[kesavan]

Why rain clouds are black?

Clouds temperature is below 0 degree celsius. When Ice temperature is below 0 , that canot Penetrate light .

[rudi_in]

Thank you for posting your question to the Ask Me Help Desk.

As clouds develop vertically, such as the cumulonimbus, more sunlight is reflected and does not penetrate to reach our eyes and therefore appears dark. In other words, the sunlight just doesn't make it all the way through the cloud. It is just too thick.

Also, as cloud droplets near the base of a cloud become larger, they become better absorbers of light and less effective at scattering it. This means that the little light that may reach this part of the cloud is now absorbed rather than scattered and so the cloud can look even darker as there are fewer wavelengths in the visible spectrum to reach our eyes.

If the droplets become large enough they will fall in the form of rain.

[Capuchin]

I don't understand your explanation of droplets becoming better absorbers of light, rudi. That doesn't add up in my mind.

[rudi_in]

We have to look at the difference between scattering, reflection, and absorption.

Scattering is when the light hits the object and white light is basically sent in all different directions.

Reflection is when the light comes in and bounces off at the same angle.

Absorption is when the energy becomes stored internally and there is little or no reflection or scattering that takes place.

OK then, in the atmosphere, scattering of light is caused by the very small particles such as the dust, air molecules, etc.

Typical cloud droplets are of a size that they can effectively scatter the different wavelengths of light rather equally. This is why the basic cloud appears white. Keep in mind though that there is some reflection and absorption that does take place too but mostly scattering.

Now as the cloud develops vertically and becomes much thicker, the white light must pass through more of the cloud before reaching the base. As such, more of the light becomes either reflected or absorbed in the process allowing much less light to penetrate the bottom directly so it appears darker.

Now as far as the cloud droplets go...

Recall we said that the scattering is caused by the very small particles. So as cloud droplets become bigger, the scattering and absorbing properties begin to change. The larger particles become less effective at scattering and better at absorbing.

This is why when any light that just might penetrate to the base of the cloud much of it would be absorbed rather than scattered because the bigger drops are near the base and are better at absorbing causing the cloud base to appear darker yet.

I hope that helped clear it up a bit more.

[Capuchin]

I guess my question was that I don't see the difference between a "cloud droplet" and a rain droplet, surely they're the same but the rain droplet is bigger.

Water is certainly not a very good absorber of light, and I don't see why dust particles should be any more dense than in normal air, where is just scatters according to Rayleigh scattering.

I had always assumed that black clouds were just shadows, because of the thicker clouds and the lack of light scattered from nearby clouds.

I hope you understand I'm just trying to learn and understand it from physics principles... I'm certainly no expert in meteorology

[Capuchin]

Just want to clear up for the OP, It's nothing to do with ice! :) (and ice is translucent anyways)

[rudi_in]

Perhaps the definitions difference between physics and meteorology makes this somewhat confusing.

In meteorology, each of these things has specific definitions.

The typical cloud droplet is about 0.02 mm in diameter while the typical rain drop has a diameter of about 2 mm. However, falling water is not classified as rain until it reaches a minimum of 0.5 mm in diameter. Falling water smaller than that is referred to as drizzle. Various size particles may or may not remain suspended depending on the strength of local updrafts.

Water vapor condenses onto what are called condensation nuclei. These are the dust particles, sea salt, etc. that provide a surface for the water vapor to condense on. The condensation nuclei have a rough diameter of 0.0002 mm. So you can see the relative sizes of these things.

The cloud droplets grow bigger through either collision and coalescence or the Bergeron process.

Rayleigh scattering does refer to scattering of light by comparing the size of the particle vs the wavelength of light.

However, this type of scattering is really only valid for particles that have radius that is much smaller than the wavelength of light. This helps explain blue sky from scattering by the really small air molecules.

For larger particles, it is Mie scattering that is involved. This is when all wavelengths are pretty much scattered equally with the larger cloud droplets (larger than air molecules). This is for the white appearance of clouds.

Rayleigh scattering -- selective -- colors visible -- blue sky

Mie scattering -- equal -- white light visible -- white clouds

Thick clouds -- lack of penetration of light -- most is either scattered or reflected above -- dark appearance.

[Capuchin]

Yes that's exactly how I understand it...

The bit I don't understand is the absorption :)

[rudi_in]

Well, the best way I can think of to say it is just that once the particles become too big, the scattering is less effective and instead is a better absorber than scatterer.

While the density does not necessarily change, since there are more cloud droplets which have coalesced, there are more condensation nuclei which are present. So the frequency of occurrence of a dust particle for example is greater. This could account for greater absorption over scattering.

My apologies for not being able to explain the mechanisms very well.