As Arctic air plunged into sunny Florida this past weekend, it made for a mesmerizing sight in a satellite video: parallel lines of clouds colloquially called “cloud streets” streamed from the land over the ocean.
Cloud streets—or horizontal convective rolls, as scientists call them—form when cold, dry air flows above warm surface water—in this case, the Gulf of Mexico and the Atlantic waters near Florida.
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That cold air gets warmed up as it moves over the ocean’s warmer surface and then rises. That rising air cools, and water vapor condenses. Eventually, the air hits a layer of warmer air above that acts like a lid and causes the rising air to roll over—“forming parallel cylinders of rotating air,” according to a 2019 description of the phenomenon from NASA. That air, now cooler and wrung of its moisture, then flows back down to the surface on the sides of the rolled clouds. There, the air remains clear, creating the long, parallel lines seen in the new video.
It takes time for the onrush of cold air to pick up moisture from the water below, hence the gap between the shoreline and when the rolls begin. And the cloud streets generally align with the direction of the wind.