1. Introduction

United States of America, popularly referred to as the United States or as America, a federal republic on the continent of North America, consisting of 48 contiguous states and the noncontiguous states of Alaska and Hawaii. In addition, the United States includes a number of outlying areas, such as the Commonwealth of Puerto Rico¹and the Virgin Islands of the United States², which are located on the Caribbean Sea, and the islands of American Samoa³and Guam⁴, located in the Pacific Ocean. The national capital is Washington, D.C., located along the banks of the Potomac River between the states of Maryland and Virginia.

The 50 U.S. states vary widely in size and population. The largest states in area are Alaska at 1 717 854 sq km （663 267 sq mi）, followed by Texas, and California. The smallest state is Rhode Island, with an area of 4 002 sq km （1 545 sq mi）. The state with the largestpopulation is California （36 553 215, 2007 estimate）, followed by Texas, and New York. Only 522 830 people （2007 estimate） live on the plateaus and rugged mountains of Wyoming, the least populous state.

The United States is a land for which superlatives come easily. Its nearly 3.8 million square miles （9 826 630 square kilometers） in area rank it third among the world’s countries. Only Russia and Canada are larger, but much of their vast lands lie in regions of poorly developed harsh northern climates. The adjoining 48 states lie squarely within the relatively mild and easily developed middle latitudes. With slightly more than 300 million people, the United States also ranks third in population, behind China and India. Unlike in those countries, the American population enjoys one of the world’s highest standards of living. With few exceptions, nature has been kind to the United States. No country can even closely match its environmental diversity. Within its borders can be found all of the world’s major climates, ecosystems, and land conditions.