Living in an alpine envirionment, I know that the weather can change quite quickly. In the summer, (we have the best summer weather on the planet) if you like to go hiking to a mountain top, start early in the morning and try to finish before the clouds arrive. Some afternoons have quick thunderstorms blow through and you don't want to be on top of a mountain if this occurs. In the winter, dress in layers and always wear a hat of some kind.
If you are not used to being very high above sea level- be careful for altitude sickness!! Just be sure to drink lots and lots of water. This helped out a lot, they also sell herbal pills for this at many of the stores in town and lodges on the slopes. You might feel light-headed and get a headache as one of our friends did, and he was out for almost a whole entire day!!
I am from Georgia, so when I visited Breckenridge I was not used to the altitude. I was warned to drink a lot of water before going to ski but I did not take it seriously because I do not get sick easily. After a long day of skiing, I felt too horrible to go anywhere that night. I had much trouble breathing, chest pains, and just an overall horrible feeling. I instantly regretted not listening to those who warned me. I took some pills made for altitude sickness and drank lots a water and I soon felt better the next day. Any traveler who is not used to the altitude MUST drink a lot of water, because no one would want to feel the way I did!