When long rows of cumulus clouds are oriented parallel to the direction of the wind, these are called cloud streets. We rarely see them in photos taken from the ground, as in the photo above. Instead they’ve been seen most often over the past few decades in satellite photos. You can recognize cloud streets as long rows of puffy cumulus or cumulus-type clouds. They are most often straight, but might form patterns when the wind driving the clouds hits an obstacle.
Cloud streets are formed by what are called convection rolls of rising warm air and sinking cool air. Rising warm air cools gradually as it ascends into the atmosphere. When moisture in the warm air mass cools and condenses, it forms clouds.
Meanwhile, sinking cool air on either side of the cloud formation zone creates a cloud-free area. When several of these alternating rising and sinking air masses align with the wind, cloud streets develop.
Cloud streets are technically called horizontal convective rolls.
NASA has taken some amazing photographs of cloud streets over the past few years with the MODIS (Moderate Resolution Imaging Spectroradiometer) instruments on board the Terra and Aqua satellites. The satellite images on this page are from these instruments.
Cloud streets typically form fairly straight lines over large flat areas such as the ocean. When geological features like islands disrupt the flow of the wind, this disruption can create spiral patterns in the cloud streets similar to the way in which large boulders create downstream eddies in rivers. The spiral patterns in clouds, called von Karman vortex streets, were named after Theodore von Kármán, a co-founder of NASA’s Jet Propulsion Laboratory. He was one of the first scientists to describe this type of atmospheric phenomenon.
Meteorological phenomena such as cloud streets and von Karman vortices are a manifestation of Earth’s atmosphere in motion.
Bottom line: Cloud streets are an atmospheric phenomenon, consisting of long rows of cumulus clouds oriented parallel to the direction of the wind. Photos of cloud streets here.
Deanna Conners is an Environmental Scientist who holds a Ph.D. in Toxicology and an M.S. in Environmental Studies. Her interest in toxicology stems from having grown up near the Love Canal Superfund Site in New York. Her current work is to provide high-quality scientific information to the public and decision-makers and to help build cross-disciplinary partnerships that help solve environmental problems. She writes about Earth science and nature conservation for EarthSky.