AIR MAIL
Where today’s air came from
The air arriving over Aetna about 5,000 feet above the ground was over the Pacific Northwest three days ago, and over the Great Lakes a day ago, about 1,740 miles back along its path. It came down from about 12,600 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. It feels dry all the same. Traced back 72 hours through NOAA’s hourly RAP analyses, nine paths at each of three heights.