AIR MAIL
Where today’s air came from
The air arriving over Grand Forks about 5,000 feet above the ground was over the southern Plains three days ago, and over the northern Plains a day ago, about 960 miles back along its path. The nine paths traced from around the city fan out, so treat that as a best guess. It rose from near the ground on the way. Near the ground the air came from the southern Plains; at 10,000 feet, from Baja California. Traced back 72 hours through NOAA’s hourly RAP analyses, nine paths at each of three heights.