Despite scientific progress and free access to many sources of information, it is rare that a person can correctly answer the question of why the sky is blue.
Why is the sky blue or blue during the day?
White light - which is what the Sun emits - is made up of seven parts of the color spectrum: red, orange, yellow, green, blue, indigo and violet. The little rhyme known from school - “Every Hunter Wants to Know Where the Pheasant Sits” - precisely determines the colors of this spectrum by the initial letters of each word. Each color has its own wavelength of light: red is the longest and violet is the shortest.
The sky (atmosphere) familiar to us consists of solid microparticles, tiny drops of water and gas molecules. For a long time, there have been several erroneous assumptions trying to explain why the sky is blue:
- the atmosphere, consisting of tiny particles of water and molecules of various gases, allows the rays of the blue spectrum to pass through well and does not allow the rays of the red spectrum to touch the Earth;
- Small solid particles - such as dust - suspended in the air scatter blue and violet wavelengths the least, and because of this they manage to reach the Earth's surface, unlike other colors of the spectrum.
These hypotheses were supported by many famous scientists, but research by the English physicist John Rayleigh showed that solid particles are not the main cause of light scattering. It is the molecules of gases in the atmosphere that separate light into color components. A white ray of sunlight, colliding with a gas particle in the sky, scatters (scatters) in different directions.
When it collides with a gas molecule, each of the seven color components of white light is scattered. At the same time, light with longer waves (the red component of the spectrum, which also includes orange and yellow) is scattered less well than light with short waves (the blue component of the spectrum). Because of this, after scattering, eight times more blue spectrum colors remain in the air than red.
Although violet has the shortest wavelength, the sky still appears blue due to the mixture of violet and green waves. In addition, our eyes perceive blue color better than violet, given the same brightness of both. It is these facts that determine the color scheme of the sky: the atmosphere is literally filled with rays of blue-blue color.
Why then is the sunset red?
However, the sky is not always blue. The question naturally arises: if we see blue skies all day, why is the sunset red? We found out above that red color is least scattered by gas molecules. During sunset, the Sun approaches the horizon and the sun's ray is directed towards the Earth's surface not vertically, as during the day, but at an angle.
Therefore, the path it takes through the atmosphere is much longer than what it takes during the day when the Sun is high. Because of this, the blue-blue spectrum is absorbed in a thick layer of the atmosphere, not reaching the Earth. And longer light waves of the red-yellow spectrum reach the surface of the Earth, coloring the sky and clouds in the red and yellow colors characteristic of sunset.
Why are the clouds white?
Let's touch on the topic of clouds. Why are there white clouds in the blue sky? First, let's remember how they are formed. Moist air containing invisible steam, heated at the surface of the earth, rises and expands due to the fact that the air pressure is less at the top. As the air expands, it cools. When water vapor reaches a certain temperature, it condenses around atmospheric dust and other suspended solids, resulting in tiny droplets of water that coalesce to form a cloud.
Despite their relatively small size, water particles are much larger than gas molecules. And if, when meeting air molecules, the sun's rays are scattered, then when they meet water droplets, the light is reflected from them. In this case, the initially white ray of sunlight does not change its color and at the same time “colors” the molecules of the clouds white.
When the wind throws a white fluffy transparent cape over the beautiful blue sky, people begin to look up more and more often. If at the same time it also puts on a large gray fur coat with silver threads of rain, then those around it hide from it under umbrellas. If the outfit is dark purple, then everyone is sitting at home and wants to see the sunny blue sky.
And only when the long-awaited sunny blue sky appears, which puts on a dazzling blue dress decorated with golden sun rays, people rejoice - and, smiling, leave their houses in anticipation of good weather.
The question of why the sky is blue has worried human minds since time immemorial. Greek legends have found their answer. They claimed that this shade was given to it by the purest rock crystal.
During the time of Leonardo da Vinci and Goethe, they also sought an answer to the question of why the sky is blue. They believed that the blue color of the sky is obtained by mixing light with darkness. But later this theory was refuted as untenable, since it turned out that by combining these colors, you can only get tones of the gray spectrum, but not color.
