AIR MAIL
Where today’s air came from
The air arriving over South San Francisco about 5,000 feet above the ground was over the Pacific off California three days ago, and over the California coast a day ago, about 970 miles back along its path. The nine paths traced from around the city fan out, so treat that as a best guess. It came down from about 11,100 feet up, warming and drying as it sank. Near the ground the air came from the California coast; at 10,000 feet, from the open Pacific. Traced back 72 hours through NOAA’s hourly RAP analyses and the GFS beyond them, nine paths at each of three heights.