That would be the clearest day of all time in DC.
I found DC to be 5 miles visibility with low overcast about 250 days a year for the 15 years I flew and lived there and rarely saw high visibility that I see about 300 days a year here in florida.
Well that's what the weather was.
On days like that from a mere 10,000' you can see NYC, Philly, Baltimore and Washington DC in the same view on the west arrival into Philly.
Having flown into DCA for only a year you can see it from 100 miles out when it's that clear. There was no haze on September 11th and every account you'll hear from pilots flying that day was how remarkably crystal clear it was.