We do not see the world in grey scale, but we see world in color. How things look as they are and how we interpret their appearances is a complex mechanism that needs a lot of context from physics and biology. I will not talk much about the color of light, because it would be too long of a tutorial and I won’t be able to meet the deadline (I didn't meet anyway :(, because I am kazakh). The main goal of this tutorial is to study how light travels and how it impacts the scene under different weather conditions. I will have mainly examples of light travel in grey scale, since we will only emphasize the general properties of light.

Light

In order to have vision, we need to have light. The images we perceive are our visual system’s way of processing the rays of light that enter our eyes and strike our retinas. Picture 1 shows how a few rays of light might travel through our eyes, resulting in the perceived image of a flower. Of course, the rays of light do not originate at the flower. Rather, they must come from some source of light. In any scene that is not pitch black, there must be a source of light, whether it be the sun, a lamp, or the display that you’re reading from now.

 

[Picture 1. looking at a flower (A.) light reflected off of it (B.) enters our eyes (C.).

Picture 2. set up with lamp and geometric objects]

 

 

 

Let’s consider the following scene: a room containing a few objects, with a lamp as the only source of light. This is shown in Picture 2.

 

Everything that we can see will be lit up by the lamp. In the Picture 3 below, we light up only the parts that face the lamp directly. This picture is missing an important physical component. Light will be reflected off of all the surfaces that it hits, making the objects in the scene a bit luminous. As illustrated in the Picture 4 below, the light will be reflected somewhat randomly, with the amount of reflection and angles of scattering depending on the properties of the surfaces.

 

[Picture 3. The set up lighted up only in areas where light can reach.

Picture 4. In the current set up light will bounce off in complicated way.]

 

 

Unfortunately, calculating how exactly light will be reflected throughout a whole scene is a complicated task. A lamp sends out many photons, and their trajectories will depend on many specific properties of the objects in the scene. Rendering software take a long time to render light well for this reason.

 

[Picture 5. Oversimplified way looking at how light is bouncing around.

Picture 6. Using oversimplified perception to redraw shadows to account for reflected light off the surfaces]

 

 

Let's oversimplify this complicated process so our minds can comprehend it. Bigger, brighter or more reflective surfaces contribute more light to their surroundings, acting in a way as light sources themselves. In our example we have drawn ridiculously reflective white objects, so they all reflect light equally. However, larger in area bright spots will have more impact on their surroundings than will smaller spots. As shown in Picture 5, the wall on the left has a big surface and receives a lot of direct light from the lamp. Light reflected off of it will bounce onto the dark parts of the cubes and the sphere, as well as into the shadows on the floor cast by these objects. In general, you can think about the following questions when trying to draw light in a complex scene:

 

*Where is the source of light?

*Which surfaces does the light source illuminate directly?

*How do these surfaces reflect light?

*Where will this reflected light reach?

 

As the result of briefly thinking like that, I got Picture 6. There is probably no limit in how long one could spend thinking about calculating directions of light, as the real world is not limited by the resolutions of our screens or the size of our brushes.

 

Now let’s move the right and left walls closer to the objects, as shown in Picture 7. This will result in more light getting onto the dark sides of sphere and cubes since the light does not have to travel as far from the walls. Also, less light will reach into the cast shadows, when comparing to Picture 8, since the objects are now closer to left wall and there is less room for light from the right wall or from the floor to bounce into that shadow.

[Picture 7: Moved left and right walls closer to objects.

Picture 8: Same as picture 6 to compare with picture 7.]

 

 

 

Everything we see is light except for pitch black. Every non-black object you observe reflects light, and this light reaches your eyes, basically making the object luminous and having some impact on the surroundings. Now, let’s see how we can apply this principle to light in different weather conditions.

 

Sunny Day

On a sunny day, we will have no clouds, and a bright Sun (1.), which is the main source of light. Let's consider a situation where we have a ball (2.) in an open space on a sunny day at noon. The sun will light up the scene very strongly.

[Picture 9: Set up of a sunny day.

Picture 10: Sun shining in all directions]

 

 

I mean very, very strongly. You can feel the sun on your skin, you can feel how much energy it emits, and how much warmer it makes our world. Sometimes, probably a bit uncomfortably warm.

 

Such direct light will create a fairly sharp shadow (3.) of the ball. It turns out though, the shadow will not be absolutely pitch black outdoors, even there is nothing behind a ball that will cause light to bounce back like in the scene that we looked at before.

[Picture 11: sun strongly shines and screams.

Picture 12: sunlight create cast shadow]

 

 

Before reaching us, sunlight travels quite a distance through space and through our atmosphere. In the atmosphere, scattering happens, and mostly warm colored light will reach us directly. Cool colored light will be more likely to be scattered by air (this scattering is called Rayleigh scattering) . As the result, the sky is blue and very luminous. The light emitted from the sky is quite uniform and it will lightly light up shadows in a sunny outdoor environment.

[Picture 13: Sun light traveling through space into atmosphere (4.). As light passes through a lot of air molecules in the atmosphere, cool light has a chance to scatter (Rayeigh scattering) (5.), while warm light barely scatters and reaches the ground (6.) in a more direct path.

Picture 14: After traveling a long distance through the atmosphere, a lot of cool light will have scattered and eventually reached ground uniformly (7.), lighting up the ground with sky blue light. Some of this cool light will light up the shadow of the ball.]

