If you look at the Sun when it is partially obscured by clouds and hidden behind these clumps of atmospheric water, you may see a familiar sight: rays of light breaking through the clouds and falling to the ground. Sometimes they seem parallel, sometimes they seem to diverge. Can sometimes see the shape of the Sun through the clouds. Why is this happening? Our reader this week asks:
Can you explain to me why on a cloudy day you can see the sun's rays breaking through the clouds? It seems to me that since the Sun is much larger than the Earth, and since its photons reach us along roughly parallel paths, we should see the entire sky evenly illuminated, rather than seeing a small ball of light.
Most people don't even think about the amazing fact that the sun's rays exist.
On a typical sunny day, the entire sky is lit up. The rays of the Sun fall almost parallel to the Earth because the Sun is very far away and it is very large compared to the Earth. The atmosphere is transparent enough for all sunlight to reach the Earth's surface or be scattered in all directions. The last effect is responsible for the fact that on a cloudy day something can be seen outside - the atmosphere perfectly scatters sunlight and fills the surrounding space with it.
This is why on a bright sunny day your shadow will be darker than the rest of the surface on which it falls, but will still remain illuminated. In your shadow, you can see the earth in the same way as if the Sun had disappeared behind the clouds, and then everything else becomes as dim as your shadow, but still illuminated by diffused light.
With this in mind, let us return to the phenomenon of solar rays. Why is it that when the Sun hides behind the clouds, you can sometimes see rays of light? And why do they sometimes look like parallel columns, and sometimes like diverging ones?
The first thing to understand is that the scattering of sunlight, when it collides with atmospheric particles and is redirected in all directions, always works - whether the Sun is hidden behind the clouds or not. Therefore, during the day there is always a basic level of lighting. That is why it is “day”, and therefore, to find darkness during the day, you need to go deeper into the cave.
What are rays? They come from gaps or thin sections of clouds (or trees or other opaque objects) that do not block sunlight. This direct light appears brighter than its surroundings, but is only noticeable if it contrasts with a dark, shadowy background! If this light is everywhere, there will be nothing remarkable about it, our eyes will adapt to it. But if a bright beam of light is lighter than its surroundings, your eyes notice this and tell you the difference.
What about the shape of the rays? You might think that clouds act like lenses or prisms, deflecting or refracting rays and causing them to diverge. But that's not true; Clouds absorb and re-emit light equally in all directions, which is why they are opaque. The ray effect only occurs where clouds do not absorb most of the light. When taking measurements, it turns out that these rays are actually parallel, which corresponds to a large distance to the Sun. If you observe rays directed neither towards you nor away from you, but perpendicular to your line of sight, this is exactly what you will find.
The reason why it seems to us that the rays “converge” towards the Sun is the same as why it seems to us that the rails or road surface converge at one point. These are parallel lines, one part of which is closer to you than the other. The sun is very far away, and the point from which the beam comes is further from you than the point of its contact with the Earth! It's not always obvious, but that's why the beams take on the shape of beams, which is clearly visible when you see how close you are to the end of the beam.
Therefore, we owe the presence of a ray to the perspective of the shadows surrounding it and the ability of our eyes to distinguish between the brightness of direct light and the relative darkness surrounding it. And the reason the rays appear to converge is due to perspective, and because the landing point of these actually parallel rays of light is closer to us than their starting point at the bottom of the clouds. That's the science behind sunrays, and that's why they look the way they do!
The Sun is a star in the solar system, which is the source of enormous amounts of heat and dazzling light. Despite the fact that the Sun is located at a considerable distance from us and only a small part of its radiation reaches us, this is quite enough for the development of life on Earth. Our planet revolves around the Sun in an orbit. If you observe the Earth from a spaceship throughout the year, you will notice that the Sun always illuminates only one half of the Earth, therefore, there will be day there, and on the opposite half at this time there will be night. The earth's surface receives heat only during the day.
Our Earth is heating unevenly. The uneven heating of the Earth is explained by its spherical shape, so the angle of incidence of the sun's ray in different areas is different, which means that different parts of the Earth receive different amounts of heat. At the equator, the sun's rays fall vertically, and they greatly heat the Earth. The further from the equator, the smaller the angle of incidence of the beam becomes, and therefore the less heat these areas receive. A beam of solar radiation of the same power heats a much smaller area, since it falls vertically. In addition, rays falling at a smaller angle than at the equator, penetrating, travel a longer path in it, as a result of which some of the sun's rays are scattered in the troposphere and do not reach the earth's surface. All this indicates that with distance from the equator to the north or south it decreases, as the angle of incidence of the sun's ray decreases.
