Looking down on a knife-edge ridge on the flank of Koʻolau Volcano on the island of Oʻahu, Hawaiʻi, US, Kit Beuret (Cloud Appreciation Society Member 63,425) spotted a banner cloud. Winds blow over O‘ahu after they’ve picked up moisture on their journeys across the Pacific Ocean. If they encounter steep topography like this, their flow over the ridge can cause the air to be sucked up the leeward slope. That’s because the air flowing over the abrupt ridge results in low pressure beyond. This encourages moist air down in the foothills below to be lifted up to fill the void. It cools as it rises, sometimes enough for its moisture to form into cloud droplets. To a cloudspotter like Kit, a banner cloud advertises a special offer of the low atmosphere as wind and moisture meet in the lee of precipitous terrain.
In Album: Roger's Timeline Photos
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