The world around us is full of amazing wonders, but we often do not pay attention to them. Admiring the clear blue of the spring sky or the bright colors of the sunset, we do not even think about why the sky changes color with the change of time of day.
We are accustomed to bright blue on a fine sunny day and to the fact that in autumn the sky becomes hazy gray, losing its bright colors. But if you ask a modern person about why this happens, then the vast majority of us, once armed with school knowledge of physics, are unlikely to be able to answer this simple question. Meanwhile, there is nothing complicated in the explanation.
What is color?
From a school course in physics, we should know that differences in the color perception of objects depend on the wavelength of light. Our eye can only distinguish a fairly narrow range of wave radiation, with blue being the shortest and red being the longest. Between these two primary colors lies our entire palette of color perception, expressed by wave radiation in different ranges.
A white sunbeam actually consists of waves of all color ranges, which is easy to verify by passing it through a glass prism - you probably remember this school experience. In order to remember the sequence of changing wavelengths, i.e. the sequence of colors in the spectrum of daylight, invented a funny phrase about a hunter that each of us learned in school: Every Hunter Wants to Know, etc. 
Since red light waves are the longest, they are the least susceptible to scattering during transmission. Therefore, when you need to visually highlight an object, they use mainly red color, which is clearly visible from afar in any weather.
Therefore, a stop signal or any other warning light is red, not green or blue.
Why does the sky turn red at sunset?
In the evening hours before sunset, the sun's rays fall on the surface of the earth at an angle, and not directly. They have to overcome a much thicker layer of the atmosphere than in the daytime, when the surface of the earth is illuminated by the direct rays of the Sun.
At this time, the atmosphere acts as a color filter, which scatters the rays of almost the entire visible range, except for the red ones, which are the longest and therefore most resistant to interference. All other light waves are either scattered or absorbed by water vapor and dust particles present in the atmosphere.
The lower the sun drops in relation to the horizon, the thicker the layer of the atmosphere the light rays have to overcome. Therefore, their color is increasingly shifted towards the red part of the spectrum. A folk sign is associated with this phenomenon, saying that a red sunset portends a strong wind the next day. 
The wind originates in the high layers of the atmosphere and at a great distance from the observer. Oblique solar rays highlight the outlined zone of atmospheric radiation, in which there is much more dust and vapor than in a calm atmosphere. Therefore, before a windy day, we see a particularly red, bright sunset.
Why is the sky blue during the day?
Differences in the length of light waves also explain the pure blue of the daytime sky. When the sun's rays fall directly on the surface of the earth, the layer of the atmosphere they overcome has the smallest thickness.
Scattering of light waves occurs when they collide with gas molecules that make up air, and in this situation, the short-wavelength light range is the most stable, i.e. blue and purple light waves. On a fine windless day, the sky acquires amazing depth and blueness. But why do we then see blue and not purple sky?
The fact is that the cells of the human eye, which are responsible for color perception, perceive blue much better than purple. Yet purple is too close to the edge of the perceptual range.
That is why we see the sky as bright blue if there are no scattering components in the atmosphere, except for air molecules. When a sufficiently large amount of dust appears in the atmosphere - for example, in a hot summer in a city - the sky seems to fade, losing its bright blue.
Gray sky of bad weather
Now it is clear why the autumn bad weather and winter slush make the sky hopelessly gray. A large amount of water vapor in the atmosphere leads to the dispersion of all components of the white light beam without exception. Light rays are crushed in the smallest droplets and water molecules, losing their direction and mixing over the entire range of the spectrum. 
Therefore, light rays reach the surface, as if passed through a giant diffuser. We perceive this phenomenon as a grayish-white color of the sky. As soon as moisture is removed from the atmosphere, the sky turns bright blue again.
Everyone knows that depending on the celestial point in which we observe the Sun, its color can vary greatly. For example, at the zenith it is white, at sunset it is red, and sometimes even crimson. In fact, this is only an appearance - it is not the color of our luminary that changes, but its perception by the human eye. Why is this happening? 
