question: Why are rain clouds darker then regular clouds?
comment: Because there's more water in them. They're thicker and contain bigger droplets of water, so they block more sunlight and appear darker.
passages: The whiteness or darkness of clouds is a function of their depth. Small, fluffy white clouds in summer look white because the sunlight is being scattered by the tiny water droplets they contain, and that white light comes to the viewer's eye. However, as clouds become larger and thicker, the white light cannot penetrate through the cloud, and is reflected off the top. Clouds look darkest grey during thunderstorms, when they can be as much as 20,000 to 30,000 feet high.
   All cloud varieties fall into one of two main groups. One group identifies the opacities of particular low and mid-level cloud structures and comprises the varieties "translucidus" (thin translucent), "perlucidus" (thick opaque with translucent or very small clear breaks), and "opacus" (thick opaque). These varieties are always identifiable for cloud genera and species with variable opacity. All three are associated with the stratiformis species of altocumulus and stratocumulus. However, only two varieties are seen with altostratus and stratus nebulosus whose uniform structures prevent the formation of a perlucidus variety. Opacity-based varieties are not applied to high clouds because they are always translucent, or in the case of cirrus spissatus, always opaque.
   There is essentially no direct sunlight under an overcast sky, so all light is then diffuse sky radiation. The flux of light is not very wavelength-dependent because the cloud droplets are larger than the light's wavelength and scatter all colors approximately equally. The light passes through the translucent clouds in a manner similar to frosted glass. The intensity ranges (roughly) from of direct sunlight for relatively thin clouds down to of direct sunlight under the extreme of thickest storm clouds.
  
  Section::::As a part of total radiation.
  
  One of the equations for total solar radiation is:
   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.
   Stratiform clouds are a layer of clouds that covers the entire sky, and which have a depth of between a few hundred to a few thousand feet thick. The thicker the clouds, the darker they appear from below, because little of the sunlight is able to pass through. From above, in an airplane, the same clouds look perfectly white, but from the ground the sky looks gloomy and grey.
  
  Section::::In the sciences, nature, and technology.:The greying of hair.
   Clouds are white for the same reason as ice. They are composed of water droplets or ice crystals mixed with air, very little light that strikes them is absorbed, and most of the light is scattered, appearing to the eye as white. Shadows of other clouds above can make clouds look gray, and some clouds have their own shadow on the bottom of the cloud.
   The luminance or brightness of a cloud is determined by how light is reflected, scattered, and transmitted by the cloud's particles. Its brightness may also be affected by the presence of haze or photometeors such as halos and rainbows. 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. High thin tropospheric clouds reflect less light because of the comparatively low concentration of constituent ice crystals or supercooled water droplets which results in a slightly off-white appearance. However, a thick dense ice-crystal cloud appears brilliant white with pronounced grey shading because of its greater reflectivity.
   The color of a cloud, as seen from the Earth, tells much about what is going on inside the cloud. Dense deep tropospheric 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 and non-tropospheric clouds appear mostly white if composed entirely of ice crystals and/or supercooled water droplets.
   Thick clouds (such as stratocumulus) reflect a large amount of incoming solar radiation, meaning they have a high albedo. Thin clouds (such as Cirrus) tend to transmit most solar radiation, so have low albedo.
  
  Section::::What can change cloud albedo?
  
  Section::::What can change cloud albedo?:Liquid Water Path (LWP).
   When the precipitation reaches the ground without completely evaporating, it is designated as the feature "praecipitatio". This normally occurs with altostratus opacus, which can produce widespread but usually light precipitation, and with thicker clouds that show significant vertical development. Of the latter, "upward-growing" cumulus mediocris produces only isolated light showers, while "downward growing" nimbostratus is capable of heavier, more extensive precipitation. Towering vertical clouds have the greatest ability to produce intense precipitation events, but these tend to be localized unless organized along fast-moving cold fronts. Showers of moderate to heavy intensity can fall from cumulus congestus clouds. Cumulonimbus, the largest of all cloud genera, has the capacity to produce very heavy showers. Low stratus clouds usually produce only light precipitation, but this always occurs as the feature praecipitatio due to the fact this cloud genus lies too close to the ground to allow for the formation of virga.
   Stratocumulus Opacus is a dark layer of clouds covering entire sky without any break. However, the cloud sheet is not completely uniform, so that separate cloud bases still can be seen. This is the main precipitating type, however any rain is usually light. If the cloud layer becomes grayer to the point when individual clouds cannot be distinguished, stratocumulus turn into stratus clouds.
  
