# Really cool clouds this evening - what causes this?

**URL:** <https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714>\
**Category:** Factual Questions\
**Created:** [November 18, 2018, 12:27am UTC](https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714 "2018-11-18T00:27:23Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [November 18, 2018, 12:27am UTC](https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714/1 "2018-11-18T00:27:23Z")

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Almost the entire sky is filled with very uniform cloudlets. What causes this phenomenon?

Photo:  
[![CloudApp](https://f.v1.n0.cdn.getcloudapp.com/items/380j2o0p21392P463X3S/DSC_5090.jpg "DSC\_5090") ](https://cl.ly/E0u8Nb94)

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**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\
**Post date:** [November 18, 2018, 12:47am UTC](https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714/2 "2018-11-18T00:47:25Z")

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Those look like they might be mammatus clouds - though an unusually extensive display of them.

Mammatus clouds are generated when cold air descends into warmer air.

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**Author:** ![nearwildheaven](https://avatars.discourse-cdn.com/v4/letter/n/90db22/32.png) [@nearwildheaven](https://boards.straightdope.com/u/nearwildheaven)\
**Post date:** [November 18, 2018, 5:29am UTC](https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714/3 "2018-11-18T05:29:45Z")

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I don’t think those are mammatus; those are very round and project from the bottom of a cloud, often one associated with severe thunderstorms or even tornadoes. The ones the OP pictured look like something we see all the time in the upper Midwest, where one can safely assume s/he is not because of the palm trees.

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**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\
**Post date:** [November 18, 2018, 5:36am UTC](https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714/4 "2018-11-18T05:36:02Z")

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Ho high were they? It looks like Altocumulus floccus to me.

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**Author:** ![Twoflower](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/twoflower/32/22_2.png) [@Twoflower](https://boards.straightdope.com/u/Twoflower)\
**Post date:** [November 18, 2018, 5:54am UTC](https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714/5 "2018-11-18T05:54:27Z")

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[Mackerel sky](https://en.wikipedia.org/wiki/Mackerel_sky)

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [November 18, 2018, 6:04pm UTC](https://boards.straightdope.com/t/really-cool-clouds-this-evening-what-causes-this/824714/6 "2018-11-18T18:04:46Z")

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> [@Twoflower](#):
>
> [Mackerel sky](https://en.wikipedia.org/wiki/Mackerel_sky)

Yes, that’s it.  
Pretty unusual out here…