Painting lessons: water colours

Night is getting ready for another painting on the sky. Moonlight brush is here, transparent clouds are used as canvas, so it's all ready for the start...
Hmmm... I want to draw a circle first. Fine, bright as moonlight itself, big as two or three Moons added aside. The Moon is almost full, and I'll take a layer of Altocumulus, subtype translucidus. I'll cover a part of the sky with these clouds and I'll dip the moon brush into a paint... Wow, the desired colour easily spreads, just as I imagined! Water colours are so wonderful when they spread all over...
But look! These beautiful, bright, nearly blueish moonlight colour gently turns into yellowish, while at the edges of the circle it is nearly red. Outside the circle, red colour gradually turns into the darkness of the night beyond the clouds.
The whole image reminds me on a saucer with a cup of coffee... Break time!
Just to give some nice name to this picture... Hmmm.. Oh yes! Let it be corona.

What is, actually, happening here?

See what I painted! An image for television!

The bright circle on the clouds around the Moon position in the sky is a result of dispersion of the moonlight rays. When moonlight rays reach a transparent cloud layer, they disperse, every ray on a tiny water droplet. That's why these colours are the water colours. This is a reliable sign of recognizing cloud layer consisted of water droplets. Even when it's cold, with temperatures even below freezing point, water droplets might remain liquid while they are so tiny. This liquid state of water droplets on temperatures below the freezing point is called supercooled water. Such droplets may turn into ice crystals only in contact with some solid particles, such as dust particles. Since larger particles usually float on lower altitudes (as heavier), the state of supercooled water may remain hor tens of hours. Turning the liquid water phase into the ice is still possible if the temperature drops, unless there are no particles that will turn them into the ice first. However, very low temperatures are required for this change of phase, even -40 degrees, which do not occur at mid-level could altitudes. Such temperatures rule at higher altitudes, regularly at high-level clouds altitudes.

Any colour can be derived from white light

The transition to a red color is the result of scattering of light rays under a slightly greater angle. As in the famous picture with prism decomposes light in rainbow colors, first appears blue, and yellow coming over to the red color which exceeds the end. Due to the different wavelengths of individual components of light that we call colors, there is a different scattering of light rays. The least colours that scatter are violet and blue, while red scatter the most. That is the exact order of changing colours.

Why does Moon corona sometimes have an irregular shape?

This image is somewhat egg-shaped... (photo Claudia Hinz)

Partially opaque clouds prevent the penetration of moonlight rays, and in those places corona looks partly damaged. If the clouds are only semi-transparent, irregularity of corona shape is caused by intercloud gap that is a part of Altocumulus layer. If there is no such gap in the cloud layer, it would no longer be Altocumulus, but Altostratus . In any case, the corona can be observed over the middle, and less often over low-altitude clouds (mainly Stratocumulus translucidus and Stratus nebulosus in the stage of creation).
The regularity of the corona form depends on the uniformity of the size of the water droplets that the elements of cloud are consisted of. If all the droplets are of similar size, the corona is regular and clear. If there are differences in the size of supercooled water droplets in a cloud, corona deforms and blurs with increasing difference in the dimensions of cloud droplets. It is sufficient to have 10% of the droplets different from the majority, and this image of corona is no longer beautiful.

Very rarely, it can happen that the corona has an irregular, almost oval shape, even if at first glance there is no cloud to cause the irregularity. However, droplets can exist and not to be seen. In such case, they are just not in a sufficient amount to show up as a cloud.

Can  corona be made of some other 'material'?

Of course it does. The condition is that the material must cause the diffraction of light. The material may even be made of small solid particles such as dust, pollen, algae (if the corona is reflected in the water), and even volcanic ash. In the latter case, the corona gets a special name - Bishop's ring. In these rare cases, the water droplets would only be an obstacle.

A source of light can also be replaced, using Sun instead of the Moon. But the Sun was is an awkward painter, pressing the brush so hard, while the glare of the Sun makes the corona very difficult to be seen.

Very rarely, some very bright stars (and the brightest planets such as Venus and Jupiter) can add  some tiny coronae, but these images are a privilege of people with excellent vision. Eyes with very little vision problems that do not require the use of glasses are no longer able to perceive such a small corona. On the other hand, due to the light pollution, such coronae are no longer visible from the city, but only from those places where the view of the sky like in the ancient times of before civilization. 
This class painting may be over, but we'll only to expect more artistic feats in the sky. So, wait patiently, the time to turn on a new technique of painting on sky canvas will come.