 

 

On a sunny day, strong sunlight will be reflected off the surfaces of objects we observe. This will cause a scene to look very bright, since there is a lot of light to reflect. Picture 15 is an example. Here we have a light, matte textured ball outdoors in open space on a sunny day. As we can see, there is a lot of light and the shadows are not pitch black, since there is light from sky to light them up. Closer to the ground on the ball you can observe a light spot, since some light from ground can reach there as well. In Picture 16 we have added a wall behind the ball. Now some sunlight will bounce off the wall and light up the cast shadow and the back of the ball.

[Picture 15: Light matte ball on a sunny day in open space.

Picture 16: Same ball but with a wall behind.]

 

 

Let’s introduce more objects to our scene and observe how sun will light them up. In picture 17, we have a dark matte ball, the same ball as from previous example, and a shiny ball. The dark ball absorbs a lot of light but not all of it. It still reflects some light from ground, but less than the ball in a middle, since the dark ball absorbs more light than the middle ball. I assumed that the shiny ball is located in such a way that it reflects direct light from sun, resulting in a halo around a white spot of reflection. This is caused by light dispersion and scattering in your eye, which happens if you stare at something really bright.

 

[Picture 17: Balls with different colors and material proportion on a sunny day in open space.

Picture 18: Same balls but with a wall behind.]

 

 

So the takeaway is that on a sunny day, there is a lot of light, first of all from the sun, second from the sky itself, and lastly from all nearby objects that reflect light and impact how the scene will appear.

 

Cloudy day

When it is cloudy, before sun light can reach us, it goes through a cloud. Clouds consist of many water tiny droplets that are big enough to cause a lot of scattering (Mie scattering). Due to this scattering, some sunlight will be reflected back into space and some will get deeper into the cloud. As it travels through the cloud, more scattering happens, so on the way out you will get fairly uniform light. This basically makes clouds the main source of light on a cloudy day, and when it is very cloudy, they will appear grayish.

 

[Picture 19: Light from the sun gets scattered by water droplets (7.) of a cloud. As a result light emitted from the clouds is uniform.

Picture 20: Sun emits a lot of light, and uniform light coming out of clouds will be weaker. ]

 

 

Consider the previous setup with balls, as shown in picture 21. We can see that edges are very distinct and light in the environment is uniform and weak. Under each ball, we have a dark shadow, because light barely reaches there. Adding a wall behind, as in picture 22, does not change how light appears that much, but it does slightly lighten the shadows of the balls and the sides of balls that face that wall.

[Picture 21: same three balls on a cloudy day

Picture 22: same three balls but with a wall behind them]

 

 

Light on a cloudy day is a lot weaker than direct sunlight. Things on the ground will not be as shiny, shadows will be softer, and things on the ground will appear as if they were more uniformly lit. Edges of objects will still appear sharp, as your senses can read image information more clearly.

[Gif 1: comparing sunny day and cloudy]

 

Foggy/smoky day

So far, we’ve only talked about when the medium between the viewer and the objects in the scene is just air. Air causes scattering of light in the atmosphere, but only at a very long distance can you see impact of such scattering. So very far objects, like those close to the horizon, might appear bluish. That’s quite far. But when we have larger(but still very small) particles in air, like the particles that make up fog, smoke, or pollution, visibility changes. You will probably not see horizon at all in such conditions. Such bigger particles in the air scatter light very evenly indifferent to its color, resulting in uniform light being given to the surroundings.

 

As shown in Picture 23, when we have such big particles in air, likelihood that light from an object will be scattered by it is higher. If light needs to pass through many layers of such particles, most of the light will be scattered. The further you are from an object, the less light you will observe from it.

 

In picture 24 we see a representation of this. Fog will make a white background, because it scatters light evenly and doesn’t absorb any light. So, the lightness of the fog in background will depend on how much light is in the area. In this picture, we only consider light on a cloudy day, so there is not a lot of light and the fog is lit uniformly. As you can see, light from the far ball is scattered heavily and that ball has blurry edges. Meanwhile, closer balls have more and more distinct edges.

 

[Picture 23: Cloudy day, Fog (8.). Observing 3 balls at various distance from observer. Light from red ball barely reaches to the eye.

Picture 24: same three balls as in picture 21, under foggy weather condition.]

 

 

Comparing to fog, smoke does absorb light, resulting in overall less light in the surroundings. More dense smoke will make what you see darker. It could even be dense enough that you see no light at all.

 

[Picture 25: same 3 balls in smoky weather

Picture 26: same 3 balls in fog, for comparison]

 

 

The way your visibility is impacted is all about how far are objects from your eyes. The further the object is from your eyes, the more fog it needs to go through before it can reach your eyes. This is shown in Gif 2, as we increase the density of fog.

 

[Gif 2: increasing density of fog ]

 

[Picture 27: Something bright shining through fog, light will be heavily scattered around the source of light]

 

 

When there are big particles in air, they will impact visibility. Edges of far objects will appear blurred into environment. Objects will disappear at a closer distance than they would on a clear day. Light will be scattered around, making uniform lighting. Transparent particles, like water, in the air won’t absorb much light, but solids, such as in smoke particles, will absorb light and cause less light to be available in the scene.

I hope now you can think more about light in different weather conditions!

Everything was drawn and animated in Clip Studio Paint.