<p>I suspect that the image is due to 'bubbles' of hotter air in the valleys inter dunes. They have a different IOR (Index Of Refraction) then the surrounding areas and the light path deviates from the 'straigth' line trajectory. In addition the light path length between the bottom of the valley and the equi-IOR plane above is larger than the one from the top of the hill to the same plane and the amount of deviation is proportional to the path length.  This atmospheric effect can appear in the absence of wind and in hot days.<br>
The center of the valleys are hotter then the top of the hills because light is reradiated from the surfaces and more surface amount is pointing to the central volume, the same as with parabolic antennas. </p>

CC BY-SA 4.0
2018-06-14T17:42:30.680Z
I'm skeptical that the change in Ri could get big enough to cause that.  In the usual mirage, the change in Ri is responsiple for only a small change in light direction, with the effect that sky appears to be below the horizon, and thus seems to be water.
Sherwood Botsford
CC BY-SA 3.0
2015-02-27T04:21:39.280Z
2015-02-27T04:21:39.280Z
Helder Velez
<p>The 'bluish' areas are exposed gypsum and gravel across which the sand dunes are moving. The colour is likely an artifact of the 'natural' colour palette used when processing the SPOT image. To understand just how arbitrary and variable the results of that processing can be, consider the following image captured at a fairly high zoom level in one of your 'blue' areas:
<img src="https://i.sstatic.net/eNwKw.jpg" alt="enter image description here"></p>

<p>Meanwhile just to the south-west, there is at least one identifiable lake (at 20.896022N ,54.007201E, perhaps an up-welling of ground water, possibly transient and apparently highly saline, judging from the lack of vegetation around it) and, as you can see here....<img src="https://i.sstatic.net/dE80q.jpg" alt="enter image description here"> it has an entirely different appearance to the blueish areas you've identified.</p>

CC BY-SA 3.0
2017-09-10T20:52:00.557Z
Change in sun and camera position could change surface appearance - somehow like as it happens in water.
Pere
CC BY-SA 3.0
2015-02-04T19:27:57.700Z
Thank you for the response.  Do you have any comments on the changes we see in the Landsat images I posted?
Glacialis
CC BY-SA 3.0
2015-02-04T09:19:59.187Z
2015-02-04T09:19:59.187Z
Mick Wilson
<p>I found some blue patches in the Arabian desert in Google Earth.  It looks like as if they were lakes with sand dunes rising out of the water.  But there isn't water in the desert, at least not this much, or is there?  So what do we see on the image?  I realize that the colour we see in the image may not be accurate.</p>

<p><a href="https://i.sstatic.net/dzjN2.jpg" rel="noreferrer"><img src="https://i.sstatic.net/dzjN2.jpg" alt="enter image description here"></a></p>

<ul>
<li><a href="https://www.google.com/maps/@20.641166,54.3972003,23537m/data=!3m1!1e3" rel="noreferrer">Link to Google Maps</a></li>
</ul>

<hr>

<p><strong>In response to Fred's comment about whether it might show water after a rain</strong></p>

<p>I took some time to browse <a href="http://landsatlook.usgs.gov/" rel="noreferrer">Landsat archives</a> and see if things are changing.  Here's a comparison of the same region on two different days (note the different dates, about 1.5 months apart).  The comparison shows that things are changing, i.e. it doesn't permanently look like this.  Most of the time it's like the first image.  This is not the exact same area as above, but it's "nearby".</p>

<p><img src="https://i.sstatic.net/TJ35K.png" alt="enter image description here"></p>

<p><img src="https://i.sstatic.net/EjJCJ.jpg" alt="enter image description here"></p>

<p><strong>Are there satellite images freely available taken at wavelengths that can clearly show water?</strong></p>

<hr>

<p><strong>Update 2</strong>  Here's some <a href="http://1.usa.gov/1DvKjT4" rel="noreferrer">daily MODIS images</a> from the same time as the two photos above, I'm not really sure what to make of them though.  See the clouds passing through 2013 Aug 3-4.  The precipitation layers in the same tool do confirm rain during those two days.</p>