The solar spectrum is a combination of seven primary colors - remember the rainbow and the famous saying about the hunter and the pheasant, which determines the color sequence: red, yellow, green, and so on until purple. But in an atmosphere filled with various types of aerosol suspensions (water vapor, dust particles), each color scatters differently. For example, violet and blue are best scattered, and red is worse. This phenomenon is called dispersion of sunlight.
The reason is that color, in fact, is an electromagnetic wave of a certain length. Accordingly, different waves have different wavelengths. And the eye perceives them depending on the thickness of the atmospheric air that separates it from the source of light, that is, the Sun. Being at the zenith, it appears white, because the sun's rays fall on the Earth's surface at a right angle (naturally, that place on the surface where the observer is located is meant), and the thickness of the air that affects the refraction of light is relatively small. A white person seems to be a combination of all colors at once. 
By the way, the sky appears blue also due to the dispersion of light: since blue, violet and blue colors, having the shortest wavelengths, scatter in the atmosphere much faster than the rest of the spectrum. That is, passing red, yellow and other rays with longer wavelengths, atmospheric particles of water and dust scatter blue rays in themselves, which give the sky its color.
The farther the Sun makes its usual daily path and descends to the horizon line, the greater the thickness of the atmospheric layer becomes, through which the sun's rays have to pass, and the more they scatter. Red is the most resistant to scattering because it has the longest wavelength. Therefore, only he is perceived by the eyes of an observer who looks at the setting star. The remaining colors of the solar spectrum are completely scattered and absorbed by the aerosol suspension in the atmosphere.
As a result, there is a direct dependence of the scattering of spectral rays on the thickness of atmospheric air and the density of the suspension it contains. Vivid evidence of this can be observed with global emissions into the atmosphere of substances denser than air, for example, volcanic dust. So, after 1883, when the famous eruption of the Krakatau volcano took place, for quite a long time in the most diverse places on the planet one could see red sunsets of extraordinary brightness.
On a clear sunny day, the sky above us looks bright blue. In the evening, the sunset colors the sky in reds, pinks and oranges. So why is the sky blue and what makes a sunset red?
What color is the sun?
Of course the sun is yellow! All the inhabitants of the earth will answer, and the inhabitants of the moon will disagree with them.
From Earth, the Sun appears yellow. But in space or on the Moon, the Sun would appear white to us. There is no atmosphere in space that scatters sunlight.
On Earth, some of the short wavelengths of sunlight (blue and violet) are absorbed by scattering. The rest of the spectrum looks yellow.
And in space, the sky looks dark or black instead of blue. This is the result of the absence of an atmosphere, hence the light does not scatter in any way.
But if you ask about the color of the sun in the evening. Sometimes the answer will be the sun is RED. But why?
Why is the sun red at sunset?
As the Sun moves towards sunset, the sunlight has to travel a greater distance in the atmosphere to reach the observer. Less direct light reaches our eyes and the Sun appears less bright.
Since sunlight has to travel longer distances, more scattering occurs. The red part of the spectrum of sunlight passes through the air better than the blue part. And we see a red sun. The lower the Sun goes down to the horizon, the larger the air "magnifying glass" through which we see it, and the redder it is.
For the same reason, the Sun seems to us to be much larger in diameter than during the day: the air layer plays the role of a magnifying glass for an earthly observer.
The sky around the setting sun can be painted in different colors. The sky is most beautiful when the air contains many small particles of dust or water. These particles reflect light in all directions. In this case, shorter light waves are scattered. The observer sees light rays of longer wavelengths, and so the sky appears red, pink, or orange.
Visible light is a form of energy that can travel through space. Light from the sun or an incandescent lamp appears white when in reality it is a mixture of all colors. The main colors that make up the white color are red, orange, yellow, green, blue, indigo and violet. These colors continuously change into one another, therefore, in addition to the primary colors, there is also a huge number of various shades. All these colors and shades can be observed in the sky in the form of a rainbow that occurs in areas of high humidity.
The air that fills the entire sky is a mixture of minute gas molecules and small solid particles such as dust.
The sun's rays, coming from outer space, begin to dissipate under the influence of atmospheric gases, and this process occurs according to the Rayleigh Scattering Law. As light travels through the atmosphere, most of the long wavelengths of the optical spectrum pass through unchanged. Only a small part of the red, orange and yellow colors interact with the air, bumping into molecules and dust.