After some time, the answer to the question of why the sky is blue was attempted to be explained in the 18th century by Marriott, Bouguer and Euler. They believed that this was the natural color of the particles that made up the air. This theory was popular even at the beginning of the next century, especially when it was found that liquid oxygen is blue and liquid ozone is blue.
Saussure was the first to come up with a more or less sensible idea, who suggested that if the air were completely pure, without impurities, the sky would turn out to be black. But since the atmosphere contains various elements (for example, steam or water drops), they, reflecting color, give the sky the desired shade.
After this, scientists began to get closer and closer to the truth. Arago discovered polarization, one of the characteristics of scattered light that bounces off the sky. Physics definitely helped the scientist in this discovery. Later, other researchers began to look for the answer. At the same time, the question of why the sky is blue was so interesting to scientists that a huge number of different experiments were carried out to find out, which led to the idea that the main reason for the appearance of the blue color is that the rays of our Sun are simply scattered in the atmosphere.
Explanation
The first to create a mathematically based answer for molecular light scattering was the British researcher Rayleigh. He hypothesized that light is scattered not because of impurities in the atmosphere, but because of the air molecules themselves. His theory was developed - and this is the conclusion the scientists came to.
The sun's rays make their way to the Earth through its atmosphere (a thick layer of air), the so-called air envelope of the planet. The dark sky is completely filled with air, which, despite the fact that it is completely transparent, is not empty, but consists of gas molecules - nitrogen (78%) and oxygen (21%), as well as water droplets, steam, ice crystals and small pieces of solid material (for example, particles of dust, soot, ash, ocean salt, etc.).
Some rays manage to pass freely between gas molecules, completely bypassing them, and therefore reach the surface of our planet without changes, but most rays collide with gas molecules, which become excited, receive energy and release multi-colored rays in different directions, completely coloring the sky, resulting in us seeing sunny blue skies.
White light itself consists of all the colors of the rainbow, which can often be seen when it is broken down into its component parts. It so happens that air molecules scatter blue and violet colors the most, since they are the shortest part of the spectrum because they have the shortest wavelength.
When blue and violet colors are mixed in the atmosphere with a small amount of red, yellow and green, the sky begins to “glow” blue.
Since the atmosphere of our planet is not homogeneous, but rather different (near the surface of the Earth it is denser than above), it has different structure and properties, we can observe blue tints. Before sunset or sunrise, when the length of the sun's rays increases significantly, blue and violet colors are scattered in the atmosphere and absolutely do not reach the surface of our planet. The yellow-red waves, which we observe in the sky during this period of time, successfully reach.
At night, when the sun's rays cannot reach a certain side of the planet, the atmosphere there becomes transparent, and we see “black” space. This is exactly how astronauts above the atmosphere see it. It is worth noting that the astronauts were lucky, because when they are more than 15 km above the surface of the earth, during the day they can simultaneously observe the Sun and the stars.
Color of the sky on other planets
Since the color of the sky largely depends on the atmosphere, it is not surprising that it is of different colors on different planets. It’s interesting that Saturn’s atmosphere is the same color as our planet’s.
The sky of Uranus is a very beautiful aquamarine color. Its atmosphere consists mainly of helium and hydrogen. It also contains methane, which completely absorbs red and scatters green and blue colors. Neptune's skies are blue: in the atmosphere of this planet there is not as much helium and hydrogen as ours, but there is a lot of methane, which neutralizes red light.
The atmosphere on the Moon, the Earth's satellite, as well as on Mercury and Pluto, is completely absent, therefore, light rays are not reflected, so the sky here is black, and the stars are easily distinguishable. The blue and green colors of the sun's rays are completely absorbed by the atmosphere of Venus, and when the Sun is near the horizon, the skies are yellow.
On a clear sunny day, the sky above us looks bright blue. In the evening, the sunset colors the sky in red, pink and orange. So why is the sky blue and what makes the 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 to scatter sunlight.
On Earth, some of the short wavelengths of sunlight (blue and violet) are absorbed by scattering. The rest of the spectrum appears yellow.
And in space, the sky looks dark or black instead of blue. This is the result of the absence of an atmosphere, therefore the light is not scattered in any way.
But if you ask about the color of the sun in the evening. Sometimes the answer is the sun is RED. But why?
Why is the sun red at sunset?
