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.
Get an Arches-area forcast from the National Weather Service.
Look up sunrise/sunset times at www.discovermoab.com/sunshine.htm.