In the Chinese province of Yunnan, soaring mountain ridges flank a series of deep river gorges. The ridges rise well above 5000 meters (16,000 feet), while the lower parts of the gorges lie just a few hundred meters above sea level.
When the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite observed the region on January 6, 2021, one of the most striking features was an ephemeral one—long rows of parallel clouds that traced some of the ridges. The second image shows land elevation data for the region from the Shuttle Radar Topography Mission (SRTM). Slide between the two images to see how closely the clouds align with the ridges.
These orographic clouds form when the shape of the landscape (in this case, the ridges) forces moist air up to altitudes high enough and cold enough for the water vapor to condense. After the air passes over the ridges, it sinks downward again, allowing the air to warm as it descends and preventing clouds from forming until another ridge forces the air upward again. Orographic clouds can take many shapes and forms, but they move slowly and often appear stationary because their formation is so dependent on the shape of the land surface below.
Between the rows of clouds, look for glimpses of the headwaters of three of Asia’s major rivers: the Jinsha (Yangtze), Lancang (Mekong), and Nujiang (Salween). Parts of two large lakes (Erhai and Chenghai) are also visible. The rivers and rugged terrain comprise part of the Three Parallel Rivers of Yunnan Protected Areas—a UNESCO World Heritage Site.
The site is known as one of the most biodiverse areas in a temperate climate. According to one estimate, the area hosts at least 6,000 species of plants, 173 species of mammals, and 417 species of birds.
NASA Earth Observatory images by Joshua Stevens, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview, and topographic data from the Shuttle Radar Topography Mission (SRTM). Story by Adam Voiland.