Why are there white clouds and dark clouds?

Why are there white clouds and dark clouds?

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White clouds and dark clouds are all made up of small water droplets. Rayleigh law does not apply to water droplets, because it is based on the secondary emission of gas molecules, and water droplets are colorless and transparent spherical lenses larger than nanoparticles. Its so-called "scattered light" is actually reflected light and transmitted light after several refractions.

There are some differences between white clouds and dark clouds in terms of "water content", but the formulation of "water content" is a bit vague and the words used are not satisfactory. It can be understood as the water content of the whole cloud or the water content of a single water drop. Both of these understandings have certain basis. Dark clouds can cover the whole sky, but white clouds can't. Therefore, as far as the general trend is concerned, the water content of dark clouds is generally greater than that of white clouds. However, a cloud in the sky can be both white and dark, and a large cloud can also be "white" or "black". Huge white clouds in summer can instantly become rolling clouds, which is not explained by water content. If "water content" means "water content of a single drop", it is accurate.

Judging from the formation process of clouds, if they don't float from other places, then the dark clouds must come from white clouds. White clouds can not only come from other places, but also change from dark clouds, and can also be suddenly "created" on the background of clear skies in Wan Li. In the process of studying solar energy, I pay great attention to the changes of clouds in the sky. I have seen many times that the blue sky background can change from blue to pink blue, then to faint white clouds with blurred edges, and even to white clouds with clear edges in a few minutes. I've never seen dark clouds suddenly appear against a blue sky background. I also noticed that white clouds turned into dark clouds mostly before it rained, and dark clouds turned into white clouds mostly after it rained. The explanations for this phenomenon are as follows: in summer, the surface moisture evaporates rapidly under the scorching sun, making the air humidity higher and higher; The temperature at high altitude is lower than the surface temperature, so the water vapor first reaches saturation and supersaturation at high altitude; There will always be some dust in the sky, which will become the condensation center, so that saturated steam and supersaturated steam will condense into fine water droplets; When the fog drops are dense enough, they become white clouds visible to the naked eye; As the fog drops get bigger and bigger, the white clouds turn into dark clouds; The water drops in the dark clouds continue to get bigger and become raindrops; After the rain, the humidity of the air becomes smaller, the water vapor returns to the unsaturated state, and the small water droplets in the dark clouds begin to evaporate and become smaller and smaller, thus turning the dark clouds into white clouds; The water droplets in the white clouds continue to evaporate. Once they all evaporate, the white clouds disappear and the sky reappears.

Why are white clouds "white"? Why are dark clouds "black"? The example that white clouds suddenly turn into dark clouds in summer is the most telling. In this mutation, the total amount of water and the projection angle of sunlight remain basically unchanged, and the conspicuous change is "from white to black". This kind of thing always happens when a thunderstorm is coming, which shows that "from white to black" is the result of water droplets "from small to large". Dark clouds are not dull "black clouds", but bright, and their scattered light is actually white light, which is no different from that of white clouds in spectrum. This is because both large water droplets and small water droplets are colorless and transparent spherical lenses for visible light, and the color of scattered light is determined by the color of incident light. Once this is clear, we can use "brightness" to make a quantitative comparison between white clouds and dark clouds.

For a single water drop, the ratio of scattered light flux to incident light flux in a specific direction should be a constant, regardless of the size of the water drop. But as far as the whole cloud cluster is concerned, the ratio of the total flux of scattered light to the total flux of incident light is not constant. The scattering cross section of a single water drop is proportional to the square of the linear, and the volume and mass are proportional to the cubic of the linear. This means that if the radius of water droplets is doubled, the scattering cross section of a single water droplet will be expanded to four times, while the total number of water droplets will be reduced to 1/8, which means that the total scattering cross section is half of the original. In this way, if the water droplets in the white clouds belong to micron level and those in the dark clouds belong to sub-millimeter level (i.e. 100 micron), then the brightness of the white clouds should be 100 times that of the dark clouds.

Why are there white and black clouds in the sky?

Most of the clouds in the sky are gray, but sometimes there are black, red and purple clouds.

The color of clouds is caused by the reflection of sunlight by clouds, and is closely related to the size, thickness, range and formation time of clouds.

On cloudy days, the clouds cover a large area, covering the sky almost or completely, and it is difficult for sunlight to penetrate, so the clouds are gray.

