Seasons do change in Arches, but not as drastically as those in parks farther north. Of course, those parks don't endure the high heat that can make Arches feel like an oven in July and August.
Perhaps the most exquisite season for visiting the park is the fall, from late September into November, when summer's high heat is history and the switch in seasons brings a sharpness to the air and the smell of fall to the park.
Spring, though, also is quite enticing for a visit to Arches, as the days typically are mild enough for shorts and T-shirts, though a squall can blow through for a day and knock the temperatures down temporarily.
Summer, when most folks seem to visit the park, is the hottest time of year. The redrock landscape soaks up the sun's heat and the dry air whisks away any sweat you generate, creating the real potential for heat stroke if you don't stay hydrated and pay attention to how hot it really is and how your body is responding.
Winters can be delightful in the park, if for no other reason than the fact that comparatively few folks visit during December, January and February. Snow can fall and create icy sections that you definitely need to watch for when you're walking in the park. But the biggest problem seems to be the shorter days that limit how long you can stay outside.
Southeast Utah is part of the Colorado Plateau, a "high desert" region that experiences wide temperature fluctuations, sometimes over 40 degrees in a single day. The temperate (and most popular) seasons are spring (April through May) and fall (mid-September through October), when daytime highs average 60 to 80 F and lows average 30 to 50 F.
Summer temperatures often exceed 100 F, making strenuous exercise difficult. Late summer monsoon season brings violent storm cells which often cause flash floods.
Winters are cold, with highs averaging 30 to 50 F, and lows averaging 0 to 20 F. Though large snowfalls are uncommon (except in nearby mountains), even small amounts of snow or ice can make local trails and roads impassable.