  Stratocumulus Perlucidus is a layer of stratocumulus clouds with small spaces, appearing in irregular pattern, through which clear sky or higher clouds can be seen.
   Studies have shown that cloud liquid water path varies with changing cloud droplet size, which may alter the behavior of clouds and their albedo. The variations of the albedo of typical clouds in the atmosphere are dominated by the column amount of liquid water and ice in the cloud. Cloud albedo varies from less than 10% to more than 90% and depends on drop sizes, liquid water or ice content, thickness of the cloud, and the sun's zenith angle. The smaller the drops and the greater the liquid water content, the greater the cloud albedo, if all other factors are the same.
   Clouds obscure the view of other objects in the sky, though varying thicknesses of cloudcover have differing effects. A very thin cirrus cloud in front of the moon might produce a rainbow-colored ring around the moon. Stars and planets are too small or dim to take on this effect, and are instead only dimmed (often to the point of invisibility). Thicker cloudcover obscures celestial objects entirely, making the sky black or reflecting city lights back down. Clouds are often close enough to afford some depth perception, though they are hard to see without moonlight or light pollution.
  
  Section::::Visual presentation.:Other objects.
   Red, orange and pink clouds occur almost entirely at sunrise and 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.
   Scattering also occurs even more strongly in clouds. Individual water droplets exposed to white light will create a set of colored rings. If a cloud is thick enough, scattering from multiple water droplets will wash out the set of colored rings and create a washed-out white color.
   As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets, which may combine to form droplets large enough to fall 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, it may be that a percentage of the light which enters the cloud is not reflected back out before it is absorbed. A simple example of this is 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.
   BULLET::::- Rain is characterized by high reflectivity values, high falling velocities and a broadening of the spectra.
  
  BULLET::::- Liquid layers within mixed-phase clouds can be detected by slightly higher peak width values, in accordance to the higher turbulence within.
  
  Section::::Cloud radar systems.
   Polar mesospheric clouds form at an extreme-level altitude range of about . They are given the Latin name noctilucent because of their illumination well after sunset and before sunrise. They typically have a bluish or silvery white coloration that can resemble brightly illuminated cirrus. Noctilucent clouds may occasionally take on more of a red or orange hue. They are not common or widespread enough to have a significant effect on climate. However, an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.
   Section::::Luminance, reflectivity, and coloration.
   When compared to normal clouds, the number of water droplets per volume of air in ship tracks is more than doubled, the radius of the drops is decreased by roughly six percent and the total volume of liquid water per volume of air is increased twofold. In other words, this excessive cloud seeding from the ship causes the clouds to retain more water. Normally, rain forms when cloud drops coagulate and reach a size at which gravity can pull them to the ground. Yet, in ship tracks, the cloud seeding makes the drops so small that they can no longer easily merge to reach the size needed to escape. Since no drizzle comes out of the seeded clouds, the liquid water just keeps building in the cloud. This makes the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum.
   Some possible mechanisms responsible for increasing the lifetime of an HVC include the presence of a magnetic field that induces a shielding effect and/or the presence of dark matter; however, there is no strong observational evidence for dark matter in HVCs. The most accepted mechanism is that of dynamical shielding, which increases the Kelvin-Helmholtz time. This process works due to the HVC having a cold neutral interior shielded by a warmer and lower-density exterior, causing the HI clouds to have smaller relative velocities with respect to their surroundings.
  
  Section::::Origins.
   Ultraviolet radiation from the Sun breaks water molecules apart, reducing the amount of water available to form noctilucent clouds. The radiation is known to vary cyclically with the solar cycle and satellites have been tracking the decrease in brightness of the clouds with the increase of ultraviolet radiation for the last two solar cycles. It has been found that changes in the clouds follow changes in the intensity of ultraviolet rays by about a year, but the reason for this long lag is not yet known.
   Cloud droplets normally form on aerosol particles that serve as CCN. Increasing the number concentration of CCN can lead to formation of more cloud droplets, which, in turn, have smaller size. The increase in number concentration increases the optical depth of the cloud, which results in increase in the cloud albedo making clouds appear whiter. Satellite imagery often shows trails of cloud or of enhanced brightness of cloud behind ocean-going ships due to this effect. The decrease in global mean absorption of solar radiation due to increases in CCN concentrations exerts a cooling influence on climate; the global average magnitude of this effect over the industrial era is estimated as between -0.3 and -1.8 Wm. 
   Section::::What can change cloud albedo?:The Twomey Effect (Aerosol Indirect Effect).
  
  Addition of cloud nuclei by pollution can lead to an increase in solar radiation reflected by clouds. Increasing aerosol concentration and aerosol density increases cloud droplet concentration, decreases cloud droplet size, and increases cloud albedo. In macrophysically identical clouds, a cloud with few larger drops will have a lower albedo than a cloud with more smaller drops.
  
  Section::::What can change cloud albedo?:Zenith Angle.
   The first indirect effect is also known as the Twomey effect. Sulfate aerosols can act as cloud condensation nuclei and this leads to greater numbers of smaller droplets of water. Lots of smaller droplets can diffuse light more efficiently than just a few larger droplets.
  