The traveler from outer space who sights the distant earth for the first time knows at once he has come upon something unusual. In his long passage across the solar system, beginning at Neptune, nearly three billion miles out, he has seen nothing like this. The other planets looked cold and lifeless, many shrouded in wispy atmospheres or swirling dust storms.

Swinging closer as he passed, he found them pitted with craters. They were creased with long, tortuous trenches and ridged with saw tooth mountain ranges.

In short, crossing the solar system, the space traveler found the world of the sun to be a desert, a corner of the universe where neither plant nor animal could live.

But what he now sees coming up before him in the distance is clearly something different. This planet is green. It looks like an oasis in the solar wasteland. As his ship streaks toward the burgeoning earth, he begins to make out details. He sees the glimmer of water, the ice caps at the poles, glittering in the sunlight; the green masses of the continents breaking up, the blue of the seas-all of it coming and going in endlessly shifting patterns behind drifting white clouds.

Indeed, this is a garden in the solar desert.

As he draws closer, the visitor had better slow up, for there's a fence around the garden-in the form of a very unusual atmosphere. If he comes in too fast, his ship will catch fire from the heat of friction.

He will go the way of the meteorites, ranging from specks the size of a grain of sand to monsters weighing tons, which streak in from space by the millions every hour, only to vanish in long scars of eerie white flame against the dark cosmic void This shield is as effective as steel armor plating fifty inches thick. But for it, the visitor would find the earth as battered a; Mercury and Mars.

The remarkable envelope which keeps the earth from being stoned to death is held in place by gravity. It presses down with the weight of nearly fifteen pounds to every square inch of the earth's 197 million square miles of surface-six quadrillion tons all told.

Each of us carries nearly ten tons on his head and shoulders We don't feel it because the weight is equalized throughout our bodies, in all directions, the same as the pressures of the deep sea on the creatures that live there.

In fact, we are bottom dwellers ourselves and, as with the deep-sea fish, we soon get into trouble as we rise above the floor of our atmospheric ocean. Nearly all land life - man, animals, the birds, bees, and plants-is confined to the narrow space between the level of the sea and a mile up.

Even before we get to the top of this space, only a little more than 5,000 feet up, our breath is already shorter. A 10,000 feet we are breathing like a runner, and at 20,000 feet where half the atmosphere now lies below, we need consider able time to get used to the altitude-if we are able to do so at all.

Forty thousand feet up, where airliners travel, our lungs would burst but for the homemade atmosphere we bring along from below. At twenty-five miles, only one percent of the air is left and the no-man's-land of space is well begun.

But the atmosphere is more than a fence around the garden It has much to do with the garden being there. It is made up almost entirely of nitrogen and oxygen, both gases vital to life.

Nitrogen is the largest component, comprising about eighty percent of the atmosphere. There are roughly twenty million tons of it above every square mile of the earth.

Every farmer knows how important nitrogen is to his soil When he buys fertilizer, what he wants most is nitrogen. This is the true staff of life, for it is the bread of the plants, which came first in the long process of evolution, animal life follow ing long afterward. Since the animals eat the plants, nitrogen indirectly is the bread of the animals as well.

The rest of the atmosphere is almost all oxygen, the "gas of life." Without oxygen coursing through our bodies in the blood stream we would die in minutes.

As nitrogen and oxygen sustain the plants and animals, so the plants and animals keep the supply of nitrogen and oxygen replenished. The animals, breathing in oxygen, breathe out carbon dioxide, which is carbon and oxygen mixed. The plants, in turn, take in the carbon dioxide, keeping the carbon and throwing off the oxygen, once more to be breathed by the animals.

From the carbon, by the mysterious process of photosyn- thesis ("photo" meaning light and "synthesis," put together), the plants manufacture growth material-stems, leaves, fruits, seeds. This growth material amounts to two hundred billion tons a year, botanists have estimated. The animals eat the growth material, breathing out more carbon dioxide for the plants, and so the cycle goes on.

The plants get almost none of their bulk from the soil, but from the air. Thus, since the food chain begins with the plants, and since the animals are the offspring as well as the dependents of the plants, it may be said that all life literally comes "out of thin air"-the magic curtain which is the fence around our garden in space.

For all the wonders of the atmosphere, however, there would be no life but for another thing unique to the planet earth. This is water. The ocean reservoirs occupy more than two thirds of the garden.

From the oceans the water is distributed about the earth by an awesomely complicated system of irrigation which no man yet fully understands. The power plant operating the irrigation system, using more than one hundred thousand times more energy than all the generating plants in the United States put together, is the sun, that atomic furnace which sits out in space, ninety-three million miles away.

By evaporation the sun pulls up water at the rate of nine teen million tons a second, more than one thousand billion tons of water a day, in the process changing it from liquid into vapor and suspending it in the atmosphere. The water comes to earth as rain, snow, sleet, hail, and dew. Nurturing plants and animals, the water finds its way by brook and creek to the rivers, and by the rivers back to sea.

Along with this cycling of the water between sea and earth goes a great deal of commotion: wind, thunder and lightning For example, there are about 2,000 thunderstorms in progress all the time in various parts of earth. There are rising and fall ing temperatures, shifting pressures, overcast, and sunshine This is weather. The sum of the weather over a period of time is climate.

