction of a unit or as a percentage.
Albedo earth surface depends on its properties and state: color, moisture, roughness, presence and nature of the vegetation cover. Dark and rough soils reflect less than light and smooth soils. Wet soils reflect less than dry soils as they are darker. Consequently, with an increase in soil moisture, the fraction of total radiation absorbed by it increases. This has a great influence, for example, on the thermal regime of irrigated fields.
Freshly fallen snow has the highest reflectivity. In some cases, the snow albedo reaches 87,%, and in the Arctic and Antarctic, even 98%. Caked, melted and more polluted snow reflects much less. The albedo of different soils and vegetation cover differs relatively little.
The albedo of natural surfaces changes somewhat during the day, with the highest albedo observed in the morning and evening, and in the daytime the albedo slightly decreases. This is explained by the dependence of the spectral composition of the total radiation on the height of the sun and the unequal reflectivity of the same surface for different wavelengths. At a low sun altitude, the proportion of scattered radiation is increased in the total radiation, and the latter is reflected from a rough surface more strongly than a straight line.
The albedo of water surfaces is, on average, less than the albedo of the land surface. This is explained by the fact that the sun's rays penetrate much deeper into the upper layers of water that are transparent to them than into the soil. They are scattered and absorbed in water. In this regard, the albedo of water is influenced by the degree of its turbidity: for polluted and turbid water, the albedo noticeably increases in comparison with pure water. The reflectivity of clouds is very high: on average, their albedo is about 80 %.
Knowing the albedo of the surface and the total radiation, it is possible to determine the amount of short-wave radiation absorbed by a given surface. The 1-A value is the absorption coefficient for shortwave radiation by a given surface. It shows how much of the total radiation arriving at a given surface is absorbed by it.
Albedo measurements of large areas of the earth's surface and clouds are carried out with artificial satellites Earth. Information about the albedo of clouds allows us to estimate their vertical extent, and knowledge of the albedo of the sea makes it possible to calculate the height of the waves.
From the Bouguer formula, it is seen that with a constant transparency of the atmosphere, the intensity of direct solar radiation depends on the optical mass of the atmosphere, i.e. ultimately from the height of the sun. So during the day solar radiation should first quickly, then more slowly increase from sunrise to noon and at first slowly, then quickly decrease from noon to sunset .
But the transparency of the atmosphere during the day varies within certain limits. Therefore, the curve of the daily course of radiation, even on a completely cloudless day, reveals some irregularities. However, in the average conclusions, the irregularities of individual diurnal curves are smoothed out, and the change in radiation during the day appears to be more uniform.
Differences in radiation intensity at noon are primarily associated with differences in midday sun altitude, which is lower in winter than in summer. Minimum intensity in temperate latitudes ah falls in December, when the sun is at its lowest. But the maximum intensity occurs not in the summer months, but in spring. The fact is that in spring the air is the least cloudy with condensation products and a little dusty. In summer, dusting increases, and the content of water vapor in the atmosphere also increases, which somewhat reduces the intensity of radiation.
The maximum values of the radiation intensity grow very little with decreasing geographic latitude in spite of the rise in the height of the sun. This is due to an increase in moisture content, and partly to dusty air in southern latitudes. At the equator, the maximum radiation values do not greatly exceed the summer maximums of temperate latitudes. In the dry air of subtropical deserts (Sahara), however, values of up to 1.58 cal / (cm2 · min) were observed.
With height above sea level, the maximum values of radiation increase due to a decrease in the optical mass of the atmosphere at the same height of the sun. For every 100 m altitude, the radiation intensity in the troposphere increases by 0.01-0.02 cal / (cm2 min). We have already said that the maximum values of the radiation intensity observed in the mountains reach 1.7 cal / (cm2 min) and more.
Intensity scattered radiation, measured, as mentioned above, for the unit horizontal surfaces also change during the day.
It rises until noon as the sun rises and decreases in the afternoon. It also depends on the transparency of the atmosphere; however, decreasing transparency, i. e. an increase in the number of clouding particles in the atmosphere does not decrease, but increases scattered radiation. In addition, scattered radiation varies over a very wide range depending on the cloudiness; the radiation reflected by the clouds is also partially scattered, and therefore the overall intensity of the scattered radiation increases. For the same reason, the reflection of radiation by the snow cover increases the scattered radiation.
