Free BART, free buses and free ferries are back again for an unprecedented fifth full day Thursday as a smog-prone weather forecast triggered the rejuvenated Spare the Air program.

``It's just so surprising, because 2004 and 2005 were just so cold and we had such clean summers,'' said Luna Salaver, spokeswoman for the Bay Area Air Quality Management District. The district runs the program and yesterday approved $800,000 to help finance three more free-transit days.

The bulk of the funding comes from the Metropolitan Transportation Commission, which isn't scheduled to fully approve its $5.3 million in extra funding until July 26. An MTC committee empowered the organization's executive director to advance money if a Spare the Air Day was declared beforehand, however.

And the city of Alameda, which runs the Harbor Bay-Alameda and Oakland-Alameda ferry services, has agreed to rejoin the program after refusing to offer free rides during this summer's fourth Spare the Air Day on Monday.

``We are very pleased that they are joining us on Day Five,'' Salaver said.

As with the four previous free transit days, ridership on all modes of transit was up Monday, rising 8 percent on BART and nearly swamping ferry operators. The Sausalito-to-San Francisco ferry service saw a fivefold increase in passengers.

The free days are being used up ``a lot faster than anyone anticipated,'' Salaver said. The smog season runs from June 1 through October 13, and already the free transit program has only one day left.

Having so many hot, low-wind, high-pressure forecasts is not unusual, however. The surprise comes from the major free-transit program starting in the first of two mild summers, Salaver explained.

As recently as 2003, there were nine smog-prone days that prompted calls from the air district for people to avoid driving or using chemicals and paint that give off toxic vapors. The record was set in 1996, when there were 25 such days.

``People are more aware of it now because of the free fares,'' Salaver said.