Denver is known as the "Mile High City" because it is at an elevation of 5280 feet above sea level. What does that mean to visitors of the city and the nearby mountains?
First off, the air is thinner. Hot days feel less hot and cold days feel less cold as there is both less energy and less moisture in thinner air. That's the good news. The bad news is that there is also less oxygen per breath. Unless you have a medical condition, you shouldn't really notice the difference when you visit Denver itself. Sadly, that explains why our sports teams really don't have much of an advantage at home.
When you visit the mountains, that is a different story. Above approximately 10,000 feet, mountain visitors may notice some altitude sickness. Symptoms may include, dizziness, fatigue, nausea, and shortness of breath. Fortunately, the locals are immune to this condition as they acclimatize to the altitude after a few weeks. Otherwise, mountain living would be a bizarre lifestyle indeed.
To reduce your risk of mountain sickness, or to ease it's effects, follow these tips:
1. Avoid smoking as the carbon monoxide inhaled will exacerbate your body's difficulty absorbing oxygen at altitude.
2. Stay well hydrated. People in Colorado should to drink more water than people at lower elevations. That is because dehydration makes you more susceptible to altitude sickness. In addition, our thin, dry air draws moisture from your breath more quickly than in lower, more humid climates.
3. Moderate your alcohol intake as that too correlates with dehydration.
4. Choose lower altitude lodging. Staying in Denver for a day or two before going to the high country will allow you body to acclimate somewhat. When you are choosing mountain lodging, take altitude into account. Lodging at 9000 feet will be noticeably more comfortable than at 11,000 feet.
The other effect of the altitude is that the sun is more intense. It is not because we are closer to the sun. The sun is over 92 million miles away, so few thousand feet isn't even a drop in the ocean. Rather, the sun's intensity at altitude is an effect of the thinner atmosphere above us. Therefore, more of the sun's rays will reach you on a sunny day in Denver than they would on a similar day in New York. Compounding the problem is the fact that Denver has low humidity and an average of 300 sunny days a year.
To make a long story short, wear plenty of sunscreen when you visit Colorado, even if you do not normally need to.
I hope I am not scaring you away from visiting. Personally, I enjoy the cool, dry, and sunny days. Visitors, however, should be properly prepared to enjoy their vacation at altitude.