What is Cloud?????????????

I think everyone know what is cloud. But, do you know where is it come from? or what is the material of cloud?? Okay, let me tell you something about cloud!

Actually, a cloud is a visible mass of liquid droplets or frozen crystal made of water or various chemicals suspended in the atmosphere above the surface of a planetary body. These suspended particles are also known as aerosol. Clouds in Earth's atmosphere are studied in Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology. Two processes, possibly acting together, can lead to air becoming saturated; cooling the air or adding water vapor to the air. In general,precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology. Two processes, possibly acting together, can lead to air becoming saturated. There are cooling the air or adding water vapor to the air. In general,precipitation will fall to the surface and an exception is virga, which evaporates before reaching the surface.
As established by Howard and his successors, clouds are commonly grouped into physical categories that can be up to five in number. There are cirriform, cumuliform, cumulonimbiform, stratocumuliform, and stratiform. These designations distinguish a cloud's physical structure and process of formation.

Cirriform
Cirriform-category clouds generally have a wispy fibrous appearance and form at high tropospheric altitudes along the very leading edges of a frontal or low-pressure weather disturbance and often along the fringes of its other borders. In general, they are non-convective but occasionally acquire a tufted or turreted appearance caused by small-scale high-altitude convection. These high clouds do not produce precipitation as such but are often accompanied or followed by lower-based clouds that do.
Cumuliform]
Cumuliform clouds typically have flat bases and puffy domed tops. They are the product of localized free-convective lift except in the case of small fair weather cumuliform clouds that do not grow vertically due to more limited convective activity. Incoming short-wave radiation generated by the sun is re-emitted as long-wave radiation when it reaches Earth's surface. This process warms the air closest to ground and increases air mass instability by creating a steeper temperature gradient from warm or hot at surface level to cold aloft. Moderate instability allows for the formation of cumuliform clouds of moderate size that can produce light showers if the airmass is sufficiently moist. The more the air is heated from below, the more unstable it tends to become. This may cause large towering cumuliform clouds to form in the lower half of the troposphere with tops growing into the upper levels. These buildups can cause moderate to occasionally heavy showers. They tend to be more concentrated and intense when they are associated with fast-moving unstable cold fronts.
Cumulonimbiform
The largest free-convective cumuliform clouds often have complex structures that include cirriform tops and multiple accessory clouds and are sometimes classified separately as cumulonimbiform. At maturity, they have very strong updrafts that can penetrate the tropopause. They can produce thunderstorms and a variety of types of lightning including cloud-to-ground that can cause wildfires. Other convective severe weather may or may not be associated with thunderstorms and include heavy rain or snow showers, hail, strong wind shear, downbursts,and tornadoes.
Stratiform
In general, stratiform-category clouds have a flat sheet-like structure and form at any altitude in the troposphere where there is sufficient condensation as the result of non-convective lift of relatively stable air, especially along warm fronts, around areas of low pressure, and sometimes along stable slow moving cold fronts. In general, precipitation falls from stratiform clouds in the lower half of the troposphere. If the weather system is well-organized, the precipitation is generally steady and widespread. The intensity varies from light to heavy according to the thickness of the stratiform layer as determined by moisture content of the air and the intensity of the weather system creating the clouds and weather. Unlike free convective cumuliform and cumulonimbiform clouds that tend to grow upward, stratiform clouds achieve their greatest thickness when precipitation that forms in the middle level of the troposphere triggers downward growth of the cloud base to near surface level. Stratiform clouds can also form in precipitation below the main frontal cloud deck where the colder air is trapped under the warmer airmass being forced above by the front. Non-frontal low stratiform cloud can form when advection fog is lifted above surface level during breezy conditions.
Stratocumuliform
Clouds of this physical structure have both cumuliform and stratiform characteristics and generally form as a result of limited convection in slightly unstable air. They can form at any altitude in the troposphere wherever and whenever there is sufficient moisture and lift. High stratocumuliform clouds also tend show some cirriform characteristics or form in association with cirriform clouds. If a poorly organized low-pressure weather system is present, virga or weak intermittent precipitation may fall from those stratocumuliform clouds that form mostly in the low and lower-middle height ranges of the troposphere.

Striking cloud colorations can be seen at many altitudes in the homosphere, which includes the troposphere, stratosphere, and mesophere. The first recorded colored cloud was seen by Nathan Ingleton in 1651, he wrote the event in his diary but the records were destroyed in 1666, in the Great Fire of London. The color of a cloud, as seen from Earth, tells much about what is going on inside the cloud.

In the troposphere, dense, deep clouds exhibit a high reflectance (70% to 95%) throughout the visible spectrum. Tiny particles of water are densely packed and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color, especially when viewed from the top.  Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases. As a result, the cloud base can vary from a very light to very-dark-grey depending on the cloud's thickness and how much light is being reflected or transmitted back to the observer. Thin clouds may look white or appear to have acquired the color of their environment or background. High tropospheric clouds appear mostly white if composed entirely of ice crystals or supercooled water droplets.

As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets. If the droplets become too large and heavy to be kept aloft by the air circulation, they will fall from the cloud as rain. By this process of accumulation, the space between droplets becomes increasingly larger, permitting light to penetrate farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, a percentage of the light that enters the cloud is not reflected back out but is absorbed giving the cloud a darker look. A simple example of this is one's being able to see farther in heavy rain than in heavy fog. This process of reflection/absorption is what causes the range of cloud color from white to black. 

Other colors occur naturally in tropospheric clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, whereas red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-size droplets in the cloud. A cumulonimbus cloud that appears to have a greenish/bluish tint is a sign that it contains extremely high amounts of water; hail or rain. Supercell type storms are more likely to be characterized by this but any storm can appear this way. Coloration such as this does not directly indicate that it is a severe thunderstorm, it only confirms its potential. Since a green/blue tint signifies copious amounts of water, a strong updraft to support it, high winds from the storm raining out, and wet hail; all elements that improve the chance for it to become severe, can all be inferred from this. In addition, the stronger the updraft is, the more likely the storm is to undergo tornadogenesis and to produce large hail and high winds. Yellowish clouds may occur in the late spring through early fall months during forest fire season. The yellow color is due to the presence of pollutants in the smoke. Yellowish clouds caused by the presence of nitrogen dioxide are sometimes seen in urban areas with high air pollution levels.

Within the troposphere, red, orange, and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. When the angle between the sun and the horizon is less than 10 percent, as it is just after sunrise or just prior to sunset, sunlight becomes too red due to refraction for any colors other than those with a reddish hue to be seen. The clouds do not become that color; they are reflecting long and unscattered rays of sunlight, which are predominant at those hours. The effect is much like if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads, this can produce blood-red clouds. Clouds look darker in the near-infrared because water absorbs solar radiation at those wavelengths.

In high latitude regions of the stratosphere, nacreous clouds occasionally found there during the polar winter tend to display quite striking displays of mother-of-pearl colorations. This is due to the refraction and diffusion of the sun's rays through thin clouds with supercooled droplets that often contain compounds other than water. At still higher altitudes up in the mesospere, noctilucent clouds made of ice crystals are sometimes seen in polar regions in the summer. They typically have a silvery white coloration that can resemble brightly illuminated cirrus.

So, that's all about cloud. Well, not all actually ^^