The air is smokeless because it's early in the morning and no one's woken up yet. London is still asleep as the speaker passes over Westminster Bridge. He acknowledges to himself,
Dear God! the very houses seem asleep.
All is calm and still in the early morning sunlight. The ships, towers, domes, theaters, and temples, all of them appear to wear the morning like a garment; it's as if they've been completely transformed by the still and the silence of the break of day. Later on, the city will come to life and be shrouded in the smoke of countless chimneys and factories. But for now, everything is bright and glittering in the smokeless air. As dawn breaks, the sun doesn't have to compete with the hanging cloud of smoke that will gradually wreathe London in a thick, dull vapor as the day progresses. For an all too brief, fleeting moment, it can shine undisturbed upon the city beneath, bathing this wondrous scene in a pure, golden light.