What is a black and white cloud? Why is a cloudy sky gray and a clear sky blue? How a mirage is formed
Many of us, even in our distant childhood, have at least once asked ourselves a similar question: why is the sky above us blue? After all, the light that reaches us straight from the Sun is essentially white, not blue. However, as people grew older, they probably did not lose interest in this issue. Many would not mind pacifying their curiosity and still finding out why the vault of heaven is exactly blue.
Why the sky is blue: physics
Physics will help you delve into all the intricacies of this issue in more detail, where scientists have long figured out the reasons and studied everything thoroughly. We can only taste the fruits of their painstaking work.
So, let's start with what distinguishes our planet from others - the presence of air that living organisms can breathe. Although its composition includes nitrogen, oxygen, carbon dioxide, water vapor, and various dust particles that are constantly in motion, it nevertheless remains absolutely transparent. But sunlight has a huge number of colors, they are familiar to us as the colors of the rainbow. Each of the seven colors has its own wave, ray, and they are all different in length.
In order for sunlight to reach the earth, it needs to break through a huge layer of air. It is during the passage of light through the air that the rays begin to scatter, and this process occurs more intensely with the blue color. This is explained very simply - the wavelength of blue light is the shortest. This is how, from a physics point of view, the blueness of the sky is explained.
Physics answered our question "why the sky is blue", but what about the clouds, because they have a white tint, even though they are in the air. It would be logical to assume that rays of sunlight would be scattered through clouds in the same way as when passing through the air. But this issue needs to be approached from a slightly different angle.
Clouds, first of all, are tiny particles of water that have evaporated from the surface of the earth and grouped into a gaseous mass. But no matter how small these particles are, they will still be much larger in size than an air molecule. It is in the size of the molecules that the answer to this question lies.
As we know, the sun's rays, penetrating into the Earth's atmosphere, encounter an obstacle on their way - air, which can only be overcome by dispersion. But this method does not work with clouds because of the same size of molecules. Light, meeting a micro-droplet of water, is not refracted, but, on the contrary, reflected from its surface.
This allows the sun's ray to remain in its original color palette, that is, white, while coloring the cloud molecules white.
On a fine summer day, it is sometimes so pleasant to admire snow-white clouds flying across the high sky, taking on the bizarre shapes of strange animals, fairy-tale castles or other images suggested by imagination.
But as soon as a black rain cloud appears at the edge of the sky, the carefree mood is immediately replaced by an unconscious feeling of anxiety. Why is this happening? Why do clouds, so white and light, when they become a thundercloud, immediately darken, acquiring a deep, almost black color?
What are clouds?
Without exception, all clouds are huge, sometimes several tens of kilometers in size, accumulations of tiny water droplets and ice crystals, moved by the force of the wind above the surface of the earth at an altitude of 0.5 to 30 kilometers.
The microscopic dimensions of cloud elements - droplets or pieces of ice - enable them to stay in the air for a long time. But as soon as conditions are created for the enlargement of cloud elements, they become too heavy, sink and fall out of the cloud.
Clouds are:
- feathery - spread across the sky in the form of huge feathers or ribbons, straight or curved;
- layered - as if consisting of several layers laid one on top of the other, often of slightly different shades;
- cumulus - resembling huge snowdrifts or lumps of snow-white cotton wool flying high in the sky.
These are the main forms of clouds, in practice they are often combined with each other in the most bizarre way, becoming cirrostratus, cirrocumulus, stratocumulus, etc.
How are clouds formed?
As mentioned above, clouds are formed by droplets of water vapor. Every day, tens of thousands of tons of water evaporate from the surface of oceans, seas, lakes and rivers, and simply from the surface of the earth. Initially, this water is found in warm layers of air located close to the surface.
This warm air, according to the laws of physics, rises, but the further it moves away from the ground, the more it cools. Water molecules, losing energy, begin to transition from a gaseous state to a liquid state, condensing in the form of droplets.
But since the air that has risen upward is more rarefied than the lower layer located near the surface, water droplets cannot condense to a sufficiently large size and remain in the atmosphere in the form of a suspension, a tiny aerosol of liquid or solid ice particles. These are the clouds.
Experts say that water in them is often present in both states at the same time - both in the form of drops and in the form of ice floes.
Why do clouds and clouds have different colors?
Meteorologists don't use the term "cloud", instead they say "rain cloud" or "thunder cloud" . We all know that if a dark cloud appears on the horizon and is approaching us, then it will definitely rain soon. There is a very direct connection between the color of a cloud and its ability to shed drops of water.
The fact is that the structure of white clouds flying in the sky at an altitude of six kilometers or more is very sparse and loose. Therefore, sunlight passes through them almost unhindered, like through a frosted lamp shade. It gives the impression of a snow-white air mass, light and radiant.
When, for various reasons, a cloud descends to a height of two kilometers or less, it becomes denser, and it is more difficult for the sun's rays to pass through its thickness.
