Now, just the sky is selected.



With just the sky selected, I'm going to run Neat Image to reduce the noise in the sky only. I can access Neat Image by going to the "Filter" menu, sliding down to "Neat Image" and then over to "Reduce Noise".



I'll click on "Auto Profile" near the top to automatically create a noise profile. Then I'll click on the "Noise Filter Settings" tab.



I'll select the "Remove only half of noise" preset and click on the "Apply" button near the top.



The sky is now noise free and the rest of the image is left untouched.