The degree of heating of the earth's surface is also influenced by the fact that the earth's axis is inclined to the orbital plane, along which the Earth makes a full revolution around the Sun, at an angle of 66.5° and is always directed with its northern end towards the North Star.
Let's imagine that the Earth, moving around the Sun, has an earthly axis perpendicular to the plane of the orbit of rotation. Then the surface at different latitudes would receive a constant amount of heat throughout the year, the angle of incidence of the sun's ray would be constant all the time, day would always be equal to night, and there would be no change of seasons. At the equator, these conditions would differ little from the present ones. It has a significant influence on the heating of the earth's surface, and therefore on the entire tilt of the earth's axis, precisely in temperate latitudes.
During the year, that is, during the entire revolution of the Earth around the Sun, four days are especially noteworthy: March 21, September 23, June 22, December 22.
The tropics and polar circles divide the Earth's surface into zones that differ in solar illumination and the amount of heat received from the Sun. There are 5 light zones: the northern and southern polar zones, which receive little light and heat, the zone with a hot climate, and the northern and southern zones, which receive more light and heat than the polar ones, but less than the tropical ones.
So, in conclusion, we can draw a general conclusion: uneven heating and illumination of the earth’s surface is associated with the sphericity of our Earth and with the inclination of the earth’s axis to 66.5° to the orbit around the Sun.
The most important source from which the Earth's surface and atmosphere receive thermal energy is the Sun. It sends a colossal amount of radiant energy into cosmic space: thermal, light, ultraviolet. Electromagnetic waves emitted by the Sun travel at a speed of 300,000 km/s.
The heating of the earth's surface depends on the angle of incidence of the sun's rays. All the sun's rays arrive at the surface of the Earth parallel to each other, but since the Earth is spherical, the sun's rays fall on different parts of its surface at different angles. When the Sun is at its zenith, its rays fall vertically and the Earth heats up more.
The entire set of radiant energy sent by the Sun is called solar radiation, it is usually expressed in calories per unit surface area per year.
Solar radiation determines the temperature regime of the Earth's air troposphere.
It should be noted that the total amount of solar radiation is more than two billion times the amount of energy received by the Earth.
Radiation reaching the earth's surface consists of direct and diffuse.
Radiation that comes to Earth directly from the Sun in the form of direct sunlight under a cloudless sky is called direct. It carries the greatest amount of heat and light. If our planet had no atmosphere, the earth's surface would receive only direct radiation.
However, passing through the atmosphere, approximately a quarter of solar radiation is scattered by gas molecules and impurities and deviates from the direct path. Some of them reach the surface of the Earth, forming scattered solar radiation. Thanks to scattered radiation, light penetrates into places where direct sunlight (direct radiation) does not penetrate. This radiation creates daylight and gives color to the sky.
Total solar radiation
All the sun's rays reaching the Earth are total solar radiation, i.e., the totality of direct and diffuse radiation (Fig. 1).
Rice. 1. Total solar radiation for the year
Distribution of solar radiation over the earth's surface
Solar radiation is distributed unevenly across the earth. It depends:
1. on air density and humidity - the higher they are, the less radiation the earth’s surface receives;
2. depending on the geographic latitude of the area - the amount of radiation increases from the poles to the equator. The amount of direct solar radiation depends on the length of the path that the sun's rays travel through the atmosphere. When the Sun is at its zenith (the angle of incidence of the rays is 90°), its rays hit the Earth through the shortest path and intensively give off their energy to a small area. On Earth, this occurs in the band between 23° N. w. and 23° S. sh., i.e. between the tropics. As you move away from this zone to the south or north, the path length of the sun's rays increases, that is, the angle of their incidence on the earth's surface decreases. The rays begin to fall on the Earth at a smaller angle, as if sliding, approaching the tangent line in the area of the poles. As a result, the same energy flow is distributed over a larger area, so the amount of reflected energy increases. Thus, in the region of the equator, where the sun's rays fall on the earth's surface at an angle of 90°, the amount of direct solar radiation received by the earth's surface is higher, and as we move towards the poles, this amount sharply decreases. In addition, the length of the day at different times of the year depends on the latitude of the area, which also determines the amount of solar radiation reaching the earth's surface;
3. from the annual and daily movement of the Earth - in the middle and high latitudes, the influx of solar radiation varies greatly according to the seasons, which is associated with changes in the midday altitude of the Sun and the length of the day;
4. on the nature of the earth's surface - the lighter the surface, the more sunlight it reflects. The ability of a surface to reflect radiation is called albedo(from Latin whiteness). Snow reflects radiation especially strongly (90%), sand weaker (35%), and black soil even weaker (4%).