The lower the cloud descends, the darker it appears: at first it takes on a slight grayish tint, then becomes distinctly gray, blue or purple.
At the same time, the air mass becomes denser, and the smallest droplets move closer to each other. This causes water droplets to increase in size, which further impairs the passage of sunlight through the cloud. It becomes more and more heavy and sinks lower and lower, and then a moment comes when the cloud can no longer hold drops of water, and they begin to fall down to the ground.
The condensation process continues until the cloud becomes less dense and begins to rise into the upper atmosphere.
Thus, it becomes clear that the color of a cloud depends mainly on its density: the higher this indicator, the darker the cloud.
Rain and thunderclouds have the highest density, the water droplets in which have already condensed almost to the point of being ready to spill onto the ground. This is why a rain cloud looks heavy and black, and a cloud looks white and light.
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Why are clouds white and clouds black, how are clouds and mirages born, and is it true that there is a Society of Cloud Lovers? Let's talk.
This is the motto of The Cloud Appreciation Society, an international organization founded in 2004 in England by Gavin Praetor-Pinney. It unites 30,000 cloud lovers from 94 countries. These are people who dream of revealing the beauty of the cloudy sky to others. Join us!
You just need to lie on the grass and watch the clouds. Or just look up. Anytime you want.
How is a cloud born?
When hot air mixes with cold air, it cools and can reach its dew point. Condensation occurs. Water vapor, settling on a particle in the air, turns into drops or ice crystals, which, when collected together, form a cloud.
Most often this happens when hot air rises from the ground and meets cold air above - in the atmosphere. A cloud-like phenomenon can also be observed at the surface of the earth. Earth or water, heated during the day, cools more slowly than air. When cold night air comes into contact with warm air near the surface of the earth or water, fog is formed.
Illustration from the book
Yes, yes, fog also consists of cloud elements. It is essentially a large cloud lying on the ground.
Why are clouds white and clouds black?
If clouds are made of droplets, why are they white? Because cloud elements reflect light: crystals and droplets glisten in the rays of the sun. And the smaller the elements and the greater the number, the whiter the cloud.
Gray, bluish and black thunderclouds consist of the same drops. It’s just that, in heavy clouds, they can cast shadows on each other (and even on themselves), which is why they appear darker. There are also denser clouds - they consist of large crystals and droplets, so the sun's rays cannot penetrate through them. They appear dark and ominous when viewed from the ground.
Illustration from the book
But if you fly over them, for example on an airplane, they will be completely white.
How a mirage is formed
A cloud forms when warm air rises. This hot rising air current is called a thermal. Birds and gliders soar on it.
The thermal can be seen if you look at a paved road on a hot day. It seems that the air above the asphalt is shaking, and the road seems to be covered with puddles. This phenomenon is called a mirage.
A mirage can be seen when hot and cold air that have different densities meet. At the boundary of media with different densities, a ray of light is refracted, and we see a mirage.
Clouds are not just wisps of cotton that block the sun. They are no less beautiful than the stars. After reading this book, you will see for yourself.
If, looking out the window, you see how the sky is covered with lead clouds, and you cannot understand the reason for what happened, then it’s okay. You might just need to fill in some knowledge gaps or refresh your memory so you're more aware of where clouds come from in the first place. And even then you will understand whether you should be afraid of them.
What are clouds
No matter how the clouds look in the sky, whether they are almost transparent, like a veil or impenetrable, like a lead cloud, they are all made of water. The fact is that when the air is heated, the moisture located on the surface of the earth takes on a gaseous state and rises to the top, where, due to the lower air temperature, it condenses. However, there is one detail necessary for the formation of clouds - dust. Even at the beginning of the process of their formation, water molecules stick to its smallest particles, after which drops and ice crystals are formed, which will later become rain. When conditions are favorable for growth, clouds gain volume, become heavier, sink lower and lower, and eventually their contents fall out as precipitation.
The height of clouds can vary from 100 m from the Earth to 30 km, depending on weather conditions, climate and the stage of their development. But they are formed precisely at an altitude of up to 14 km, between the upper layers of the troposphere and the surface of the Earth. The height at which clouds first form and subsequently settle depends on their type. To finally understand which of them the so-called lead clouds belong to, let’s move on to their description.
Cloud classification
When looking at the sky, you can see three types of clouds:
- Cirrus. As a rule, they are white, look like huge ribbons, curved or straight, spread across the sky. They are located at an altitude of 6-10 km, their thickness ranges from 100 m to 2 km, and their structure is usually crystalline.
- Layered. The name speaks for itself; clouds of this type seem to be superimposed on each other in a neat layer, and they are often of different shades, which makes them even more beautiful. They are located at an altitude of 0.1-0.7 km, have a thickness of 0.2-0.8 km, and have a mainly droplet structure.
- Cumulus. They resemble large snow-white snowdrifts floating high in the sky. Usually at an altitude of 800-1500 m, width from 100 m to 2 km.