<hr>

<p><strong>Update 3</strong>  <a href="http://www.nasa.gov/multimedia/imagegallery/image_feature_1200.html" rel="noreferrer">Yet another image showing more clearly what looks like lakes.</a>  Click for description and full-size image showing larger area.  This is the eastern part of <em>Rub' al Khali</em>.</p>

<p><a href="http://www.nasa.gov/multimedia/imagegallery/image_feature_1200.html" rel="noreferrer"><img src="https://i.sstatic.net/WWvjg.jpg" alt="enter image description here"></a></p>

<hr>

<p><strong>Update 4</strong>  These are likely old and dried up lake beds.  <a href="https://en.wikipedia.org/wiki/Rub%27_al_Khali#Lake_beds" rel="noreferrer">Mention on Wikipedia.</a>  Quote from <a href="https://www.saudiaramcoworld.com/issue/198903/lakes.of.the.rub.al-khali.htm" rel="noreferrer">article</a>:</p>

<blockquote>
  <p>All that remains of the lakes are their eroded basins, standing up in forlorn little buttes a meter or so high, <strong>in stark white or gray against the Rub' al-Khali's infinity of rusty-red sand</strong>. </p>
</blockquote>

<p>It doesn't explain the bluish colour, but it's likely these dried up lake beds that I see on the images.</p>

<p><a href="http://www.sidetracked.com/the-empty-quarter/" rel="noreferrer">Some ground-level photos here.</a></p>

CC BY-SA 3.0
2015-02-04T01:30:24.863Z
It is not clear to me whether "what's this thing I saw in Google Earth" types of questions are on topic here, so please let me know.
Glacialis
CC BY-SA 3.0
2015-02-04T02:10:03.410Z
It would be interesting if the photo was taken shortly after rain. I know you can't answer that. The white patches in the bottom right remind me of salt that I've seen in salt lakes regions and when salt scars develop after land has been deforested for agricultural purposes in semi-arid locations.
Fred
CC BY-SA 3.0
2015-02-04T02:38:42.717Z
@Fred See my update.
Glacialis
CC BY-SA 3.0
2015-02-04T05:08:05.837Z
I think it's clearly on-topic, assuming it's a natural phenomenon. Even if it's not, it's something to do with remote sensing systems, which are also on-topic. The fact that the shadows are quite strong and on the north west indicates that the photos were taken on a winter morning. Might be frost, or mist that hasn't burnt off?
naught101
<geomorphology><desert><satellite-oddities>
What are these lake-like blue patches in the desert, visible in satellite image?
Glacialis
# What are these lake-like blue patches in the desert, visible in satellite image?

(Asked by: username_0 on 2015-02-04)

I found some blue patches in the Arabian desert in Google Earth. It looks like as if they were lakes with sand dunes rising out of the water. But there isn't water in the desert, at least not this much, or is there? So what do we see on the image? I realize that the colour we see in the image may not be accurate.

[![enter image description here](https://i.sstatic.net/dzjN2.jpg)](https://i.sstatic.net/dzjN2.jpg)

* [Link to Google Maps](https://www.google.com/maps/@20.641166,54.3972003,23537m/data=!3m1!1e3)

---

**In response to username_1's comment about whether it might show water after a rain**

I took some time to browse [Landsat archives](http://landsatlook.usgs.gov/) and see if things are changing. Here's a comparison of the same region on two different days (note the different dates, about 1.5 months apart). The comparison shows that things are changing, i.e. it doesn't permanently look like this. Most of the time it's like the first image. This is not the exact same area as above, but it's "nearby".

![enter image description here](https://i.sstatic.net/TJ35K.png)

![enter image description here](https://i.sstatic.net/EjJCJ.jpg)

**Are there satellite images freely available taken at wavelengths that can clearly show water?**

---

**Update 2** Here's some [daily MODIS images](http://1.usa.gov/1DvKjT4) from the same time as the two photos above, I'm not really sure what to make of them though. See the clouds passing through 2013 Aug 3-4. The precipitation layers in the same tool do confirm rain during those two days.

