Dark Clear Blue

Was driving home this afternoon and happened upon a lovely site. The sun shining brightly upon the ground, clear blue skys...but up ahead...a vast dark ominous overcast, upon which I was entering. The clouds formed a near perfect line across the sky, separating the dark clouds from the clear sky. There was a clear and obvious difference once I dove under those clouds, for the sun disappeared, and the temp dropped by at least 5 degrees. The air all around was foggy and misty, and sort of creepy, yet at te same time, calming.