On cloudy or sunny days, there are several clouds in the sky, and the sun illuminates them. Clouds are usually near white.

In summer, before the thunderstorm comes, a large range of thunderstorm clouds will form. This cloud is usually black because it is so thick that sunlight can hardly penetrate it.

In the morning and evening clouds, its color is often red, because at sunrise and sunset, the sun shines obliquely through the thick atmosphere, and the relatively strong red-orange light shines on the clouds, making them appear red and orange.

There are clouds of different colors in the sky, some as white as floc, some as black, some as gray, some as red and purple. How are these clouds of different colors formed?

The thickness of the clouds we see varies greatly, from seven or eight kilometers thick to only a few tens of meters thin. There are many kinds of layered clouds, isolated cumulus clouds and fluctuating clouds.

Thick layered clouds, or cumulonimbus clouds, the light from the sun and the moon is difficult to transmit, and the clouds look black; Slightly thinner layered clouds and wave clouds look gray, especially in wave clouds, and the edges of the clouds are grayer; Very thin clouds, light can easily penetrate, especially thin clouds made of ice crystals. Clouds are especially bright and silky in the sun. Even if there are such layered clouds in the sky, objects on the ground will still reflect shadows under the illumination of the sun and the moon.

Sometimes the clouds are so thin that you can hardly see them, but as long as you find one or several big rings near the sun and the moon, you can still tell that there are clouds. This cloud is called "cirrostratus with a thin curtain". Isolated cumulus clouds, because the clouds are thick, almost all the light is reflected to the sunny side, so it looks white; On the backlight side and its bottom, the light is not easily transmitted and looks gray and black.

At sunrise and sunset, due to the oblique light from the sun, molecules, water vapor and impurities in the air pass through the thick atmosphere, which makes the short-wave part of light scatter a lot, while the long-wave part of red and orange does not scatter much. Therefore, when it irradiates the lower atmosphere, long-wave light, especially red light, accounts for an absolute majority. At this time, not only the sky in the direction of sunrise and sunset is red, but also the bottom and edge of the cloud illuminated by it turn red.

Because clouds are composed of water droplets, ice crystals and a mixture of the two, when the sun and the moon pass by, they will also cause various beautiful rings of light or iridescence.

Why are there white clouds and dark clouds?

The upper cloud is about 10 km high. The air here is thin, and most of the clouds are ice crystals, so sunlight can penetrate, so the clouds are white. The clouds in the lower layer are water droplets and dust, which are generally thicker, especially in nimbostratus, which is thicker, so the sunlight is completely opaque, so it is black. The color of clouds depends on their height and thickness. There are also white clouds at the lower level, which are too small to stop the sun, but punishment can be reflected.

Clouds with rich water vapor are thick and thin, with reduced transparency and black color, while clouds with thin water vapor have high transparency and white color. Dark clouds are extremely rich in water vapor, so the color is darker. Sometimes a dark cloud can contain dozens of tons of water.

Why are clouds black and white?

Clouds in the sky are made up of small water droplets and dust in the air. Their diameter is much longer than the wavelength of sunlight of any color, so Rayleigh scattering rarely occurs. Part of the sunlight is reflected into the air; Part of it is scattered by Michael, and then the scattered light hits the earth, but Michael scattering does not change any color of sunlight; Some directly penetrate the gaps between water droplets. The above three conditions have no effect on the composition of sunlight, so it seems that the clouds in the sky are white. But the clouds are getting thicker and thicker, and there are more and more water droplets, which are almost connected. Sunlight and scattered light cannot or rarely penetrate clouds, so white clouds become dark clouds.

It is in the process of sunlight incident on the earth's surface through the atmosphere that air molecules or other particles in the atmosphere will absorb, reflect and transmit sunlight, thus forming the brilliant afterglow of blue sky, white clouds, sunset and morning glow. If there is no atmosphere and other particles, even during the day, the sun looks like a solitary bright ball, and the sky will be dark, so the air not only provides us with living conditions, but also makes our sky colorful.

Red clouds are also called flaming clouds because the light emitted by the sun in the evening is red.

White clouds are because clouds reflect light.

Dark clouds are too thick to transmit light.

Gray clouds are partly transparent and partly opaque, so white+black = gray.