in Thoreau’s Journal:

The white anemone is withering with drought; else would probably have opened. Return while the sun is setting behind thunder-clouds, which now over-shadow us. Between the heavy masses of clouds, mouse-colored, with dark-blue bases, the patches of clear sky are a glorious cobalt blue, as Sophia calls it.

How happens it that the sky never appears so intensely, brightly, memorably blue as when seen between clouds and, it may be, as now in the south at sunset? This, too, is like the blue in snow. For the last two or three days it has taken me all the forenoon to wake up.