Given the above climatic tendencies, hiking is most comfortable during the spring (March through early May) and autumn (September through mid November) as day temperatures are comfortably in the 70s and 80s though night temperatures can be in the 50s or even down to freezing. That way, you can avoid ice (for the most part) in winter (though it's not necessarily a bad time of year to go outdooring at this time) and definitely avoid the dangerous desert heat of the summer (where there's also flash floods and lightning from afternoon thunderstorms to contend with). They're also good times to go waterfalling.
To give you an idea of what extremes to expect, day time temperatures usually reach and exceed 100 degrees F. Under such conditions, heat stroke is a very real danger and strenuous activity should be avoided. On the flip side, night temperatures are cold enough even to freeze water. If you're not prepared to endure such extremes, hypothermia becomes a problem.
Our first experiences in the deserts were in the Grand Canyon in late August of 2000 and in the Grand Circle in late June 2001. Temperatures were consistently above 95 degrees F (I recalled how difficult hiking just 5 miles was in Arches NP) and afternoon thunderstorms were common. Other times, we were wise to hike late in the afternoon or early in the morning.
Since then, we generally explored the deserts in the shoulder seasons (March, April, September, November [cool but did encounter brief winter rain], May [though some summer weather shows up at this time], and even late June [to do the Zion Narrows]). As you can see, we purposely avoid coming in summer unless it's a cool hike like the Narrows.
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