It is cool in the desert at night and there is an air of unreality about it.

We are sitting in a Chinese restaurant listening to "The Stars and Stripes Forever" on Muza. A Pakistan waiter bring non-alcoholic beer made in Switzerland.

Outside, a Bedouin in traditional dress roars down the street in a General Motors truck. Huge cranes loom nearby in the darkness. The sound of jackhammers pounding rock carries over the noises of traffic.

By day, things are more unreal, if reality is defined as the storybook pictures many Americans carry around in their heads about Saudi Arabia - pictures of a nomdaic life of poverty, piety and camels.

The streets of Riyadh are like the arterials of Washington or Los Angeles at rush hour, wildly dangerous, deafening and incredibly dogget.

Along every roadway, a new dry is rising - miles of office buildings, palaces,hotels, villas, housing complexes, highrise apartments, schools, hospitals. Fried Chicken is here and the stalls in the souk, the old market place, are crammed with the bounty of the Western world, from candy bars to washing machines.

The physical change is so profound that there is talk at the archeological museum of preserving old mud buildings before the buildozers have gottea them all.

And in all the government ministries there is talk of preserving the Saudi culture and traditions, which means preserving, among other things, Islamic puritanism, the subjugation of women and the preservatiion of the monarchy.

The official concern is wellfounded, because the pace of social change here is probably unequaled in the experience of any nation in this century. It was only 15 years ago that slavery was abolished in Saudi Arabis by decrea and the payment of about $3.5 million for the freedom of 1,682 men, women and children. It was only 25 years ago that a drunke son of KIng Ibn Saud killed a British diplomat and the king offerred to the widow the son's head, is accordance with the old rule of a life for a life. The offer was declined.

There are still public executions and the severing of hands for thievery afte trials by religious courts. Audlteres and adultresses may still be stoned to death. In Jiddah, a few months, ago a young man and a royal princess were executed for a sexual liaison; an intermediary in the liaison died with them.

THESE CUSTOMS come out of the old Arabia. The new Abria is something else.

It claims one of the most advanced welfare states in the wold. "We are doing things," said the minister of industry, Ghazi Al Qusaybi, "that no socialist state would consider."

Medical care and education are free, with pocked money for high school students and $200amonth stipends for college students. A faimly wanting a house is given land by the government and interestfree loans of up to $80,000 for construction. These "loans" may, in fact, be gifts because no payback system has been established.

For the old and disabled there are government allowances more generous than in the United States. Subsidies pour out to farmers and businessmen. The average wage, according to the Ministry of Industry, is $7,200 a year. There are no taxes and no involuntary unemployment. Instead, Saudi Arabis is importing workers on a vast scale. At least a million Yements have come in as laborers; there are a quarter of a million Indians, Pakistanis, Egyptians, South Koreans, Americans and Eurooeans in technical and skilled jobs.

Gaeth Pharoan, the tycoon whose business empire includes $3 billion in construction contracts a nd 60 per cent of Bert Lance's Atlanta bank, boasted at a dinner party at the American Embasy this week that he employs 12,000 South Koreans, 3,600 Taiwanese, 6,500 Filipinos and 6,000 Yemenis in addition to Saudi nationals. Even the Yemenis,who occupy the lowest rung of the economic ladder here, are earning $6 to $15 a day.

A British journalist, David Holden, sensed what was coming more than a decade ago. He began a book with these words: "Arabis. The name has always evoked an exotic image. Because it signified for so long a land so desolate, isolated and unknown, it conveys even now a ring of loneliness and a promise of escape calculated ot stir in all ofus some flicker of romance . . . This book writes goodbye to all that, in a story of change so swift and embracing as to merit the title of revolution."

Holden did not live to see the end of it. He was murdered last month near the Cairo airport. The Saudi haven't seen an end to it either. The current fiveyear reformation plan, budgeted at $142 billion, will be followed by other fiveyear plans financed by the kindgom's oil resources. The face of the country and the living style and standards of it people will be more thoroughly and permanently transformed.