The weather is a wheel within the several wheels of the climate, all turning at a different rate. The weather changes seasonally as the earth moves around the sun, tilting on its axis and presenting a changing aspect to the sun's radiation as it goes-a little like a roast on a rotisserie.

Thus, we have the cycle of the seasons: spring, summer. autumn, and winter-and then spring again.

Meanwhile, the climate within which the weather is turn ing through its four seasons is changing too, though far more slowly. It is often said, usually with a note of concern. that the climate is changing: It's getting colder, warmer, drier, as the case may be.

The cycles run from tens of years to thousands and perhaps millions of years. The trouble is, man hasn't been here long enough to have seen much of what's going on firsthand.

He didn't start to keep track of the weather until the seven teenth century, when the Royal Society of London and the Accademia del Cimento of Florence, Italy, began to measure pressures, snow, wind, storms, and the like. By such direct observation a dozen possible cycles, ranging up to two hundred years, seemed to be made out.

At the same time that he was learning about the cyclical nature of the climate with his own eyes, man has also been finding out how to read the record of what went on before he arrived on the scene. The story is there in abundance, all the way back to the very morning of time, some forty-five million centuries ago.

Much of the climate record is written in the remains of the things that have lived on earth since life began. From the very firstborn, all are still here-plants, insects, birds, fish, and animals, buried in the sea and soil.

Living forms have waxed and waned and altered in rhythm with the climate as it changed from cold to warm and back to cold, from dry to wet and back to dry. The story of the climate is told by how much and what kind of plant and animal remains are found in a given place.

One of the most detailed of such records lies out of sight at the bottom of the oceans. The seas teem with microscopic plants and animals called plankton. Able to double their number in a single day, plankton are the first link in the food chain of the ocean's creatures. The vegetable plankton are eaten by the animal plankton, which in turn are eaten by the fish. Plankton are the favorite food of herring, mackerel and whalebone whales.

One of the animal plankton, Foraminifera, is quite unusual in a number of ways. No larger than a grain of sand, it sheds the shell it lives in when it reproduces, and the shell sinks to the bottom. So many of these shells have rained to the ocean floor through the ages that in many parts of the oceans the name of this plankton has been given to the ooze on the bottom.

One kind of Foraminifera coils its snail-like shell to the right in warm water, to the left in cold water. Also, some kinds thrive in cold water, becoming more abundant. Others do poorly, decreasing in numbers.

These facts make for a record of the climate as good as if the little creatures had put it in writing. All that is needed is to bring it up and read it. The story is contained in cores taken from the ocean floor, each holding perhaps several hundred thousand years of climate history.

By studying samples every few inches along the length of the core and seeing how much there is of what kind of shell, whether coiled to right or to the left, the scientist can tell the nature of the climate as each layer was put down. The layers record the story much as the growth rings in a tree tell by their width what the weather was like each year of the tree's life.

The age of the different layers can be told by other materials present in the core-volcanic ash, for example-which are radioactive and therefore may be dated. The shells them selves contain carbon 14, likewise radioactive, so they may be dated, too.

Another way to learn about the weather long ago is to study the soil in cross section. Each kind of vegetation- forest or grass land-puts down its own kind of layer as it decomposes, and since temperature and rainfall govern the kinds of plants prevailing at any given time, the successive soil layers are like the pages of a climatic history book.

From studying the information stored in nature's archives, scientists generally agree that there have been three great ice ages in the last five hundred million years, covering a third of the earth's land area with ice more than a mile thick, with several smaller ice ages in between.

Year after year the snow kept piling up in the north until it got so heavy that it compressed itself into ice underneath. Finally, with the help of gravity, the ice began to move under the weight. In summer warm weather sometimes caused the southbound ice sheet to stop temporarily, or even to retreat a little, but only to move on again as winter returned.

In North America the last time the ice came down, leveling hills and mountains and scooping out the Great Lakes, the ice reached from the state of Washington all the way across the continent to Long Island and New Jersey, bulging in the middle to a point just below the southern tip of Indiana. It covered Europe to the south of England.

That was only about eighteen thousand years ago-only yesterday as geologic times goes.

With so much of the earth's water tied up in ice, the sea lay some three hundred feet lower than it does today. Land connected Alaska and Siberia, now separated by the Bering Sea. There were other such land bridges about the globe, long since covered with water.

Then all at once the climate reversed itself. It stopped getting colder and instead began to get warmer. Each summer the ice retreated just a little more than it advanced in winter. About 8,000 years ago, the ice was gone from Europe, and 5,000 years ago from the North American continent. The last great ice age was over.

The evidence from the sea and soil shows that these major climate cycles lasted roughly 100,000 years, getting colder for the first half of that time, warmer for the second half. But within these great cycles were strong subcycles, averaging 20,000 years. For 10,000 years there was cooling, when the forests gave way to badlands and animal life changed; then 10,000 years of warning, when the forests returned and animal life changed yet again.

With the climate always changing-for reasons still gen- erally unknown-no part of the earth has stayed the same. All the cold places have been warm, the warm places cold. The dry areas have been wet, the wet regions dry-over and over again.

This, then, is the climate-the grand design of the condi- tions that make our earth an oasis in the desert of solar space. Man accepts the climate.

The weather is something else.