Given the different elevations and climates in Utah, there's something to experience year-round, whether hiking the southern canyons in spring or fall, skiing in the mountains in winter, or golfing, biking, fishing, or swimming in summer. The national parks can become crowded in summer (and also hot). Early to mid-October is an ideal time to visit nearly any part of Utah, though the extreme high country can get chilly and even experience a surprise early snowstorm on occasion. Fall color displays can be found in nearly every canyon in the state. Cottonwood trees turning brilliant gold along river bottoms with a backdrop of red rock make particularly good photographs.
The Wasatch Front, including Salt Lake City, experiences four distinct seasons, and temperatures can vary wildly, reaching triple digits in summer and falling below zero occasionally in winter. Valleys experience winter inversions, when hot air above traps fog and smog below for days. When this happens, head to the mountains where the skies will be blue and the temperatures warmer. Expect afternoon thundershowers in the mountains, especially the High Uintas, in July and August.
Southern Utah's weather can be ideal any time of year, though heat in summer can be unpleasant. Spring break and Easter vacation bring some of the biggest crowds to places such as Moab, St. George, and Zion National Park. But always be prepared for the unpredictable. Be especially aware of the potential for flash floods in late summer in southern Utah parks, avoiding narrow canyons if there is any threat at all of a thunderstorm.