As the Sun moves toward sunset, 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 descends to the horizon, the larger the airy “magnifying glass” through which we see it, and the redder it is.
For the same reason, the Sun appears 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 have 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, which is why the sky appears red, pink or orange.
Visible light is a type of energy that can travel through space. Light from the Sun or an incandescent lamp appears white, although in reality it is a mixture of all colors. The primary colors that make up white are red, orange, yellow, green, blue, indigo and violet. These colors continuously transform into one another, so in addition to the primary colors there are 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 appears in an area of high humidity.
The air that fills the entire sky is a mixture of tiny gas molecules and small solid particles such as dust.
The sun's rays, coming from space, begin to scatter under the influence of atmospheric gases, and this process occurs according to Rayleigh's Law of Scattering. 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 interacts with the air, bumping into molecules and dust.
When light collides with gas molecules, light can be reflected in different directions. Some colors, such as red and orange, reach the observer directly by passing directly through the air. But most blue light is reflected from air molecules in all directions. This scatters blue light throughout the sky and makes it appear blue.
However, many shorter wavelengths of light are absorbed by gas molecules. Once absorbed, the blue color is emitted in all directions. It is scattered everywhere in the sky. No matter which direction you look, some of this scattered blue light reaches the observer. Since blue light is visible everywhere overhead, the sky appears blue.
If you look towards the horizon, the sky will have a paler hue. This is the result of light traveling a greater distance through the atmosphere to reach the observer. The scattered light is scattered again by the atmosphere and less blue light 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 the light could be reflected, the light travels directly. The rays of light are not scattered, and the “sky” appears 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, which 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 extreme cold, the temperature range could fluctuate from -170°C at night to +120°C during the day. The atmosphere also protects us from harmful radiation from the Sun and space, absorbing and dispersing 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 clearly defined boundaries; they change seasonally, and in addition, their parameters change at different latitudes.
Homosphere
- The lower 100 km, including the Troposphere, Stratosphere and Mesopause.
- Makes up 99% of the mass of the atmosphere.
- Molecules are not separated by molecular weight.
- The composition is fairly 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 that are unevenly distributed. As water vapor rises, it cools and condenses, then returning to the ground in the form of precipitation - snow and rain. The stratosphere itself is very dry.
- Ozone is another molecule whose distribution is uneven. (Read below about the ozone layer in the stratosphere.)
Heterosphere
- Extends above the homosphere and includes the Thermosphere and Exosphere.
- The separation of molecules in this layer is based on their molecular weights. Heavier molecules such as nitrogen and oxygen are concentrated at the bottom of the layer. Lighter ones, helium and hydrogen, predominate in the upper part of the heterosphere.
Division 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 absorption of ultraviolet light
- The degree of ionization changes with altitude.
- Different layers reflect long and short radio waves. This allows radio signals traveling 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 approximately 70,000 km altitude, this altitude depends on the intensity of the solar wind. The magnetosphere protects us from high-energy charged particles from the solar wind by keeping them in the Earth's magnetic field.
Division 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 an altitude of approximately 16 km at the equator, and to an altitude of 9 km at the North and South Poles.
- The prefix "tropo" means change. Changes in the parameters of the troposphere occur due to weather conditions - for example, due to the movement of atmospheric fronts.
- As altitude increases, the temperature drops. Warm air rises, then cools and falls back to Earth. This process is called convection, it occurs as a result of the movement of air masses. Winds in this layer blow predominantly vertically.
- This layer contains more molecules than all other layers combined.
Stratosphere- extends from approximately 11 km to 50 km altitude.
- Has a very thin layer of air.
- The prefix "strato" refers to layers or division into layers.
- The lower part of the Stratosphere is quite calm. Jet aircraft often fly into the lower stratosphere to avoid bad weather in the troposphere.
- At the top of the Stratosphere there are strong winds known as high-altitude jet streams. They blow horizontally at speeds of 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 is very effective at absorbing harmful ultraviolet rays from the sun, thereby protecting life on earth. The ozone molecule consists of three oxygen atoms. The oxygen molecules we breathe contain two oxygen atoms.
- The stratosphere is very cold, with a temperature of approximately -55°C at the bottom and increasing with altitude. The increase in temperature is due to the absorption of ultraviolet rays by oxygen and ozone.