When light collides with gas molecules, the light can be reflected in various directions. Some colors, such as red and orange, reach the observer directly by passing directly through the air. But most of the blue light is re-reflected from air molecules in all directions. In this way, blue light is scattered throughout the sky and it appears blue.
However, many shorter wavelengths of light are absorbed by gas molecules. After absorption, the blue color is emitted in all directions. It is scattered all over the sky. In whatever direction you look, some of this scattered blue light reaches the observer. Since blue light is visible everywhere overhead, the sky looks blue.
If you look towards the horizon, the sky will have a paler hue. This is a result of the fact that light travels a greater distance in the atmosphere to the observer. The scattered light is again scattered by the atmosphere, and less blue reaches the observer's eyes. Therefore, the color of the sky near the horizon appears paler or even appears completely white.
Why is space black?
There is no air in outer space. Since there are no obstacles from which light could be reflected, the light propagates directly. The rays of light do not scatter, and the "sky" looks dark and black.
Atmosphere.
The atmosphere is a mixture of gases and other substances that surround the Earth, in the form of a thin, mostly transparent shell. The atmosphere is held in place by the Earth's gravity. The main components of the atmosphere are nitrogen (78.09%), oxygen (20.95%), argon (0.93%) and carbon dioxide (0.03%). The atmosphere also contains small amounts of water (in different places its concentration ranges from 0% to 4%), solid particles, gases neon, helium, methane, hydrogen, krypton, ozone and xenon. The science that studies the atmosphere is called meteorology.
Life on Earth would not be possible without the presence of an atmosphere that supplies the oxygen we need to breathe. In addition, the atmosphere performs another important function - it equalizes the temperature throughout the planet. If there were no atmosphere, then in some places on the planet there could be sizzling heat, and in other places it would be extremely cold, the temperature range could range from -170 ° C at night to + 120 ° C during the day. The atmosphere also protects us from the harmful radiation of the Sun and space, absorbing and scattering it.
The structure of the atmosphere
The atmosphere consists of different layers, the division into these layers occurs according to their temperature, molecular composition and electrical properties. These layers do not have pronounced boundaries, they change seasonally, and in addition, their parameters change at different latitudes.
Homosphere
- Lower 100 km including Troposphere, Stratosphere and Mesopause.
- Makes up 99% of the mass of the atmosphere.
- Molecules are not separated by molecular weight.
- The composition is quite homogeneous, with the exception of some small local anomalies. Homogeneity is maintained by constant mixing, turbulence and turbulent diffusion.
- Water is one of two components distributed unevenly. When water vapor rises, it cools and condenses, then returning to the earth in the form of precipitation - snow and rain. The stratosphere itself is very dry.
- Ozone is another molecule whose distribution is uneven. (Read about the ozone layer in the stratosphere below.)
heterosphere
- Extends above the homosphere, includes the Thermosphere and the Exosphere.
- The separation of the molecules of this layer is based on their molecular weights. Heavier molecules such as nitrogen and oxygen are concentrated at the bottom of the layer. The lighter ones, helium and hydrogen, dominate in the upper part of the heterosphere.
Separation of the atmosphere into layers depending on their electrical properties.
Neutral atmosphere
- Below 100 km.
Ionosphere
- Approximately above 100 km.
- Contains electrically charged particles (ions) produced by the absorption of ultraviolet light
- The degree of ionization changes with altitude.
- Different layers reflect long and short radio waves. This allows radio signals propagating in a straight line to bend around the spherical surface of the earth.
- Auroras occur in these atmospheric layers.
- Magnetosphere is the upper part of the ionosphere, extending to about 70,000 km, this height depends on the intensity of the solar wind. The magnetosphere protects us from the high-energy charged particles of the solar wind by keeping them in the Earth's magnetic field.
Separation of the atmosphere into layers depending on their temperatures
Top border height troposphere depends on seasons and latitude. It extends from the earth's surface to a height of about 16 km at the equator, and to a height of 9 km at the North and South Poles.
- The prefix "tropo" means change. The change in the parameters of the troposphere occurs due to weather conditions - for example, due to the movement of atmospheric fronts.
