Jorge slowly unfurled his index finger and pointed to the dashboard. The van's thermometer showed 5 degrees Celsius. I didn't know the formula for converting Celsius to Fahrenheit, but I knew that it was chilly. And that it had been much warmer 30 minutes ago when Jorge, my chauffeur for the day, picked me up at my seaside hotel on Tenerife, one of the seven Canary Islands.

The finger was pointing again. The thermometer read 4 degrees. A small smile threatened to break across Jorge's face. When he spotted a thin rime of ice coating the road a few minutes later, he couldn't hold it in. "Hielo! Hielo! (Ice! Ice!)" he said with a big grin. Then he fumbled for his cellphone, held it up to the dashboard and snapped a photo of the thermometer's latest proclamation: -1 Celsius.

All right, all right, I got it. I was going to have a cold climb up Mount Teide (pronounced TAY day). Most hikers tackle the mountain in summer, when temps are moderate. I was here in November, when the air takes on an unpleasant chill. "But I live in the Upper Midwest; I can handle the cold," I thought.

Then I remembered my house was perched a mere 984 feet above sea level. We'd be starting our climb of Teide at 7,825 feet, not stopping until we summited a full 12,195 feet above the Atlantic Ocean. I didn't know if I was wearing enough layers, or if my footwear choice (running shoes) was a wise one. But as Jorge's van lurched up the winding road, it was too late to worry about that.

Formed 30 million years ago by underwater volcanoes that belched up mountains of molten lava, rock and ash, the Canary Islands sit about 60 miles from the border between Morocco and Western Sahara, but are a part of Spain. Tenerife is the largest island in the archipelago, and home to Teide National Park. One of the most visited national parks in the world, it owes its popularity largely to its star attraction, Mount Teide.

The dormant volcano, which last erupted in 1909, towers above the island.

No one knows when people began to climb "the Teide," as locals call it. But my guide, Pedro Adán, said it was sometime after the Spanish took over the islands in the late 15th century. "They were conquistadors and explorers, you know?" he said, shrugging into his backpack at the tiny base camp parking lot where Jorge had deposited me.

Unlike the conquistadors, the typical park visitor doesn't climb the rugged, sometimes steep 6 miles up Teide. Instead, tourists usually peer at her majestic peak from the safety of a tour bus, or take the teleférico (cable car) partway up her flank, where they can disembark to stroll amid jumbled, inky piles of old volcanic rock and gaze down at barren stretches, reminiscent of a moonscape, flowing from her base.

But whether visitors ride in a lumbering tour bus or swaying cable car, or stand al fresco on a tiny footpath carved into her side, they come to pay homage to the Teide.

Starting out

Winds swirled fiercely around Pedro and me as we began our climb from the base of neighboring Montaña Blanca. The path was wide and even, and the ascent, gentle. This was the tough climb I was worried about? The winds were more challenging than the terrain.

We wound around the back of Montaña Blanca, which mercifully led us out of the wind. As we crunched along the gravel trail, Pedro pointed out the sights. There was the north flank of Las Cañadas, a neighboring peak, off to our right. These giant boulders? Volcanic bombs. Teide coughed up small basalt chunks during her eruptions, which rolled down her sides, picking up girth from the molten lava. By the time they hit the bottom, they were enormous. "It's just like rolling a snowball," Pedro said.

About 3 miles into our hike, we reached the saddle between Montaña Blanca and Teide. We'd climbed a mere 1,000 feet.

"I think of this beginning section as a warm-up," said Pedro. "Now the real climb begins."

"Bring it on," I thought bravely.

Time to climb

The change in terrain was dramatic. Now our path was a narrow, rocky scramble. My heart started hammering and I began to sweat as Pedro and I carefully climbed the treacherous path. My trekking poles, useful on the first stretch, were now a hindrance, often getting stuck in the small crevices between the jumble of rocks that comprised our path. Soon my quadriceps were screaming from the strain of the never-ending steep steps. Yet our progress was steady.

I looked over at Pedro, and found him huddled over his cellphone. He'd been texting our entire climb, it seemed. As if reading my thoughts, he suddenly turned and said, "Sorry that I've been texting. But my boss was worried you'd have trouble with the climb, and he keeps checking in to see how you're doing. I told him to stop worrying about it, because you're doing just fine."

