More than anything else, Florida's weather appears to be dictating whether astronauts John W. Young and Robert L. Crippen fly the space shuttle Columbia into earth orbit on time Friday.

If Friday's weather is a repeat of Monday's or today's, Young and Crippen will not rocket away from launch pad 39A at 6:50 a.m. as scheduled. The cloud cover was too thick early Monday and the winds too strong today to have allowed the launch to occur on time on those days.

"What this means is that the people in launch operations would have had to wait and make some real-time decisions about when they could launch," said William H. Schick, shuttle test director at the Kennedy Space Center.

The weather constraints are primarily for landing if Young and Crippen should have to abort their flight and return to Cape Canaveral.

Shuttle mission rules state that tail and cross winds not exceed 10 knots, that cloud coverage be no more than 50 percent, that no rain fall and that visibility be at least eight miles.

Shuttle planners don't want Young and Crippen to fly a crippled Columbia to a landing if tailwinds are so strong they force Columbia to overshoot the 3-mile-long runway at the Kennedy Space Center. Strong cross-winds could also force Columbia off the edge of the runway, while rain might slicken the runway enough to cause Columbia to skid on landing.

The weather at Edwards Air Force Base in California is also important, since that is where Columbia will land Sunday if it takes off Friday. Shuttle planners don't want Young and Crippen taking off in good weather at Cape Canaveral only to find rain squalls flooding the runways at Edwards.

If it's raining in California, alternate landing sites have been selected in New Mexico, Hawaii, Spain, and Okinawa.

One reason the launch is scheduled so early Friday is to give Florida's changeable weather a chance to change for the better. Shuttle planners figure they can wait six hours from the planned launch time to get Columbia into orbit, but no longer.

If they do, there might not be enough daylight for a landing in California 54 hours later. The launch would be delayed for another 48 hours so technicians could restart the countdown.

Although there are some problems, the countdown to launch was proceeding well enough that it was not causing undue concern to flight planners. One problem technicians encountered was that it took them longer than expected to purge Columbia's hydrogen and oxygen fuel cells of contaminating gases such as helium and nitrogen, which could foul the cells during flight.

"What we did was extend our built-in hold time today from eight hours to 14 hours so we could finish purging those fuel cells," Schick said. "That still leaves us 16 hours of hold time to iron out any other wrinkles we see before launch time."

Shuttle planners have given themselves 30 hours of built-in hold time in the five-day-long countdown this week. This is to assure that the launch crews get sufficient rest and to deal with unforeseen problems.

The countdown was due to begin again at 10 p.m. today, when technicians began installing batteries to power the two rocket engines that help propel Columbia into orbit. When that is done, technicians will pressurize the fuel tanks for the 46 engines that help power Columbia and maneuver it while in space.

In the next two days, Columbia's huge external tank will be powered up to receive fuel, and the three fuel cells on board the shuttle will be loaded with the hydrogen and oxygen that will supply Columbia with its electrical power in flight. Each of the three fuel cells routinely generates seven kilowatts of electricity in flight.

The most difficult part of the countdown begins early Friday, when the launch pad is cleared of technicians and 1 1/2 million gallons of super-cold liquid oxygen and hydrogen are pumped into Columbia's external tank. It takes three hours to load the tank, at which point Young and Crippen will be awakened and told that they are about to make space history.