<p>The true reflected colour of a cloud doesn't change from white unless there is serious pollution, such as Saharan dust, wildfire soot or industrial emissions. None of these produce a blue colour, so the question really becomes 'what reflected light looks blue', and that is a function of the direction of view (towards or away from the sun), the subject's perception of colour (which can be surprisingly deceptive), and how much atmospheric blue back-scatter the cloud is reflecting. The latter accounts for most of the 'blue cloud' that we see.</p>

CC BY-SA 3.0
2015-10-24T03:34:43.753Z
2015-10-24T03:34:43.753Z
Gordon Stanger
<p>The cloud is inherently white/gray (it's clear water droplets). It's <em>shadowed</em> from the red light to the west, but <em>exposed</em> to the blue light from the rest of the (apparently mostly clear) sky, so it is mostly illuminated with blue light, and that's what it will reflect.</p>

<p>As another answer observed, there are also perceptual effects. Your eyes aren't colorimeters; put a gray object next to an orange expanse, and it will look bluish. In a photo, the effect may be less pronounced; that's why the photo looks "washed out" to you in comparison to your memory.</p>

CC BY-SA 4.0
2019-04-16T16:08:43.533Z
2019-04-16T16:08:43.533Z
jeffB
<p>The color of a cloud depends primarily upon the color of the light it receives. The Earth's natural source of light is the sun which provides 'white' light. White light combines all of the colors in the 'visible spectrum', which is the range of colors we can see.</p>

<p>Each color in the visible spectrum represents electromagnetic waves of differing lengths. The colors change as the wavelength increases from violet to indigo to blue, green, yellow, orange, red and deep red.</p>

<p>Visible light is only a small portion of the full electromagnetic spectrum.
As a light wave's length increases, its energy decreases. This means the light waves that make up violets, indigo and blue have higher energy levels than the yellow, orange and red.</p>

<p>One way to see the colors of sunlight is by the use of a prism. The velocity of light decreases slightly as it moves into the prism, causing it to bend slightly. This is called refraction. The degree of refraction varies with the energy level each wave.</p>

CC BY-SA 4.0
2019-04-16T13:05:26.160Z
Thanks for responding, but this doesn't really answer the question. Why would this cloud be receiving primarily blue light, when most of the blues from the sun are getting filtered out to the west (leaving the lower frequencies of the sunset colors, as seen reflected by the clouds on the left)?
WBT
CC BY-SA 4.0
2019-04-16T03:44:27.453Z
2019-04-16T03:44:27.453Z
Dylan
<p>I have seen blue clouds, but not in connection with storms. They come about in some circumstances when the Sun is about to rise* but the local weather is overcast.</p>
<p>What happens is this: when the Sun is approaching the horizon, scattered light from the sky filters through the clouds and this filtered light appears dark blue, most notably over water away from city lights. But this indirect filtered light is faint; it will get obscured when the Sun rises and the filtered white light coming directly through the clouds takes over. The clouds gray out.</p>
<p>*I live in a city where a largw body of water is to the East, so the effect is easier for me to see at dawn than at dusk.</p>

CC BY-SA 4.0
2023-07-22T23:34:51.793Z
2023-07-22T23:34:51.793Z
Oscar Lanzi
<p>In a thunderstorm cloud about sunset time, I saw these clouds, including some (in the upper right) that were a unique shade of blue.  I don't think I've seen clouds quite that color before.  I tried to capture the color as best I could with a digital camera, though the blue is slightly washed out in the image.  I took these images facing East at sunset time, during summer. There were rivers (that you could probably throw a rock over if you had a good arm) in the general vicinity, but no large bodies of water such as lakes or oceans.</p>
<p><img src="https://i.sstatic.net/dGsHk.jpg" alt="photo of clouds with blue in upper right" /></p>
<p>Optically zooming in on the blue cloud:
<img src="https://i.sstatic.net/25gBn.jpg" alt="Zooming in on the blue cloud" /></p>
<p><a href="https://physics.stackexchange.com/a/82818/87049">This answer</a> suggests that the sunlight should be missing those wavelengths (&quot;at sunset, when only the red/yellow end of the spectrum hits&quot;) which is consistent with the brilliant warm colors to the left.  But...the cloud was a strong and unique shade of blue, different from the blue behind it.  How does this happen?  That is, what natural processes underly the occurrence?</p>
<p>(Although <a href="https://physics.stackexchange.com/questions/81464/why-do-clouds-appear-black">that question</a> is on <a href="https://physics.stackexchange.com/">Physics.SE</a>, there seem to be more <a href="https://earthscience.stackexchange.com/questions/5068/why-do-clouds-have-areas-of-darkness-instead-of-scattering-light-to-produce-a-ra">questions</a> <a href="https://earthscience.stackexchange.com/questions/4452/what-causes-these-banded-clouds">about</a> <a href="https://earthscience.stackexchange.com/questions/5044/why-are-clouds-darker-before-it-rains">cloud</a> <a href="https://earthscience.stackexchange.com/questions/2264/where-can-we-see-the-noctilucent-clouds">appearances</a> on this site.)</p>

CC BY-SA 3.0
2015-10-24T02:58:24.247Z
@naught101 which also explains why it was so washed out in his photo.
Eubie Drew
CC BY-SA 4.0
2023-07-21T12:36:44.653Z
I’ve had a similar but different blue cloud experience. Was quite an intense blue[This was over Norwich, UK, 19/7/23@ 18: 47 looking west. 1]](https://i.sstatic.net/1hkCG.jpg)
Fiona Tibbitt
CC BY-SA 3.0
2015-08-04T22:50:18.197Z
Might help if you include when/where this happened.
f.thorpe
CC BY-SA 3.0
2015-08-06T02:13:19.103Z
Your eyes will adjust to the colours they're dealing with. It's possible that the blue part appeared as a much stronger blue because of the contrast with the orange in the left of the image.
naught101
<meteorology><atmosphere><clouds>
What causes clouds to appear blue?
WBT
# What causes clouds to appear blue?

