Removed from hurricane season and the predictable afternoon showers of summer, December would seem a more reliable time for launching a space shuttle.

But winter has its own pitfalls: low clouds and strong winds.

Strong winds kept space shuttle Discovery grounded on the launch pad Friday, and low clouds scrubbed a launch on Thursday. Each launch scrub costs NASA $500,000.

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NASA was planning a second attempt at 8:47 p.m. EST on Saturday, although there was only a 30 percent chance of acceptable weather because of high winds. Technicians on Friday fueled the shuttle's power cells anyway.

"The problem with the wintertime is that we can get low clouds and we can get strong winds, which we don't normally see in the summer," said Scott Kelly, a meteorologist with the National Weather Service in Melbourne, Fla. "In the summer, it's lightning and, of course, hurricanes. So there's just different weather elements in the winter versus the summer."

NASA won't launch a shuttle with low cloud cover because it prevents the necessary observation of the spacecraft during its ascent, and the commander needs visibility if an emergency landing is required.

The top winds allowed at the launch pad, depending on the direction and range, are between about 20 and 40 miles per hour. Peak gusts in Cape Canaveral around noon Friday were at 35 mph.

The winds are due to a high pressure system building over Florida. The odds for good weather improve to 40 percent on Sunday and Monday. The best opportunity for launching over the next several days is Tuesday, with a 60 percent chance of decent weather.

The space agency can make two launch attempts in a row before standing down a third day because of the need to refresh the fuel supply, spokeswoman Jessica Rye said.

The liftoff of Discovery will be the first night launch in four years.

"We actually like to launch at night," said NASA Administrator Michael Griffin. "In general, the weather is a bit calmer. We're less vulnerable to thunderstorms down here."

NASA had required daylight launches for the three flights after the 2003 Columbia accident to make sure the agency could get good daytime photos of the external fuel tank in case debris fell from it.

Foam breaking off the tank and striking Columbia's wing at liftoff caused the damage that led to the disaster that killed seven astronauts.

But NASA officials were comfortable with the acceptable levels of foam loss during the last two liftoffs and believe radar will spot pieces falling from Discovery's tank.

During the 12-day mission, Discovery's astronauts will rewire the space station, bring up a new 2-ton addition to the space lab and rotate out one of the three station crew members.

NASA Administrator Michael Griffin said he felt no pressure to stick to the launch schedule, despite NASA's desire to go up before Dec. 17 so that Discovery is back on the ground for the new year.

Shuttle computers are not designed to make the change from the 365th day of the old year to the first day of the new year while in flight. The space agency has figured out a solution for the New Year's Day problem, but managers are reluctant to try it.

If Discovery is still grounded by Dec. 18, NASA may decide to keep trying anyway through Dec. 26.

"We've got days and days, and we're not even worrying about the clock problem," Griffin said. "The clock problem is an annoyance, but it's not a real problem in the sense that we know how to deal with it."