- As the altitude increases, the temperature drops. Warm air rises, then cools and descends back to Earth. This process is called convection, it occurs as a result of the movement of air masses. The winds in this layer blow mainly vertically.
- This layer contains more molecules than all the other layers combined.
Stratosphere- extends approximately from a height of 11 km to 50 km.
- It has a very thin layer of air.
- The prefix "strato" refers to layers or layering.
- The lower part of the Stratosphere is quite calm. Jet planes often fly in the lower Stratosphere in order to get around bad weather in the Troposphere.
- Strong winds known as high-altitude jet streams blow in the upper part of the Stratosphere. They blow horizontally at speeds up to 480 km/h.
- The stratosphere contains the "ozone layer" located at an altitude of approximately 12 to 50 km (depending on latitude). Although the concentration of ozone in this layer is only 8 ml/m 3 , it absorbs the sun's harmful ultraviolet rays very effectively, thereby protecting life on earth. The ozone molecule is made up of three oxygen atoms. The oxygen molecules we breathe contain two oxygen atoms.
- The stratosphere is very cold, its temperature is about -55°C at the bottom and increases with height. The increase in temperature is due to the absorption of ultraviolet rays by oxygen and ozone.
Mesosphere- extends to altitudes of about 100 km.
Wears the usual blue color. At night, it turns black. But during sunset, it always turns bright red. Why does this happen, for what reason does the crimson hue spread across the sky? Perhaps, many people have repeatedly asked this question, and therefore it makes sense to give an exhaustive answer to it.
The sunset is tinted with the rays of the setting sun, this is understandable to many. But why is it red, and not orange or another color?
Features of the color spectrum
Before reaching the surface of the earth, where people can contemplate it, sunlight must pass through the entire air shell of the planet. The light has a wide spectrum, in which the primary colors, the shades of the rainbow, still stand out. Of this spectrum, red has the longest wavelength of light, while violet has the shortest. At sunset, the solar disk rapidly turns red and rushes closer to the horizon.
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In this case, the light has to overcome an increasing thickness of air, and part of the waves is lost. Purple disappears first, then blue, blue. The longest waves of red color continue to penetrate to the surface of the Earth to the last, and therefore the solar disk and the halo around it until the last moments have reddish hues.
Why is the sky blue during the day?
Long light waves can penetrate deep into the atmosphere for the reason that they are almost not absorbed, not scattered by aerosols and suspensions that constantly circulate in the planet's atmosphere. When the luminary is closer to the zenith, a different situation develops, which provides the sky with blueness. Blue has shorter wavelengths than red and is absorbed more strongly. But its dispersal ability is 4 times higher compared to red.
When the sun is at or near its zenith, the sky is always blue. This is due to the fact that the layer of air between the planet and the star at this moment is small, and blue, blue waves pass freely. They have a great ability to diffuse, and therefore successfully drown out other colors and shades. Therefore, this color dominates the sky for almost the entire daylight hours.
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What changes in the evening?
Closer to sunset, the Sun rushes to the horizon, the lower it falls, the faster the evening approaches. At such times, the layer of atmosphere that separates the original sunlight from the earth's surface begins to increase dramatically due to the angle of inclination. At some point, the thickening layer ceases to transmit other light waves except red, and at that moment the sky is painted in this color. Blue is no longer present, it is absorbed in the process of passing through the layers of the atmosphere.
: at sunset, the sun and sky pass through a whole gamut of hues as one or the other of them ceases to pass through the atmosphere. The same can be observed at the time of sunrise, the causes of both phenomena are the same.
What happens at sunrise?
At sunrise, the sun's rays go through the same process, but in reverse order. That is, first, the first rays break through the thickness of the atmosphere at a strong angle, only the red spectrum reaches the surface. Therefore, the sunrise initially dawns red. Then, as the sunrise and the angle change, waves of other colors begin to pass - the sky turns orange, and then it becomes habitually blue. A half-day deep blue of the sky is observed, and then, by evening, it begins to turn again to crimson. On one side of the sky, far from the sun, there is a blue-black tint, but the closer to the setting star, the more red shades can be seen near the horizon, until the Sun disappears completely.