(Asked by: username_0 on 2015-08-04)

In a thunderstorm cloud about sunset time, I saw these clouds, including some (in the upper right) that were a unique shade of blue. I don't think I've seen clouds quite that color before. I tried to capture the color as best I could with a digital camera, though the blue is slightly washed out in the image. I took these images facing East at sunset time, during summer. There were rivers (that you could probably throw a rock over if you had a good arm) in the general vicinity, but no large bodies of water such as lakes or oceans.

![photo of clouds with blue in upper right](https://i.sstatic.net/dGsHk.jpg)

Optically zooming in on the blue cloud:
![Zooming in on the blue cloud](https://i.sstatic.net/25gBn.jpg)

[This answer](https://physics.stackexchange.com/a/82818/87049) suggests that the sunlight should be missing those wavelengths ("at sunset, when only the red/yellow end of the spectrum hits") which is consistent with the brilliant warm colors to the left. But...the cloud was a strong and unique shade of blue, different from the blue behind it. How does this happen? That is, what natural processes underly the occurrence?

(Although [that question](https://physics.stackexchange.com/questions/81464/why-do-clouds-appear-black) is on [Physics.SE](https://physics.stackexchange.com/), there seem to be more [questions](https://earthscience.stackexchange.com/questions/5068/why-do-clouds-have-areas-of-darkness-instead-of-scattering-light-to-produce-a-ra) [about](https://earthscience.stackexchange.com/questions/4452/what-causes-these-banded-clouds) [cloud](https://earthscience.stackexchange.com/questions/5044/why-are-clouds-darker-before-it-rains) [appearances](https://earthscience.stackexchange.com/questions/2264/where-can-we-see-the-noctilucent-clouds) on this site.)

**Tags:** meteorology, atmosphere, clouds

> **Comments:**
> **username_3**: Might help if you include when/where this happened.  
> **username_4**: Your eyes will adjust to the colours they're dealing with. It's possible that the blue part appeared as a much stronger blue because of the contrast with the orange in the left of the image.  
> **username_1**: @username_4 which also explains why it was so washed out in his photo.  
> **username_2**: I’ve had a similar but different blue cloud experience. Was quite an intense blue[This was over Norwich, UK, 19/7/23@ 18: 47 looking west. 1]](https://i.sstatic.net/1hkCG.jpg)  

---

## 4 answers

### Answer by username_5 on 2015-10-24. Score: 5

The true reflected colour of a cloud doesn't change from white unless there is serious pollution, such as Saharan dust, wildfire soot or industrial emissions. None of these produce a blue colour, so the question really becomes 'what reflected light looks blue', and that is a function of the direction of view (towards or away from the sun), the subject's perception of colour (which can be surprisingly deceptive), and how much atmospheric blue back-scatter the cloud is reflecting. The latter accounts for most of the 'blue cloud' that we see.


---

### Answer by username_6 on 2019-04-16. Score: 3

The cloud is inherently white/gray (it's clear water droplets). It's *shadowed* from the red light to the west, but *exposed* to the blue light from the rest of the (apparently mostly clear) sky, so it is mostly illuminated with blue light, and that's what it will reflect.

As another answer observed, there are also perceptual effects. Your eyes aren't colorimeters; put a gray object next to an orange expanse, and it will look bluish. In a photo, the effect may be less pronounced; that's why the photo looks "washed out" to you in comparison to your memory.


---

### Answer by username_7 on 2019-04-16. Score: 0

The color of a cloud depends primarily upon the color of the light it receives. The Earth's natural source of light is the sun which provides 'white' light. White light combines all of the colors in the 'visible spectrum', which is the range of colors we can see.

Each color in the visible spectrum represents electromagnetic waves of differing lengths. The colors change as the wavelength increases from violet to indigo to blue, green, yellow, orange, red and deep red.

Visible light is only a small portion of the full electromagnetic spectrum.
As a light wave's length increases, its energy decreases. This means the light waves that make up violets, indigo and blue have higher energy levels than the yellow, orange and red.

One way to see the colors of sunlight is by the use of a prism. The velocity of light decreases slightly as it moves into the prism, causing it to bend slightly. This is called refraction. The degree of refraction varies with the energy level each wave.

> **Comments:**
> **username_0**: Thanks for responding, but this doesn't really answer the question. Why would this cloud be receiving primarily blue light, when most of the blues from the sun are getting filtered out to the west (leaving the lower frequencies of the sunset colors, as seen reflected by the clouds on the left)?  

---

### Answer by username_8 on 2023-07-22. Score: 0

I have seen blue clouds, but not in connection with storms. They come about in some circumstances when the Sun is about to rise\* but the local weather is overcast.

What happens is this: when the Sun is approaching the horizon, scattered light from the sky filters through the clouds and this filtered light appears dark blue, most notably over water away from city lights. But this indirect filtered light is faint; it will get obscured when the Sun rises and the filtered white light coming directly through the clouds takes over. The clouds gray out.

\*I live in a city where a largw body of water is to the East, so the effect is easier for me to see at dawn than at dusk.


---

