After a total of four years off and on in the United States, I always feel reflective when leaving. This is a huge, diverse, beautiful, ugly land of extremes. Two months ago it was so hot the air conditioners were going full bore; today, we are clearing up after five inches of snow. Only three weeks ago several were killed in tornadoes forty miles south of Washington.
Expect constant environmental changes and you'll never be surprised.

The America I first came to in 1961 bewildered a young kiwi graduate student. In the richest country in the world, horses still drew carts of tobacco leaves to the North Carolina market. In the land of the free, the local bus station had segregated waiting rooms marked "Whites" and "Blacks". Mild-mannered people could flare at the mention of Castro, Khruschev, or Martin Luther King. The country's make-up then was vastly different from today. There was only one visible ethnic divide - whites and blacks. Sure, there were German, Polish, Scandanavian and Italian minorities. Jews wielded political power in New York City, and still do; the Irish in Boston and Chicago. But the most visible racial divide was being worked out politically in the agricultural South where most blacks didn't vote. Those who did, often supported Republicans in memory of Lincoln who freed the slaves. White southerners were Democrats. Last month, blacks who bothered to vote, supported the Democrats, while a clear majority of white southerners ticked the Republicans who now dominate all levels of southern politics.

The United States hasn't paid all its bills to the United Nations, but it is a mini-UN in itself. A recent study shows that immigrants have contributed more than 50% of the labour force growth in recent years. Today, US growth is more dependent on migrants than at any time over the last century. Two million migrated here over the last two years. In Washington where I have been for four months, Hispanics are a prominent presence. Invisible 40 years ago, they drive the buses, taxis and delivery vans; they do the cleaning, and much of the manual work. America's porous borders in the south have let millions enter without papers. When the sniper was on the loose two months ago, law enforcement agencies tried to enlist Hispanic help, only to find many promptly went to earth. In addition there are all the other immigrants. I have been driven in taxis by a Ghanaian, a Nigerian and a Kenyan; by a Cambodian, a Tamil, a Thai and a Tibetan; by a Russian, and even a Chechen. Together, they provide much of today's dynamism. Their dubious immigration status keeps them working, heads down, hoping their papers will eventually be sorted, but constantly fearing the worst. A visit to the Social Security Department is a fraught exercise; employers demand a Social Security number, but who knows what might follow from identifying oneself?

African Americans on the other hand have integrated into urban society to a remarkable extent compared with the 1960s. In Washington they hold clerical jobs, teaching and administrative positions of responsibility . The superb, graffiti-free Metro system, where everything runs like clockwork except the elevators (many are permanently on the blink) is staffed by efficient, helpful blacks. Such are their speech patterns they're hard to comprehend, but one thing is for sure: the great mass have materially improved their lives. There's less visible homelessness, too, than when I was last here ten years ago. Indeed, the level of home ownership in the US at 68% has risen steadily this last decade. Efficient charitable organisations round up the occasional stray on bleak nights.

Blacks and whites are conspicuously joined together in one disturbing feature of modern America: both are marching fatward at a punishing pace. More than 30% are obese. Moving mountains frequent streets. Endlessly stuffing their faces or carrying containers of sweet drink labelled "extra diet", they propel food about as though famine is imminent. Cars have trays and can holders, back-packs are fitted with special pouches. Portions in restaurants are enormous: at most ethnic eateries three New Zealanders could feed adequately from two servings. Some Americans seem unable to link obesity with eating. Eyes are currently fastened on two black teen-agers whose weight totals 440 pounds. They are suing McDonalds. Their mothers refuse to accept any responsibility . Blaming others if things go wrong is as prevalent here as in New Zealand.

But there's a critical mass of well-read people, too. In the square below me I can buy the Washington Post, the New York Times and the London Financial Times each morning. A better range than forty years ago. That's what I'll miss most - plus the incomparable artistic diet that confirms I've been in the capital of the world - a painting exhibition of Pierre Bonnard's, a parade of the planet's finest pianists, and operas starring Kiri Te Kanawa and Placido Domingo. It's a wonderful place - in small doses - and if you don't dwell on the terrorist threat.

Michael Bassett has been Fulbright Professor of New Zealand Studies at Georgetown University in Washington DC.