Mesosphere- extends to altitudes of approximately 100 km.
One of the hallmarks of a person is curiosity. Probably everyone, as a child, looked at the sky and wondered: “why is the sky blue?” As it turns out, answers to such seemingly simple questions require some knowledge base in the field of physics, and therefore not every parent will be able to correctly explain to their child the reason for this phenomenon.
Let's consider this issue from a scientific point of view.
The wavelength range of electromagnetic radiation covers almost the entire spectrum of electromagnetic radiation, which also includes radiation visible to humans. The image below shows the dependence of the intensity of solar radiation on the wavelength of this radiation.
Analyzing this image, we can note the fact that visible radiation is also represented by uneven intensity for radiation of different wavelengths. Thus, the violet color makes a relatively small contribution to visible radiation, and the largest contribution is made by blue and green colors.
Why the sky is blue?
First of all, this question is prompted by the fact that air is a colorless gas and should not emit blue light. Obviously, the cause of such radiation is our star.
As you know, white light is actually a combination of radiation from all the colors of the visible spectrum. Using a prism, light can be clearly separated into a full range of colors. A similar effect occurs in the sky after rain and forms a rainbow. When sunlight enters the earth's atmosphere, it begins to scatter, i.e. the radiation changes its direction. However, the peculiarity of the composition of air is such that when light enters it, radiation with a short wavelength is scattered more strongly than long-wave radiation. Thus, taking into account the spectrum depicted earlier, you can see that red and orange light will practically not change their trajectory when passing through the air, while violet and blue radiation will noticeably change their direction. For this reason, a certain “wandering” short-wave light appears in the air, which is constantly scattered in this environment. As a result of the described phenomenon, short-wave radiation in the visible spectrum (violet, cyan, blue) appears to be emitted from every point in the sky.
The well-known fact of radiation perception is that the human eye can catch, see, radiation only if it directly enters the eye. Then, looking at the sky, you will most likely see shades of that visible radiation, the wavelength of which is the shortest, since it is this that is best scattered in the air.
Why don’t you see a clearly red color when looking at the Sun? Firstly, it is unlikely that a person will be able to carefully examine the Sun, since intense radiation can damage the visual organ. Secondly, despite the existence of such a phenomenon as the scattering of light in the air, most of the light emitted by the Sun reaches the surface of the Earth without being scattered. Therefore, all the colors of the visible spectrum of radiation are combined, forming light with a more pronounced white color.
Let's return to light scattered by air, the color of which, as we have already determined, should have the shortest wavelength. Of visible radiation, violet has the shortest wavelength, followed by blue, and blue has a slightly longer wavelength. Taking into account the uneven intensity of the sun's radiation, it becomes clear that the contribution of the violet color is negligible. Therefore, the largest contribution to the radiation scattered by air comes from blue, followed by blue.
Why is the sunset red?
In the case when the Sun hides behind the horizon, we can observe the same long-wave radiation of red-orange color. In this case, light from the Sun must travel a noticeably greater distance in the Earth's atmosphere before reaching the observer's eye. At the point where the sun's radiation begins to interact with the atmosphere, the colors blue and blue are most pronounced. However, with distance, short-wave radiation loses its intensity, as it is significantly scattered along the way. While long-wave radiation does an excellent job of covering such long distances. That's why the Sun is red at sunset.
As mentioned earlier, although long-wave radiation is weakly scattered in the air, scattering still takes place. Therefore, being on the horizon, the Sun emits light, from which only radiation of red-orange shades reaches the observer, which has some time to dissipate in the atmosphere, forming the previously mentioned “wandering” light. The latter colors the sky in variegated shades of red and orange.
Why are the clouds white?
Speaking of clouds, we know that they consist of microscopic droplets of liquid that scatter visible light almost uniformly, regardless of the wavelength of the radiation. Then the scattered light, directed in all directions from the droplet, is scattered again on other droplets. In this case, the combination of radiation of all wavelengths is preserved, and the clouds “glow” (reflect) in white.
If the weather is cloudy, then little solar radiation reaches the Earth's surface. In the case of large clouds, or a large number of them, some of the sunlight is absorbed, causing the sky to dim and take on a gray color.