On cloudless days, scattered radiation is small. Even when the sun is high, i.e. in the midday hours in summer, its intensity in the absence of clouds does not exceed 0.1 cal / (cm2 · min). Cloudiness increases this value by 3 - 4 times.
Scattered radiation can thus substantially complement direct solar radiation, especially at low sun exposure.
Scattered radiation not only increases the heating of the earth's surface. It also increases the illumination on the earth's surface. The overall illumination increases especially significantly, sometimes up to 40%, if there are clouds in the sky that do not cover the solar disk.
All solar radiation coming to the earth's surface, direct and scattered together, is called total radiation... By the intensity of the total radiation we mean the inflow of its energy in one minute per one square centimeter of the horizontal surface placed under open air and not shaded from direct sunlight. Thus, the intensity of the total radiation is
sin h + I, (55)
where I- the intensity of direct radiation, i - scattered radiation intensity, h- the height of the sun.
With a cloudless sky, the total radiation has a diurnal variation with a maximum around noon and an annual variation with a maximum in summer. Partial clouds that do not cover the solar disk increase the total radiation compared to a cloudless sky; full cloudiness, on the contrary, reduces it. On average, cloudiness reduces the total radiation. Therefore, in summer, the arrival of total radiation in the pre-noon hours is on average greater than in the afternoon. For the same reason, it is higher in the first half of the year than in the second.
The noon values of the total radiation in the summer months near Moscow with a cloudless sky averaged 1.12 cal / (cm2 · min), with the sun and clouds - 1.15, with overcast - 0.37 cal / (cm2 · min).
Falling on the earth's surface, the total radiation is mostly absorbed in the upper, thin layer of soil or water and turns into heat, and is partially reflected. The magnitude of the reflection of solar radiation by the earth's surface depends on the nature of this surface. The ratio of the amount of reflected radiation to the total amount of radiation falling on a given surface is called surface albedo. This ratio is expressed as a percentage.
So, from the total flux of total radiation I sin h+ i part of it is reflected from the earth's surface (I sin h + i)A, where A - albedo of the surface. The rest of the total radiation (I sin h + i) (1- A) absorbed by the earth's surface and goes to heating the upper layers of soil and water. This part is called absorbed radiation. .
The albedo of the soil surface is generally in the range of 10 - 30%; in the case of wet chernozem, it decreases to 5%, and in the case of dry light sand, it can rise to 40%. With an increase in soil moisture, the albedo decreases. The albedo of the vegetation cover - forests, meadows, fields - is in the range of 10 - 25%. For freshly fallen snow, the albedo is 80 - 90%, for long-standing snow - about 50% and below. The albedo of a smooth water surface for direct radiation varies from a few percent at high sun to 70% at low sun; it also depends on excitement. For scattered radiation, the albedo of water surfaces is 5 - 10%. On average, the albedo of the world's ocean surface is 5 - 20%. The albedo of the upper surface of clouds is from several percent to 70 - 80%, depending on the type and thickness of the cloud cover; on average, it is 50 - 60%. The numbers given refer to the reflection of solar radiation not only visible, but in its entire spectrum. In addition, the albedo is measured by photometric means only for the visible radiation, which, of course, may differ somewhat in magnitude from the albedo for the entire radiation flux.
The predominant part of the radiation reflected by the earth's surface and the upper surface of the clouds goes beyond the atmosphere into world space. Part of the scattered radiation, about one third of it, also goes into world space. The ratio of this reflected and scattered solar radiation going into space to the total amount of solar radiation entering the atmosphere is called the planetary albedo of the Earth, or simply albedo of the earth .
The planetary albedo of the Earth is estimated at 35 - 40%; apparently, it is closer to 35%. The main part of the Earth's planetary albedo is the reflection of solar radiation by clouds.
The amount of direct solar radiation (S) arriving at the earth's surface in a cloudless sky depends on the height of the sun and transparency. The table for three latitudinal zones shows the distribution of monthly sums of direct radiation in a cloudless sky (possible sums) in the form of averaged values for the central months of the seasons and the year.
The increased arrival of direct radiation in the Asian part is due to the higher transparency of the atmosphere in this region. High values of direct radiation in summer in the northern regions of Russia are explained by a combination of high transparency of the atmosphere and long day length
Reduces the arrival of direct radiation and can significantly change its daily and annual course. However, under average cloud conditions, the astronomical factor is predominant and, therefore, the maximum direct radiation is observed at highest altitude sun.
