At dusk, the valley grew dark except for the lights from a few ger camps glowing like strings of pearls. We walked to the top of a ridge near our camp and came upon shaggy Bactrian camels grazing in the moonlight. At bedtime our hosts built fires in the wood stove at the center of the ger, and we laid on our backs watching golden light playing on the striated ceiling above us.

But even in the middle of all that summer ease, there were whiffs of something dangerous. We kept stumbling across the bleached skulls of animals that had not survived the winter. In the worst times, when temperatures can fall to 40 below zero, livestock in Mongolia sometimes freeze to death in a standing position; the herders’ families then tramp through the snow and gather the bodies together into a miserable heap.

There is a word for this kind of disaster — dzud. A white dzud buries the life-giving grass under heavy snows; an iron dzud seals it under a glaze of ice. The snows continue until April, when the animals are so weakened by their ordeal that they shudder in the wind and stop searching for grass. A black dzud comes after a dry summer, when the ground is scoured bare and winter means slow starvation.

I brought the subject up with Tumuruu, a jovial and mostly toothless herdsman who, on the afternoon we visited, was monitoring his yaks through high-resolution binoculars. Tumuruu had been serving us green tea with yak milk so rich that chunks of cream floated on the surface. When I asked about the dzud it was if a curtain came down over his face.

“It’s not a thing people like to talk about,” he said. “Springtime is the most dangerous. That’s when people get lost and die. In the morning they think the weather is nice.”

That was all he wanted to say about it.

It was a conversation that could have occurred in this century, or in the 19th or 18th. Tumuruu has awakened at 6 a.m. every day of his adult life to oversee milking before 7, when the flies come out. Between a third and a half of Mongolia’s population is still nomadic, living more or less the way their ancestors did, down to the prescribed arrangement of vermilion lacquered furniture inside their gers. They live without bank accounts — for that matter, without money. The herd is the source of everything that matters.