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The default result you get when using the Corona Skyand Corona Sunsimulates perfectly clear weather with no clouds or haze.

The scene rendered with the default values of Sky intensity 1 and Sun intensity 1:

You can simulate an overcast sky or other atmospheric phenomena, that make lighting less intensive in real life, using the built-in Corona Sky and Corona Sun controls.

To make the overall environment lighting intensity lower - lower the intensity of your Sky and Sun.

In this case, the intensity of the Sky is lowered to 0.5 and the intensity of the Sun is lowered to 0.2:

To make the Corona Sky less intensive relative to the Corona Sun - lower the Sky intensity and leave the Sun intensity at the default value. In this case, the Sky intensity is lowered to a very low value of 0.05 to demonstrate the effect:

To make the Corona Sun less intensive relative to the Corona Sky, you can also lower the Sun intensity while leaving the Sky intensity at the default value of 1. In this case, the Sun intensity was lowered to a very low value of 0.1 to demonstrate the effect:

Multiple Suns

To add multiple suns to your scene, simply create multiple Corona Sun objects.

It is also possible to add more than one sky to a scene. In this example there are two suns, and each sun is linked to its own separate sky. This way the two skies are each affected by its own sun creating realistic atmospheric effects. 

Regardless if using a single Corona Sky and Corona Sun or multiple ones, it is possible to control their colors and intensities using LightMix. This can be done using the "Open Corona Interactive LightMix Setup" button in the Corona Official Toolbar.

In this example, to demonstrate how LightMix can be used with multiple suns and skies, the colors of the first sun and sky were changed to cool blue, and the colors of the second sun and sky were changed to warm orange:

Starting from Chaos Corona 9, there is an option to add Clouds to the Corona Sky map.

Clouds Parameters

Enable clouds:

Amount – The amount of the main clouds (stratus, cumulus, cumulonimbus). A value equal to zero means there will be no such clouds in the sky, while a value equal to 1 means the sky is full of these clouds.

Variety – Random changes the appearance of main clouds.

Cirrus amount – The amount of cirrus clouds (clouds high in the sky). A value equal to zero means that no cirrus clouds will be visible, while a value equal to one means the maximum amount of cirrus clouds in the sky.

Phase – Determines minor perturbation of the clouds to simulate their change in time.

Clouds position

Altitude – Controls the altitude (measured from the ground) at which the main cloud starts.

Height – Controls the height (thickness) of the main clouds. Increasing this value will lead to heavier clouds while decreasing it will result in lighter (more transparent) clouds.

Offset X – Offset in X axis.

Offset Y – Offset in Y axis.

Contrails

Plane count – Controls how many planes are simulated to create contrails.

Time – Controls the simulation of time - increasing time moves planes along their travel direction. Increasing this property by larger values can be used as a random seeding of the planes positions.

Strenght – Determines how much are contrails visible in the sky.

Distortion – Determines how much are contrails distorted. Value of zero gives contrail lines without any distortions, while value of one results in a maximum amount of distortion.

Offset X – Offsets the contrails horizontally in the sky.

Offset Y – Offsets the contrails vertically in the sky.

Seed – Randomizes the main clouds.

Apply sun color tint – When enabled, the clouds color is tinted by the current sun color. When disabled, the clouds color is based on a realistic sun color. Note that the intensity of the clouds is not influenced by the sun intensity. It is influenced only by the sky intensity parameter.

Density scaling – Available in Corona 12 and newer. Lower values make clouds more transparent, while higher values create darker, thicker clouds with a pronounced silver lining effect. 

Clouds position

Altitude – Controls the altitude (measured from the ground) at which the main cloud starts.

Height – Controls the height (thickness) of the main clouds. Increasing this value will lead to heavier clouds while decreasing it will result in lighter (more transparent) clouds.

Offset X – Offset in X axis.

Offset Y – Offset in Y axis.

Contrails –Toggles contrails (aircraft vapor trails).

Plane count – Controls how many planes are simulated to create contrails.

Time – Controls the simulation of time - increasing time moves planes along their travel direction. Increasing this property by larger values can be used as a random seeding of the planes positions.

Strenght – Determines how much are contrails visible in the sky.

Distortion – Determines how much are contrails distorted. Value of zero gives contrail lines without any distortions, while value of one results in a maximum amount of distortion.

Offset X – Offsets the contrails horizontally in the sky.

Offset Y – Offsets the contrails vertically in the sky.

While it's not physically accurate, because the sun and sky model is accurate by default, it is possible to color-correct the Corona Sky map for artistic direction, for example using the Corona Color Correct map

Once Corona Sky and Corona Sun are added into a scene, the resulting render from a 360 panoramic Corona Camera can be saved as a 32-bit file (for example EXR) and can be then re-used as HDRI environment for other scenes. For correct rendering of a spherical panorama, the proportions of the output image should be set to 2:1 (e.g. 2000 x 1000 pixels).