AIR MAIL
Where today’s air came from
The air arriving over Alexandria about 5,000 feet above the ground was over the western Atlantic three days ago, and over Quebec a day ago, about 850 miles back along its path. It came down from about 12,900 feet up, warming and drying as it sank. Near the ground the air came from the Maritimes; at 10,000 feet, from Canada. It feels dry all the same. Traced back 72 hours through NOAA’s hourly RAP analyses and the GFS beyond them, nine paths at each of three heights.