searchlights crisscrossing the sky ; rumbling aircraft motors ; flak tracing red , green , and blue trails ; flares descending slowly in groups , suspended by their parachutes they were called christmas trees , because of the resemblance in shape and brilliance the sparkling of the bursting shells against the black depth of the night , the deep coughing of the anti-aircraft cannons , the noise of a hail of shrapnel as it fell crackling onto roofs and sidewalks , the clacking of machine guns .