I’m guessing that the snow measurement surface is cleaned off periodically and total snowfall summed. How often is that done?
We passed your question to Frank Wachowski, the Midway Airport observer, who has been measuring Chicago’s snow for more than 60 years. New snowfall is measured on a snow board — a 2 foot square piece of plywood painted white to reflect heat. The observer pushes a ruler down to the board’s surface and measures the amount of the snow to the nearest tenth of an inch. The board is cleaned off every six hours with the summation of the six-hourly totals providing the official snowfall total for the day. Snow depth is also measured to the nearest inch by averaging the depth of accumulated snow in as many as 10 sheltered locations where the effects of blowing and drifting are minimized.