Earth's surface absorbing solar radiation (absorbed radiation), heats up and radiates heat into the atmosphere (reflected radiation). The lower layers of the atmosphere largely block terrestrial radiation. The radiation absorbed by the earth's surface is spent on heating the soil, air, and water.
That part of the total radiation that remains after reflection and thermal radiation of the earth's surface is called radiation balance. The radiation balance of the earth's surface varies during the day and according to the seasons of the year, but on average for the year it has a positive value everywhere, with the exception of the ice deserts of Greenland and Antarctica. The radiation balance reaches its maximum values at low latitudes (between 20° N and 20° S) - over 42 * 10 2 J/m 2, at a latitude of about 60 ° in both hemispheres it decreases to 8 * 10 2 - 13*10 2 J/m 2.
The sun's rays give up to 20% of their energy to the atmosphere, which is distributed throughout the entire thickness of the air, and therefore the heating of the air they cause is relatively small. The sun heats the surface of the Earth, which transfers heat to the atmospheric air due to convection(from lat. convection- delivery), i.e. the vertical movement of air heated at the earth's surface, in place of which colder air descends. This is how the atmosphere receives most of its heat—on average, three times more than directly from the Sun.
The presence of carbon dioxide and water vapor does not allow heat reflected from the earth's surface to freely escape into outer space. They create greenhouse effect, thanks to which the temperature difference on Earth during the day does not exceed 15 °C. In the absence of carbon dioxide in the atmosphere, the earth's surface would cool by 40-50 °C overnight.
As a result of the growing scale of human economic activity - the combustion of coal and oil at thermal power plants, emissions from industrial enterprises, and an increase in automobile emissions - the content of carbon dioxide in the atmosphere increases, which leads to an increase in the greenhouse effect and threatens global climate change.
The sun's rays, having passed through the atmosphere, hit the surface of the Earth and heat it, which, in turn, gives off heat to the atmosphere. This explains a characteristic feature of the troposphere: a decrease in air temperature with height. But there are cases when the higher layers of the atmosphere turn out to be warmer than the lower ones. This phenomenon is called temperature inversion(from Latin inversio - turning over).
Life on our planet depends on the amount of sunlight and heat. It’s scary to imagine even for a moment what would have happened if there had not been such a star in the sky as the Sun. Every blade of grass, every leaf, every flower needs warmth and light, like people in the air.
The angle of incidence of the sun's rays is equal to the height of the sun above the horizon
The amount of sunlight and heat that reaches the earth's surface is directly proportional to the angle of incidence of the rays. The sun's rays can strike the Earth at an angle of 0 to 90 degrees. The angle of impact of the rays on the earth is different, because our planet is spherical. The larger it is, the lighter and warmer it is.
Thus, if the beam comes at an angle of 0 degrees, it only glides along the surface of the earth without heating it. This angle of incidence occurs at the North and South Poles, beyond the Arctic Circle. At right angles, the sun's rays fall on the equator and on the surface between the South and
If the angle of the sun's rays hitting the ground is straight, this indicates that
Thus, the rays on the surface of the earth and the height of the sun above the horizon are equal. They depend on geographic latitude. The closer to zero latitude, the closer the angle of incidence of the rays is to 90 degrees, the higher the sun is above the horizon, the warmer and brighter it is.
How the sun changes its height above the horizon
The height of the sun above the horizon is not constant. On the contrary, it is always changing. The reason for this lies in the continuous movement of the planet Earth around the star Sun, as well as the rotation of the planet Earth around its own axis. As a result, day follows night, and seasons follow each other.
The territory between the tropics receives the most heat and light; here day and night are almost equal in duration, and the sun is at its zenith 2 times a year.
The surface above the Arctic Circle receives less heat and light; here there are such concepts as night, which last about six months.
Days of autumn and spring equinox
There are 4 main astrological dates, which are determined by the height of the sun above the horizon. September 23 and March 21 are the days of the autumn and spring equinox. This means that the height of the sun above the horizon in September and March on these days is 90 degrees.
