The Indonesian island of Java is famous for its volcanoes, which run through the island’s center like a spine. Many are active, intermittently spewing plumes of ash into the atmosphere. But the white areas surrounding the volcanoes in the top image do not have a fiery origin: they are clouds. However, they were influenced by the volcanic topography.

Both images were acquired on May 19, 2017. At 12:51 p.m. local time (05:51 Universal Time), the Visible Infrared Imaging Radiometer Suite (VIIRS) on Suomi NPP captured the top image of clouds ringing the volcanoes. Earlier that day, when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra passed over at about 10 a.m. local time (03:00 Universal Time), the clouds had not yet formed. Turn on the image comparison tool to see the difference.

According to Joshua Qian of the University of Massachusetts, the clouds are the result of a meteorological phenomenon called “valley wind.” During the day, the Sun warms the air in the valleys and by afternoon causes it to rise up the slopes of the mountains. As the warm, moist air rises, it cools and condenses into clouds over the volcanoes.

Sea breezes could have contributed to the cloud growth, too. As the land warms during the day, the temperature difference between the land and sea surfaces causes winds to blow onshore. Moist sea breezes combine with valley winds and move up toward the mountaintops.

NASA Earth Observatory images by Joshua Stevens, using MODIS and VIIRS data from LANCE/EOSDIS Rapid Response. Story by Kathryn Hansen.