Southwest makes it easier than other airlines to know when the number of flights to each city it serves goes up or down. Yesterday, the airline announced that effective May 9, it will add 19 flights and eliminate 32 flights, a net decrease of 13. PHL breaks even, with one less flight per day than it will have in March to Orlando and Fort Lauderdale, the kind of normal cuts in Florida service most carriers do in late spring. Southwest is adding one roundtrip a day to Chicago, for a total of eight, and one daily roundtrip to Denver, for a total of three. A chart of all the schedule changes is on Southwest's Web site.