Gradient mask
A gradient mask lets an adjustment fade gradually from full effect to none, along a straight line. The classic use is a darker sky: you drag the gradient from the top to the horizon. It simulates the effect of a graduated ND filter, but after the fact.
Drag the gradient across your photo and everything on one side gets full effect, the other side none, with a soft transition between. Perfect for skies, but also for subtly lifting a foreground. In Lightroom you can combine a gradient mask with a color or luminance range, so the gradient hits only the sky and not the buildings sticking up into it. Keep the transition wide, so it stays natural.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.