Colorado encompasses most of the Rocky Mountains. The state was named for the Colorado River, which early Spanish explorers named the Rio Colorado for the red colored (Spanish: Colorado) silt the river carried from the mountains. On August 1, 1876, U.S. President Ulysses S. Grant signed a proclamation admitting Colorado as the 38th state. Colorado is nicknamed the "Centennial State" because it was admitted to the Union as the 38th state in 1876, the centennial year of the United States Declaration of Independence.
Colorado's most populous city, and capital, is Denver. The Denver-Aurora-Boulder Combined Statistical Area with an estimated 2009 population of 3,110,436, is home to 61.90% of the state's residents.As of 2005, Colorado has an estimated population of 4,665,177, which is an increase of 63,356, or 1.4%, from the prior year and an increase of 363,162, or 8.4%, since the year 2000. This includes a natural increase since the last census of 205,321 people.
The climate of Colorado is quite complex compared to most of the United States. Unlike in other states, southern Colorado is not necessarily warmer than northern Colorado. Most of Colorado is made up of mountains, foothills, high plains, and desert lands. Mountains and surrounding valleys greatly affect local climate. As a general rule, with an increase in elevation comes a decrease in temperature and an increase in precipitation. Northeast, east, and southeast Colorado are mostly the high plains, while Northern Colorado is a mix of high plains, foothills, and mountains. Northwest and west Colorado are predominantly mountainous, with some desert lands mixed in. Southwest and southern Colorado are a complex mixture of desert and mountain areas.