Between early June and early September you can expect temperatures in the high sixties all the way up to a hundred degrees Fahrenheit, depending on whether you are in the high desert of Wyoming, the plains of Idaho or the mountains of Colorado. Be prepared for wild variations in the mountains – and, of course, the higher you go the colder it gets. The altitude is high enough to warrant a period of acclimatization, while the sun at these elevations can be uncomfortably fierce. In fact, parts of Wyoming and Colorado bask in more hours of sunshine per year than San Diego or Miami Beach. Spring, when the snow melts, is the least attractive time to visit, and while the delicate golds of quaking aspen trees light up the mountainsides in early autumn, by October things are generally a bit cold for enjoyable hiking or sports. Most ski runs are open by late November and operate well into March – or even June, depending on snow conditions. The coldest month is January, when temperatures below 0°F are common.