AIR MAIL
Where today’s air came from
The air arriving over South Branch about 5,000 feet above the ground was over the Pacific Northwest three days ago, and over the Great Lakes a day ago, about 1,820 miles back along its path. It came down from about 13,800 feet up, warming and drying as it sank. Near the ground the air came from the Great Lakes; at 10,000 feet, from the California coast. Traced back 72 hours through NOAA’s hourly RAP analyses, nine paths at each of three heights.