Southern and are equally illuminated by the sun, and the length of the night is equal to the length of the day. When astrological autumn begins in the Northern Hemisphere, it is spring, on the contrary, in the Southern Hemisphere. The same can be said about winter and summer. If it is winter in the Southern Hemisphere, then it is summer in the Northern Hemisphere.
Days of summer and winter solstice
June 22 and December 22 are summer days and December 22 has the shortest day and longest night in the Northern Hemisphere, and the winter sun is at its lowest altitude above the horizon for the entire year.
Above latitude 66.5 degrees, the sun is below the horizon and does not rise. This phenomenon, when the winter sun does not rise to the horizon, is called polar night. The shortest night occurs at latitude 67 degrees and lasts only 2 days, and the longest night occurs at the poles and lasts 6 months!
December is the month of the entire year when the nights are longest in the Northern Hemisphere. People in Central Russia wake up for work in the dark and return in the dark. This is a difficult month for many, as the lack of sunlight affects people's physical and mental well-being. For this reason, depression may even develop.
In Moscow in 2016, sunrise on December 1st will be at 08.33. In this case, the length of the day will be 7 hours 29 minutes. It will be very early, at 16.03. The night will be 16 hours 31 minutes. Thus, it turns out that the length of the night is 2 times greater than the length of the day!
This year the winter solstice is December 21st. The shortest day will last exactly 7 hours. Then the same situation will last for 2 days. And starting from December 24, the day will start to make a profit, slowly but surely.
On average, one minute of daylight will be added per day. At the end of the month, sunrise in December will be exactly 9 o'clock, which is 27 minutes later than December 1st
June 22 is the summer solstice. Everything happens exactly the opposite. For the entire year, this date is the longest day in duration and the shortest night. This applies to the Northern Hemisphere.
In Yuzhny it’s the other way around. Interesting natural phenomena are associated with this day. A polar day begins above the Arctic Circle; the sun does not set below the horizon at the North Pole for 6 months. Mysterious white nights begin in St. Petersburg in June. They last from about mid-June for two to three weeks.
All these 4 astrological dates can change by 1-2 days, since the solar year does not always coincide with the calendar year. Shifts also occur during leap years.
The height of the sun above the horizon and climatic conditions
The sun is one of the most important climate-forming factors. Depending on how the height of the sun above the horizon changed over a specific area of the earth's surface, climatic conditions and seasons change.
For example, in the Far North, the sun's rays fall at a very small angle and only glide along the surface of the earth, without heating it at all. Due to this factor, the climate here is extremely harsh, there is permafrost, cold winters with freezing winds and snow.
The higher the sun's height above the horizon, the warmer the climate. For example, at the equator it is unusually hot and tropical. Seasonal fluctuations are also practically not felt in the equator region; in these areas there is eternal summer.
Measuring the height of the sun above the horizon
As they say, everything ingenious is simple. So it is here. The device for measuring the height of the sun above the horizon is simply simple. It is a horizontal surface with a pole in the middle 1 meter long. On a sunny day at noon, the pole casts its shortest shadow. With the help of this shortest shadow, calculations and measurements are carried out. You need to measure the angle between the end of the shadow and the segment connecting the end of the pole to the end of the shadow. This angle value will be the angle of the sun above the horizon. This device is called a gnomon.
Gnomon is an ancient astrological tool. There are other instruments for measuring the height of the sun above the horizon, such as the sextant, quadrant, and astrolabe.
The peculiarities of the impact of direct sunlight on the body today are of interest to many, primarily those who want to spend the summer profitably, stock up on solar energy and acquire a beautiful, healthy tan. What is solar radiation and what effect does it have on us?
Definition
Sun rays (photo below) are a flow of radiation, which is represented by electromagnetic oscillations of waves of different lengths. The spectrum of radiation emitted by the sun is diverse and wide, both in wavelength and frequency, and in its effect on the human body.
Types of sun rays
There are several regions of the spectrum:
- Gamma radiation.
- X-ray radiation (wavelength less than 170 nanometers).
- Ultraviolet radiation (wavelength - 170-350 nm).
- Sunlight (wavelength - 350-750 nm).
- Infrared spectrum, which has a thermal effect (wavelengths greater than 750 nm).
In terms of biological influence on a living organism, the most active are ultraviolet rays from the sun. They promote tanning, have a hormonal protective effect, stimulate the production of serotonin and other important components that increase vitality and vitality.
