San Francisco -- Reaching 1,070 feet into the air, San Francisco's proposed Transbay Tower would be the tallest building west of the Mississippi.

But if approved and built, it could also do something else, critics say: Its shadow, and the shadow of five other buildings proposed to rise at least 700 feet from an area near the old Transbay Terminal at First and Mission streets, could stretch across Market Street and into the parks of North Beach and Chinatown, blocking the sun and turning the parks - at least at certain times of the day - into shadowy wastelands.

That's hyperbole, city officials say. Most parks, including Union Square, would be covered with a smidgen of shadow and only at certain times of the year, like early in the morning during winter, when the sun is low in the sky and most people aren't lounging there.

During most of year, they say, the shadows will miss most parks and open spaces and blend in with every other downtown shadow.

The plan will create a new, high-density downtown core with 1.3 million square feet of office space and 16,500 square feet of retail space. If approved, the project could bring millions of dollars to San Francisco's economy and support the Transbay Terminal, the city's new transit hub.

In the shadows

A newly released environmental analysis of the proposed development explains in detail what the impacts will be: Almost 90 percent of Portsmouth Square would be covered in shade most winter mornings, a 40 percentage point increase. At certain times, Union Square would be entirely shaded by new and existing high-rises, although only for a few minutes.

"What this does is eliminate a significant amount of public sunlight that the public can never get back," said Bill Maher, a proponent of a 1984 voter-approved initiative to prevent new buildings from shading public parks.

Proposition K requires that any new shadows on city parks be approved by both the Planning and Recreation and Park commissions. The development plans will also have to be approved by the Board of Supervisors.

Let the sunshine in

The reality will be far better than the study, officials said. The environmental report analyzed the absolute worst-case scenarios, a theoretical building shaped like a refrigerator, said John Rahaim, the planning director. The planning code requires that the actual buildings be tapered to allow more light to reach the street, he said.

"It will never be that bad for two reasons: Not all the (proposed) buildings will be built, and every building will have a design," he said.

Gabriel Metcalf, co-chairman of a city committee formed to analyze Prop. K issues, said he didn't think the impacts were too severe, or surprising.

"Clearly there will be people who don't like this plan because it is San Francisco, (and) nothing like this could get done without controversy," said Metcalf, who is also head of the San Francisco Planning and Urban Research Association, a smart-growth think tank.

Most days the parks will be just as they are now, he said. The total amount of new shadow cast on parks over a year - an arcane planning statistic - will increase by only very small amounts, in some cases no more than two-tenths of a percent.

But that increase, or any increase, is still too much for Maher and others. Letting a little shadow encroach onto a park is a slippery slope to a dark, gloomy downtown, he said.

"You can never build a downtown park once you build tall buildings; you can build space, but you can't build a park," Maher said. "I am just not in favor of selling the public sunlight."