Too many people sign up for the climb through a third party, he explained, and don't realize how strenuous it is. Then they're upset and frustrated once they start. Some give up. Not me. I was determined to reach the top.

Soon Pedro gave a little whoop — we'd arrived at Refugio Altavista, elevation 10,696 feet. This mountain hut was a landmark in the climb, and meant it was time for a little rest. A few other hikers had gathered, and we joined them.

Most people who climb Teide start out in the afternoon, Pedro explained as he slid off his pack. They climb to this clean, spartan lodge, then spend the night. The newly remodeled space has about two dozen beds, plus toilets and sinks. Before dawn, the hikers rise, bundle up, then trek the rest of the way to Teide's peak to catch the sunrise.

While Pedro refueled with a snack, I chatted up hiker Maik ter Veer from Amsterdam. Ter Veer had just completed a 30-day hike along Spain's famous Camino de Santiago pilgrimage trail, and was rewarding himself with a trip to the Canaries. He'd planned to relax beachside, but once he learned about Teide, he immediately traded in his flip-flops for hiking boots. It's not often you can climb a dormant volcano.

Unfortunately, Ter Veer didn't know you needed a permit to summit Teide; he would be turned away at the checkpoint set up before the final ascent. "That's OK," he said. "I'll come back for sure. This place is mind-blowing, because you can see exactly how the Earth was made."

The summit beckons

Ter Veer took off. Glancing up at the jumbled pile of hardened lava, Pedro said ruefully, "This is always where I start to get a headache from the altitude."

I quickly took a big gulp of water, remembering the best way to avoid altitude sickness was to stay well hydrated. After walking just a few minutes, Pedro pulled up short. Did he have a headache already? No, his heel had developed a hot spot, and he wanted to tape it up. "Maybe I'd better text your boss and tell him you're having some problems with the climb," I joked.

We slowly inched upward in silence. A few minutes later, we came upon Ter Veer, leaning against some rocks. "I'm just resting," he said slowly. "I feel dizzy and tired."

After making sure he was OK and had plenty of water, we pushed on. Our path merged with another, and suddenly people were everywhere. We'd reached the cable-car station, the next landmark. The summit was within our grasp.

Pedro showed the official our summit permit, and we passed through a gate. Because the Teide's volcanic cone area is fragile, narrow and somewhat dangerous, there's a daily visitor quota of 200. In addition, no more than 50 people can be on the trail in any given two-hour period. We were authorized to hike to the summit between 1 and 3 in the afternoon.

This final leg isn't that far, but the path is so steep it typically takes a half-hour to reach the top. Huffing and puffing, we ascended at a turtle's pace. I was looking down when a ripe, eggy smell assaulted my nostrils. Sulfur. The sign of a volcano with some life left in her. Looking up, I spotted Teide's summit cone to my left. An enormous, jagged crater, it was whitish-green in color. The trail wound around its back side to the top.

The final few feet were so rocky and narrow, I ditched my cumbersome trekking poles and used my hands to scramble up. And suddenly, I was there. The view was magnificent, especially the north side and its famous "sea of clouds," an impressive pileup of white fluff that occurs when clouds are blown in off the sea, but can't make it over the mountaintops. Looking to the north, it appeared we were standing in the clouds. Or on the clouds. Or that we were part of the clouds, and the sky, and the heavens.

I'm not sure how long I stood there, awe-struck. But suddenly I heard Pedro's voice, thin and reedy in the fierce wind, saying he was freezing and I had five more minutes before it was time to leave. Snapping back to reality, I realized I was frozen, too. A fellow hiker took a quick victory photo of Pedro and me, grinning atop the Teide in triumph, and then we made our way down toward the cable-car station.

As we neared, Pedro said he was going to make another "tick" in his Teide count when he got home; today's climb would boost it to 22, he guessed. Suddenly inspired, I proposed we do a double-climb. There was just enough time to take the cable car down, then hike back up again. What a feat that would be!

Pedro turned and stared at me incredulously. "Even if you paid me 100 euros, I wouldn't take you up again today." And with that, he collapsed onto a bench to await the next cable car heading down Teide, and home.

Melanie Radzicki McManus writes about travel and fitness from her home in Sun Prairie, Wis.