---

**Update 3** [Yet another image showing more clearly what looks like lakes.](http://www.nasa.gov/multimedia/imagegallery/image_feature_1200.html) Click for description and full-size image showing larger area. This is the eastern part of *Rub' al Khali*.

[![enter image description here](https://i.sstatic.net/WWvjg.jpg)](http://www.nasa.gov/multimedia/imagegallery/image_feature_1200.html)

---

**Update 4** These are likely old and dried up lake beds. [Mention on Wikipedia.](https://en.wikipedia.org/wiki/Rub%27_al_Khali#Lake_beds) Quote from [article](https://www.saudiaramcoworld.com/issue/198903/lakes.of.the.rub.al-khali.htm):

> All that remains of the lakes are their eroded basins, standing up in forlorn little buttes a meter or so high, **in stark white or gray against the Rub' al-Khali's infinity of rusty-red sand**.

It doesn't explain the bluish colour, but it's likely these dried up lake beds that I see on the images.

[Some ground-level photos here.](http://www.sidetracked.com/the-empty-quarter/)

**Tags:** geomorphology, desert, satellite-oddities

> **Comments:**
> **username_0**: It is not clear to me whether "what's this thing I saw in Google Earth" types of questions are on topic here, so please let me know.  
> **username_1**: It would be interesting if the photo was taken shortly after rain. I know you can't answer that. The white patches in the bottom right remind me of salt that I've seen in salt lakes regions and when salt scars develop after land has been deforested for agricultural purposes in semi-arid locations.  
> **username_0**: @username_1 See my update.  
> **username_2**: I think it's clearly on-topic, assuming it's a natural phenomenon. Even if it's not, it's something to do with remote sensing systems, which are also on-topic. The fact that the shadows are quite strong and on the north west indicates that the photos were taken on a winter morning. Might be frost, or mist that hasn't burnt off?  

---

## 2 answers

### Answer by username_5 on 2015-02-04. Score: 18 | Accepted answer

The 'bluish' areas are exposed gypsum and gravel across which the sand dunes are moving. The colour is likely an artifact of the 'natural' colour palette used when processing the SPOT image. To understand just how arbitrary and variable the results of that processing can be, consider the following image captured at a fairly high zoom level in one of your 'blue' areas:
![enter image description here](https://i.sstatic.net/eNwKw.jpg)

Meanwhile just to the south-west, there is at least one identifiable lake (at 20.896022N ,54.007201E, perhaps an up-welling of ground water, possibly transient and apparently highly saline, judging from the lack of vegetation around it) and, as you can see here....![enter image description here](https://i.sstatic.net/dE80q.jpg) it has an entirely different appearance to the blueish areas you've identified.

> **Comments:**
> **username_0**: Thank you for the response.  Do you have any comments on the changes we see in the Landsat images I posted?  
> **username_6**: Change in sun and camera position could change surface appearance - somehow like as it happens in water.  

---

### Answer by username_3 on 2015-02-27. Score: 3

I suspect that the image is due to 'bubbles' of hotter air in the valleys inter dunes. They have a different IOR (Index Of Refraction) then the surrounding areas and the light path deviates from the 'straigth' line trajectory. In addition the light path length between the bottom of the valley and the equi-IOR plane above is larger than the one from the top of the hill to the same plane and the amount of deviation is proportional to the path length. This atmospheric effect can appear in the absence of wind and in hot days.  
The center of the valleys are hotter then the top of the hills because light is reradiated from the surfaces and more surface amount is pointing to the central volume, the same as with parabolic antennas.

> **Comments:**
> **username_4**: I'm skeptical that the change in Ri could get big enough to cause that.  In the usual mirage, the change in Ri is responsiple for only a small change in light direction, with the effect that sky appears to be below the horizon, and thus seems to be water.  

---

