The office parks off Winter Street are emptiest at three in the morning, which is exactly when the plow lights start sweeping across them. Cleanway works those hours through a Waltham storm — plowing lots and drives, hand-clearing walkways and entries, spreading salt so the first cars in never learn how much snow there was. Storms rarely arrive politely; they stall, slide over to sleet, come back at dawn, and the route gets run again. The measure of the night is simple: lots down to the asphalt, paths open, nobody stuck. Snow work for Waltham and nearby Weston, Watertown, Newton and Lexington.