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Why Are Clouds White?

Gianna, Age 6, asked:

Gianna’s entry was entitled “A Day in the Sun”

 
Great question, Gianna! To answer it, we have to first talk about waves, like the kind you see in the ocean. Ocean waves look sort of like this:
Scientists call the top of the wave the “peak” of the wave.
The arrows point to the peaks of the wave.

Waves can come in different sizes. The wave on the top is “shorter” than the wave on the bottom, because the distance between two peaks is bigger:

Smaller wave
Bigger wave
Believe it or not, light is a kind of wave, too! Like with ocean waves, we can say whether the light wave is big or small, depending on the distance between two peaks.
Small wave
Big wave
Now, light comes in lots of different colors, right? Well, it also comes in lots of different sizes! Some light waves are big, and some light waves are small. The size of the wave tells us what color it is. The biggest light waves are red, and the smallest light waves are purple. The whole rainbow is in between.
    
The biggest waves we can see are red, and the smallest waves we can see are purple.
In order to see something, a light wave has to bounce off of it and travel to your eye. Right now, you can see this screen because a light wave flows out of the light in your room, bounces off the screen, and enters your eye. You can see your hand because a light wave flows out of the light in your room (or the Sun!), bounces off your hand, and enters your eye. You see different colors depending on what size the light wave is when it enters your eye. If you see red, it’s because the wave is big – if you see blue or purple, it’s because the wave is small.
Light doesn’t have to be a color in the rainbow, though. Sometimes we see light that looks black, or white. What does this mean?
 
Black means that there are no light waves! None. Zero. We see black when there is no light. That’s why your room looks black when you turn out the lights. White means that ALL of the light waves are there! The big ones, the medium ones, and the small ones.  When you add up all the different light waves, you actually get white.
Now that we know this, we can talk about why clouds are white. The light that comes to Earth from the Sun is made up of all the different light waves: red waves, orange waves, yellow waves, green waves, blue waves, purple waves. That means that it’s white! Now, clouds are actually made up of tiny drops of water. When a light wave reaches one of those drops, it bounces off, before it finally reaches your eye. 
 
A red light wave travels from the Sun to your eye, bouncing off a cloud along the way

 

A blue light wave travels from the Sun to your eye, bouncing off a cloud along the way
All of the light waves do the same thing: they all bounce off the cloud, and reach your eye. Remember that in order to see something, a light wave has to bounce off of it and enter your eye. Because all of the light waves bounce off the cloud and reach your eye, you see all the colors (red, orange, yellow, green, blue, and purple) which add up to be white. That’s why clouds look white.
Keep being curious!
-Anna, a student from MIT

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