From the time man first poked his head outside the cave and found the elements forbidding, he's been wondering why he can't figure out the weather.

For all the refinement and sophistication that meteorology has undergone since then, it remains an uncertain science. You know: They call for rain and it doesn't.

But now, instead of talking about it, the world's meteorologists are going to do something about it, through what is being billed as history's largest international scientific experiment.

The United States and 146 other nations next month will go full-scale into a year-long global weather-data collection experiment to try to find out why it doesn't rain when they think it will. And vice-versa.

Appropriately enough, it's being called the Global Weather Experiment, sponsored by the World Meteorological Organization and the International Council of Scientific Unions.

Through years of planning and meetings, the participating countries have agreed to devote an unprecedented array of scientific gadgetry to a data-collection effort from pole to pole and all points between.

Officials of 10 U.S. government agencies taking part in the effort explained at a briefing yesterday that they hope their findings will allow meteorologists to develop more accurate ways of predicting the weather.

The scientific arsenal includes stationary weather satellites, orbiting polar satellites, special airplanes to take wind test, research ballons in the atmosphere, ocean buoys throughout the Southern Hemisphere, 50 wind-testing ships, plus scientific gear on commercial shipss and aircraft.

The data collected by all of that instrumentation will be fed into computers, then assessed and used by weathermen around the world to try to improve their forecasting techniques.

Beyond knowing when your picnic might be rained out, there are some very basic and important reasons for having a better fix on the weather.

Richard A. Frank, administrator of the National Oceanic and Atmospheric Administration, noted that agriculture, construction, transportation, as well as utility companies and government planners, lean heavily on accurate weather forecasting.

"There is no doubt that longer range weather forecasts would bring vast benefits to society-not only in the United States but throughout the world," Frank said.

Dr. Verner Soumi of the University of Wisconsin, head of the U.S. committee in the experiment, said it would be "a mistake" to expect better forecasting instantly after the experiment.

But he said, the idea is that the researchers will give the earth's atmosphere "its first complete physical examination"-a look that should produce new insights into the global wind and sea currents that affect weather.

And as Soumi explained it, the price- $300 million for the scientific mobilization-is not at all unreasonable. The U.S. share of $100 million, he said, comes to about "the price of a Big Mac and an order of fries for every other American."