Lenticular clouds are a fairly common sight to anyone who lives near mountains. They form as moist air flows over mountain peaks, and they can take on many shapes (disks, waves, caps and some that even look like UFOs), based on the height and shape of the mountains, the speed of the wind, and the amount of moisture in the air.
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Air cools as it rises and warms as it descends. When rising air cools enough, any water vapour in the air starts to condense out at the 'dew point', and a cloud begins to form. The cloud will persist and grow as long as there is still moisture available and the air remains at the dew point. If it runs out of moisture or the temperature rises above the dew point, the cloud stops.
For lenticular clouds, the shape and location of them will depend on exactly when the rising air reaches the dew point. If the air temperature reaches the dew point above the mountain peak, a disk or saucer shaped cloud forms overtop of the mountain or in the 'lee' of the mountain. If the dew point is reached further down, the cloud can form a wave or cap over the peak, and if there is only a limited amount of moisture in the air it can form a ring around the mountain peak.
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In the case of the cloud seen in the photograph, the air temperature is at the dew point halfway up the slope. The wind is flowing roughly from left to right, indicated by the turbulent 'eddies' visible just to the right of the peak. It flows up and around the mountain side, and anywhere the wind blows upwards enough for the temperature to drop to the dew point, cloud droplets form. As the wind flows down the right slope of the mountain, it warms enough to rise above the dew point again and the cloud stops forming. Since the wind and moisture content of the air remain fairly constant, the cloud can persist for hours.
Klyuchevskaya Sopka is the tallest active volcano in Eurasia, towering 4,750 meters above the ground.
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