Living in the Arizona desert, clouds are a rare occurrence. But in the Andean highlands, they are everything. The cloud forest (bosque nublado) that dominates these mountains can change its character rapidly, and so imposes its rhythms on its inhabitants.

The first thing to understand is where cloud forest comes from. Clouds form as moist air is heated by the sun and is pushed against mountainsides, forcing it to gain elevation. As the air rises, it begins to cool. As a result, the water vapor condenses and clouds appear.

The result is a band of clouds that persist at intermediate elevations on the sides of mountain ranges. These clouds can persist through the entire year, lending landscapes a dark gray appearance that is punctuated only by the occasional sunny day, brought about by abrupt temperature changes or ephemeral windy conditions. Living in these places is exactly like living inside of a cloud.

The constant presence of clouds means that moisture is always available to plants. The end result are ecosystems comprised of wet bogs and dark forests, rich in epiphytes like mosses and lichens, and dominated by soft soils and moisture-loving trees. Diversity is high in the cloud forest – some of the world’s highest levels of orchid and bird diversity are found in the Andean cloud forest. Climbing trees in these environments is a challenge – bark is often covered in layers of other organisms, and needs to be ‘shaved’ before attaching ropes.

The constant mist and frequent rain make for challenging work conditions. Expensive equipment has to be kept dry, and soils have to be preserved to prevent catastrophic erosion and the formation of muddy mires. Here you can see us setting up a large plastic tarp (special delivery from Cusco) to protect our drying ovens and gas analyzers. Rough as this looks, it was a notable improvement over the previous few days’ installation of the site, pushing a supply van through deep mud, huddling beneath a small clear plastic sheet in the cold rain, equipment bags piled beneath us, wondering where the cooking equipment and tents had been packed.

Clouds don’t remain in one place – they are always moving, changing from mist to rain. Sometimes they even behave like dense liquids. Here you can see a cloud that is denser than the surrounding air. It is being pushed across a valley and is slowly flowing down into another drainage, while the surrounding slopes remain sunny and dry. Clouds can also appear and disappear suddenly. On one off-trail descent into the cloud forest I was caught unawares by a building cloud and was within a few short minutes unable to see more than a few meters ahead of me. It was a very unnerving feeling of being utterly lost and without a sense of direction.

This cloud movement makes it possible to watch how a forest breathes. We work on the eastern edge of the Andes, thousands of meters above the Amazon basin. Each morning before sunrise, a sea of clouds appears over the Amazon. The moisture comes from evaporation from soils and transpiration from plants. Standing in the mountains, it feels almost possible to swim in this sea, floating just a little above the canopy of the forest. But as the day begins, transpiration increases, and so does the moisture in the air. And as the sunlight strikes the moist air, the clouds begin to rise.

Over the course of a few hours between sunrise and early morning, the clouds rise a thousand meters, shrouding the mountains in a thick gray layer. Wait some hours more, and transpiration slows down, the air begins to cool, and the clouds begin to retreat, exposing more and more of the mountainsides. Each day the forest breathes: a slow morning inhalation and a slower evening exhalation of moisture, transforming the landscape. It’s a magical experience to sit and watch a day pass from a high peak.

This daily dance of clouds brings moisture and life to the forest, but things are changing. As climates warm, the elevations at which clouds form are beginning to shift – and these shifts are occurring faster than plants and animals can keep up. The consequence is high mortality and turnover of species at low elevations that are now devoid of clouds, and invasion of high peaks by new species now shrouded in clouds. The future of Andean cloud forest is uncertain, as is the fate of the biodiversity hotspots it hosts. A new rhythm is coming to these places, and we are surely not ready for it.