Ultraviolet radiation
There are 3 classes of rays in the ultraviolet spectrum that affect the body differently:
- A-rays (wavelength - 400-320 nanometers). They have the lowest level of radiation and remain constant in the solar spectrum throughout the day and year. There are almost no barriers for them. The harmful effect of sun rays of this class on the body is the lowest, however, their constant presence accelerates the process of natural aging of the skin, because, penetrating to the germ layer, they damage the structure and base of the epidermis, destroying elastin and collagen fibers.
- B-rays (wavelength - 320-280 nm). Only at certain times of the year and hours of the day do they reach the Earth. Depending on the geographic latitude and air temperature, they usually enter the atmosphere from 10 a.m. to 4 p.m. These sun rays take part in activating the synthesis of vitamin D3 in the body, which is their main positive property. However, with prolonged exposure to the skin, they can change the genome of cells in such a way that they begin to multiply uncontrollably and form cancer.
- C-rays (wavelength - 280-170 nm). This is the most dangerous part of the UV radiation spectrum, which unconditionally provokes the development of cancer. But in nature, everything is very wisely arranged, and the sun’s harmful C rays, like most (90 percent) of B rays, are absorbed by the ozone layer without reaching the Earth’s surface. This is how nature protects all living things from extinction.
Positive and negative influence
Depending on the duration, intensity, and frequency of exposure to UV radiation, positive and negative effects develop in the human body. The first include the formation of vitamin D, the production of melanin and the formation of a beautiful, even tan, the synthesis of mediators that regulate biorhythms, and the production of an important regulator of the endocrine system - serotonin. That’s why after summer we feel a surge of strength, an increase in vitality, and a good mood.
The negative effects of ultraviolet exposure include skin burns, damage to collagen fibers, the appearance of cosmetic defects in the form of hyperpigmentation, and the provocation of cancer.
Vitamin D synthesis
When exposed to the epidermis, the energy of solar radiation is converted into heat or spent on photochemical reactions, as a result of which various biochemical processes are carried out in the body.
Vitamin D is supplied in two ways:
- endogenous - due to formation in the skin under the influence of UV rays B;
- exogenous - due to intake from food.
The endogenous pathway is a rather complex process of reactions that occur without the participation of enzymes, but with the obligatory participation of UV irradiation with B-rays. With sufficient and regular insolation, the amount of vitamin D3 synthesized in the skin during photochemical reactions fully meets all the body's needs.
Tanning and vitamin D
The activity of photochemical processes in the skin directly depends on the spectrum and intensity of exposure to ultraviolet radiation and is inversely related to tanning (degree of pigmentation). It has been proven that the more pronounced the tan, the longer it takes for provitamin D3 to accumulate in the skin (instead of fifteen minutes to three hours).
From a physiological point of view, this is understandable, since tanning is a protective mechanism of our skin, and the layer of melanin formed in it acts as a certain barrier to both UV B rays, which serve as a mediator of photochemical processes, and class A rays, which provide the thermal stage of transformation in the skin provitamin D3 into vitamin D3.
But vitamin D supplied with food only compensates for the deficiency in case of insufficient production during the process of photochemical synthesis.
Vitamin D formation during sun exposure
Today it has already been established by science that to meet the daily requirement for endogenous vitamin D3, it is enough to stay in open sunlight class UV rays for ten to twenty minutes. Another thing is that such rays are not always present in the solar spectrum. Their presence depends both on the season of the year and on geographic latitude, since the Earth, when rotating, changes the thickness and angle of the atmospheric layer through which the sun's rays pass.
Therefore, solar radiation is not always able to form vitamin D3 in the skin, but only when UV B rays are present in the spectrum.
Solar radiation in Russia
In our country, taking into account the geographical location, class B rich UV rays are distributed unevenly during periods of solar radiation. For example, in Sochi, Makhachkala, Vladikavkaz they last about seven months (from March to October), and in Arkhangelsk, St. Petersburg, Syktyvkar they last about three (from May to July) or even less. Add to this the number of cloudy days a year and the smoky atmosphere in large cities, and it becomes clear that the majority of Russian residents experience a lack of hormonotropic solar exposure.
This is probably why intuitively we strive for the sun and rush to the southern beaches, while forgetting that the sun's rays in the south are completely different, unusual for our body, and, in addition to burns, can provoke strong hormonal and immune surges that can increase the risk of cancer and other ailments .
At the same time, the southern sun can heal, you just have to follow a reasonable approach in everything.