You can often observe their combinations, such as cirrostratus, stratocumulus, etc. If your gaze fell on a lead cloud, then you probably already have a nimbostratus or cumulonimbus cloud in front of you. It might start to rain soon.
The reason for the formation of a lead cloud
Everyone knows that there is a direct connection between the color of clouds and their ability to shed rain. If a dark cloud appears on the horizon, then precipitation will probably fall in the near future, possibly accompanied by a thunderstorm. But sometimes the sight of lead clouds in the sky can sincerely amaze that even an adult will have a question about the reason for their appearance. In fact, they are no different from ordinary clouds. The right conditions simply arose for their growth, after which, due to the large volume of moisture and density, they completely stopped transmitting sunlight and seemed so intimidating. Polluted air sometimes also has an effect, due to which a large amount of soot and dust gets into the clouds, and they become blacker. And finally, regarding the necessary conditions for the formation of a lead cloud:
- Instability of the air mass rising upward;
- The presence of warm and cold air (more common in late summer, spring and early autumn).
And all that should be done when it appears is just to protect yourself from being struck by lightning.
It’s a great pleasure to watch the cumulus clouds passing by in the summer and try to explain why they are sometimes light and sometimes dark. When the cloud is illuminated by the Sun, it is dazzling white, but when the cloud passes above us, its base becomes gray or completely dark. The drops of water in the cloud are so closely located to each other that it appears as an almost completely opaque white body - white because light hardly penetrates the cloud, but is very well reflected by the numerous drops. If the Sun is covered by cumulus clouds, they appear dark, but the edges of the clouds are always light: "every cloud has a silver lining."
Thus, the distribution of light and shadow gives us interesting information about the different parts of the cloud - upper, lower, front, back and about the actual shape of this huge formation. It is not always easy to get a correct idea of its proportions, as well as the position of the cloud in relation to the Sun. If, for example, the clouds are located in front of me, and the Sun is some distance above it, I will be puzzled to see only shadows (Fig. 169, a). I can't imagine the grandeur
Rice. 169. Light and shadows on cumulus clouds: a) how we see landscapes
observer, viewed from north to south; b) erroneous personal impressions of the observer and the picture he expects to see; c) how things are in reality. In cases b) and c) the observer looks from east to west. The sun is in the south.
distance to the Sun, I unconsciously consider it very close and therefore it seems to me that part AB should be illuminated (Fig. 169, 6). In fact, the sun's rays illuminating the cloud run parallel to the line connecting the Sun with my eye (Fig. 169, c).
No matter how capricious the play of light and shadow may be, no matter how complicated the matter may be by the shadows cast by one cloud on another, it seems impossible to explain by this alone all the differences in the colors of cumulus clouds.
I. Levitan
When the sky clears after a storm and there are only a few cumulus clouds left on it, brightly illuminated by the Sun and located so that the shadow of one of them cannot fall on another, these clouds become darker and darker and, finally, before completely disappearing , turn blue-black. The general impression is that the thin parts of cumulus clouds seen against a blue sky are not "blue+white" (as one might expect), but "blue+black".
On the other hand, a cumulus cloud appears gray when seen against the background of another large cloud that is completely white, so there cannot be any question of an increase in brightness simply due to an increase in the overall thickness of the layers. The physics of these phenomena, although observed daily, are still not well understood. Of course, one must be very wary of the idea that clouds actually absorb light; one must first try to explain the phenomenon by assuming that clouds are solid white formations, then remember that they are really nothing more than light-scattering fog, and finally consider that clouds may contain dark dust particles.
It is interesting to compare clouds with the white steam (not smoke!) of a steam locomotive. In some cases, the steam appears whiter when viewed at a greater angle to the incident light, and less bright when observed from the direction of the Sun, when the eye perceives rays reflected almost in the direction of incidence. In other cases, regardless of viewing direction, the vapor is much brighter than the brightest parts of the cumulus; This may be due to the greater distance to the clouds and the attenuation of light due to scattering in the air.
Georgy Nissky. Autumn. Semaphores
Dark cumulus clouds often appear bluish at great distances. This is not the color of the clouds themselves, but the light scattered in the atmosphere between the cloud and our eye. The further away such a dark cloud is, the closer its color approaches the color of the sky. On the other hand, bright clouds near the horizon become yellowish.
We should also consider other clouds and try to explain, for example, why rain clouds are so gray, why thunderclouds have a special lead color visible next to faded orange. Isn't this dust? Our information about all this, however, is so incomplete that we will limit ourselves to calling on the reader to make independent observations.
The distribution of brightness across the vault of heaven, when it is completely and evenly covered with clouds, is very characteristic and, as it were, complements the distribution in a clear sky. Compare, for example, using a mirror, the zenith and the horizon: with a clear sky, the zenith is always darker; The brightness ratio ranges from 3 to 5 (photos X1X and XX).