In most of the continental regions of Russia in the spring-summer months, direct radiation in the pre-noon hours is greater than in the afternoon. This is associated with the development of convective cloudiness in the afternoon hours and with a decrease in the transparency of the atmosphere at this time of the day as compared to the morning hours. In winter, the ratio of pre- and afternoon radiation values is the opposite - the pre-midday values of direct radiation are less due to the morning maximum cloud cover and its decrease in the second half of the day. The difference between the pre- and afternoon values of direct radiation can reach 25–35%.
In the annual course, the maximum direct radiation falls on June-July, with the exception of areas Of the Far East, where it shifts to May, and in the south of Primorye in September, a secondary maximum is noted.
The maximum monthly amount of direct radiation on the territory of Russia is 45–65% of that possible with a cloudless sky, and even in the south of the European part it reaches only 70%. The minimum values are observed in December and January.
The contribution of direct radiation to the total arrival under actual cloudiness conditions reaches its maximum in the summer months and averages 50–60%. An exception is the Primorsky Territory, where the largest contribution of direct radiation falls on the autumn and winter months.
The distribution of direct radiation under average (actual) cloudiness conditions over the territory of Russia largely depends on. This leads to a noticeable violation of the zonal distribution of radiation in certain months. This is especially evident in the spring. So, in April, there are two maximums - one in southern regions and Amur region, the second - in the northeast of Yakutia and in the Kolyma, which is also the result of a combination of high transparency of the atmosphere, high frequency of clear skies and the length of the day.
The data shown on the maps are based on valid cloud conditions.
I would be grateful if you share this article on social networks:
Site search.
If the atmosphere let all the sun's rays pass to the surface of the earth, then the climate of any point on the Earth would depend only on the geographical latitude. So it was believed in antiquity. However, when the sun's rays pass through earthly atmosphere there is, as we have already seen, their weakening due to the simultaneous processes of absorption and scattering. Water droplets and ice crystals, which make up clouds, absorb and scatter a lot.
That part of solar radiation that enters the earth's surface after scattering it by the atmosphere and clouds is called scattered radiation. That part of solar radiation that passes through the atmosphere without scattering is calleddirect radiation.
Radiation is scattered not only by clouds, but also in a clear sky - by molecules, gases and dust particles. The ratio between direct and scattered radiation varies widely. If, with a clear sky and vertical incidence of sunlight, the fraction of scattered radiation is 0.1% direct, then
in a cloudy sky, scattered radiation may be more direct.
In parts of the world where clear weather prevails, such as Central Asia, the main source of heating of the earth's surface is direct solar radiation. Where cloudy weather predominates, as, for example, in the north and northwest of the European territory of the USSR, diffuse solar radiation becomes essential. Tikhaya Bay, located in the north, receives scattered radiation almost one and a half times more than the straight one (Table 5). In Tashkent, on the contrary, scattered radiation is less than 1/3 of direct radiation. Direct solar radiation in Yakutsk is greater than in Leningrad. This is explained by the fact that in Leningrad there are more cloudy days and less transparency of the air.
Albedo of the earth's surface. The earth's surface has the ability to reflect rays falling on it. The amount of absorbed and reflected radiation depends on the properties of the earth's surface. The ratio of the amount of radiant energy reflected from the surface of the body to the amount of incident radiant energy is called albedo. Albedo characterizes the reflectivity of a body surface. When, for example, they say that the albedo of freshly fallen snow is 80-85%, this means that 80-85% of all radiation falling on the snow surface is reflected from it.
The albedo of snow and ice depends on their purity. In industrial cities, due to the deposition of various impurities on the snow, mainly soot, the albedo is lower. On the contrary, in the arctic regions the snow albedo sometimes reaches 94%. Since the albedo of snow is the highest in comparison with the albedo of other types of the earth's surface, then with a snow cover, the heating of the earth's surface occurs weakly. The albedo of grass and sand is much less. The albedo of grass vegetation is 26%, and that of sand is 30%. This means that the grass absorbs 74% of the sun's energy and the sand 70%. The absorbed radiation is used for evaporation, plant growth and heating.
Water has the greatest absorption capacity. Seas and oceans absorb about 95% of the incoming solar energy on their surface, ie, the albedo of water is 5% (Fig. 9). True, the albedo of water depends on the angle of incidence of the sun's rays (V.V. Shuleikin). With a vertical incidence of rays from the surface pure water only 2% of the radiation is reflected, and when the sun is low, almost all.
