The dark clouds approached from the horizon, casting ominous shadows on the ground below. Their ragged, grizzly surface bubbled like a vapor coming out of a simmering kettle, overheated and ready to blow.
The trees froze immobile, stunned, looking in disbelief at this monster that was getting closer slowly and inexorably. They were so terrified that they couldn't even move their leaves.
From a distance came a rumble of thunder, not quite loud yet. Not as loud as when the lightning strikes nearby, and a deafening explosion of thunder makes me blink, and I hear the ring in my ears.
The air is always still, hot, and dense during those short periods of quietness before the thunderstorm. Everything quiets down as if in awe, even insects stop buzzing.
And in the short period of this unnatural serenity, you realize that something is about to happen. Something is approaching. You still see the clear blue sky above but turn your head, and you see that the other half of the sky is covered with an impenetrable and horrifying veil of darkness.
Darkness is moving toward you in silence. And you realize that everything is quiet because every little creature has hidden and trembles in fright.
And there are silent flashes below this dark curtain that is about to swallow the world around. Bluish rapid flashes like the lights from another world