An outbreak of extreme summer heat across the West is coinciding with the start of the summer vacation season, coming at a time when many families may be planning to explore the great outdoors. Millions of visitors flock to U.S. national parks every year, but few places compare to the raw natural beauty of the Grand Canyon.

But there’s a reason only 1 in 20 tourists ever sets foot below the rim of the canyon — the weather.

While the descent to the base of the canyon and return to the rim are exhilarating and rewarding, the extreme change in temperature and desert dryness can imperil inexperienced and unwary hikers.

Most summer visitors view the canyon from the South Rim of the park. Many are done after several minutes of looking into the deep abyss, but for the more adventurous, a short hike down into the canyon on an easily accessible trail makes the trip so much more memorable.

Hiking anywhere in the desert during the summer is difficult because of the very hot and very dry conditions, but the impacts are even more extreme in the Grand Canyon. The U.S. National Parks website notes that “there are no easy trails into or out of the Grand Canyon” and that more than 250 hikers require rescue every year.

The Grand Canyon is what I refer to as an “upside-down hike.” People are used to hikes that start out low and end up high, usually to the top of a mountain or other landmark that offers panoramic views. If you get tired hiking up, no problem — coming back down is easy.

Hiking the Grand Canyon is the opposite. Visitors may be tempted to start down the Bright Angel Trail from the South Rim of the canyon for a short walk.

Little do many people realize, however, that a one-hour hike down at a reasonable pace of 3 mph means you have descended nearly 2,000 feet into the canyon — and the only way out is up. All of the effort comes in the second half of the hike, which is drastically more difficult.

Trekking all the way down to the Colorado River reflects a descent of nearly 5,000 feet over a length of 7 to 9.5 miles depending on the trail. Making a hike like that in one day is a challenge any time of year — even for the most experienced hikers. Doing it in the summertime is a recipe for disaster for the unprepared.

Elevation and a desert climate — a lethal combination

Complicating matters is the unique climate of the canyon, which often features dramatic temperature shifts. The temperature difference that you may encounter from the rim to the base of the canyon may be the greatest you will ever feel on any hike.

It’s not out of the question to experience a 50-to-60-degree temperature change from a hike that starts out in the morning on the rim and ends by midafternoon along the Colorado River nearly a mile below. A summer hike that begins at sunrise from the rim of the canyon may feature temperatures below 50 degrees, but midafternoon temperatures at Phantom Ranch near the base of the canyon regularly top 100 degrees. Occasionally, they can get as high as 120 degrees.

The weather this weekend offers a remarkable example of these contrasts.

Saturday morning’s low at the rim of the Grand Canyon dipped to 43 degrees, while afternoon temperatures soared to a sweltering 116 degrees at the base. The situation was repeating on Sunday. After starting the day in the 40s at the rim, the afternoon high at the base was forecast to soar to around 115, with an excessive heat warning in effect.

There are two primary factors that cause these tremendous differences in temperature. The first is elevation. In the atmosphere, the temperature changes by about 3.5 degrees for every 1,000 feet of elevation. As you go higher, it gets colder; that’s why you see snow on mountaintops. The reverse is true when you descend into the canyon because of a process known as compressional warming. With a nearly 5,000-foot difference from the rim to the base of the canyon, the temperature should warm by an astounding 18 degrees.

The second factor is the desert location of the canyon. The air is exceptionally dry in this part of the world, allowing for sharper temperature swings. That means it gets colder at night and hotter during the daytime than air that has higher humidity. Sandy desert soils also heat and cool much more quickly.

The combined effects yield a double whammy that can be grueling for hikers.

That range can overwhelm unsuspecting or novice hikers. On a recent hike we did in early April from the rim to the base of the canyon, the sunrise temperature was 37 degrees. By the time we reached the Colorado River in the early afternoon, the temperature had skyrocketed to 92.

Water can mean the difference between life and death when hiking the Canyon. Extreme heat threatens to literally desiccate anyone without adequate hydration. In the dry canyon environment, you don’t feel like you are sweating. That’s because your sweat immediately evaporates into thin air. It’s hard to notice how quickly you’re losing water.

Far too often, it’s too late by the time one discovers just how dehydrated they are. Those who aren’t prepared pay the price with severe dehydration, heat stroke and, in some cases, death. It is paramount to carry enough water when hiking.

The seven-mile hike on the South Kaibab Trail from the rim to the base of the canyon has no water sources on it. I carried about a gallon of water for the hike down. That’s roughly 8.3 pounds of extra weight to carry, but it’s truly lifesaving.

The best times to hike the Grand Canyon are usually in the spring and fall when temperatures haven’t quite hit the extremes that are en route this week. If you do plan to visit the Grand Canyon this summer, I suggest a short walk down into the canyon, because it really gets you in touch with what it’s all about. That said, plan ahead, making sure you are in adequate physical shape to get back up once you hike down.

Dress appropriately, especially when it comes to footwear. Above all, bring plenty of water. It will make your trip much more enjoyable and could very well save your life.

Tom Niziol recently retired as winter weather expert at the Weather Channel after a 32-year career at the National Weather Service office in Buffalo.