President Reagan has decided to sell the nation's five weather and land satellites and its planned ocean satellites to private companies, John V. Byrne, administrator of the National Oceanic and Atmospheric Administration, confirmed yesterday.

Byrne said at a news conference that the sale is part of an overall program to "commercialize" some government functions, including parts of the weather services.

He said the satellites are being sold to cut costs and to give a boost to a new American business enterprise. France and Japan are already planning to launch commercial land-sensing satellites.

The jobs of more than 3,500 NOAA employes, or 30 percent of the agency's work force, will be reviewed to see if they should be terminated and their holders given priority when the jobs go to private companies.

Under the proposal, the weather and land satellites, which cost more than $1.5 billion to launch into Earth orbit, would be sold to the highest bidder as a package or separately.

The government would then purchase weather data from the new owner and distribute some of it. Part of it now distributed free of charge would still be free, and some would be sold. General and emergency forecasts would still be distributed at no cost, Byrne said.

Whether or not the satellites are purchased, some "specialized" forecasts--for example, frost warnings and other weather warnings for specialized crops, traditionally available to farmers--will now be sold.

Byrne said there are no reliable figures to show that such a sale would save the government money. He said the government might lose money, at the rate of hundreds of millions of dollars annually, for perhaps 10 years. "But that's only a guess . . . . We don't know," he said.

He said he hoped that, in the long run, the purchaser would make money and no longer require subsidies, but said there are no reliable figures showing that this will be the case.

Byrne was asked whether, since there apparently are no figures to support the government's proposal, "the reason this is going on now is you've been told to do it" by the White House.

"Of course, the reason you're here is that a decision has been made," Byrne told reporters, adding that the decision was made only after a study. He said there is faith in the private sector and its ability to make the proposal work.

Robert Denman of the National Farmers Union described the plan and other such "commercialization" proposals as "shortsighted and misguided." He added: "They are . . . restricting data by charging fees for it, and doing it at a time when many people, like the small farmer, can't afford those fees."

"This is symptomatic of this administration. This becomes a windfall for private weather organizations, the only other people you will be able to turn to when you need information," he said.

Consumer activist Ralph Nader said he wondered what would be charged for hurricane forecasts. "This is a grotesque giveaway. What's next? Obviously they will be selling the FBI" to some private security force, Nader said.

The dollar giveaway, he said, is "not the most serious part of this. It is taking information that everyone has free access to and turning it over to a private monopoly to traffic and profit with it."

Sen. Larry Pressler (R-S.D.) said a study completed at the end of last year by NASA and the Defense Department showed that the proposal to sell the satellites will cost the government $800 million.

The study also concluded that creating a single, government-subsidized monopoly company for weather and land-sensing information might actually inhibit the "free-market process," he said.

NOAA also is reviewing all its other parts, including weather-monitoring stations and computer weather analysis, to decide what else can be turned over to private companies by sale or contract.