This time, I will write about clouds seen from the ALMA Operations Support Facility (OSF). In astronomical observation, clouds are regarded as an obstacle, but I sometimes miss them when I see clear blue sky every day. Outside working hours, clouds of unusual shapes that appear once in a while are great fun for me, and soothe my mental fatigue.
The Atacama Desert is located in one of the driest regions in the world. On the west coast (on the Pacific side), Humboldt Current, a cold current coming up from Antarctica, brings dry air, while on the eastern side, the moist air from Bolivia and Argentina drops its moisture as snow by being blocked by the Andes Mountains as much as 6000 meters high. As a result, the Atacama Desert between these areas becomes an extremely dry region. Since millimeter and submillimeter waves observed by ALMA are susceptible to atmospheric water vapor absorption, its construction site needs to be a dry area throughout the year. Moreover, it is necessary to obtain a flat land over 10 km to install dozens of antennas. After many years of close site testing of candidate sites at 5000 meters above sea level in the Atacama Desert, this area was determined to be a rare place satisfying all the conditions above and finally selected as the ALMA site.
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Photo 1: Sheep cloud (altocumulus cloud) over the OSF |
Usually, west wind is dominant around here but occasionally moist air comes from east (from Bolivia), which becomes a factor contributing to the formation of clouds. The sky over the Operations Support Facility (OSF) is sometimes covered by a large-scale cloud, such as sheep cloud (altocumulus cloud) as if small cotton pieces are placed in rows, and cirrus cloud extending across the sky. These clouds increase their thickness in the afternoon and turn into red clouds at sunset extending over the great plain of the Atacama Desert (Photo 2). However, in a short time, such magnificent red cloud dynamically changes its shape and color and quickly fades away. This momentary scenery is too beautiful to describe it by words.
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Photo 2: Red cloud extending over the great plain of the Atacama Desert |
On the border between Chile and Bolivia, there is a mountain called Licancabur Volcano (5916 m) whose shape is similar to that of Mt.Fuji. One morning, I found the volcano wearing a hat of lenticular clouds. Though this cloud didn’t have a perfect lenticular shape, it can be called as lenticular cloud. It was very interesting to watch the cloud changing its shape, extending and breaking the right side of the lens (leeward) like glutinous rice cake. As lenticular clouds are supposed to be a precursor of strong wind, a dust storm began to blow in the afternoon as expected.
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Photo 3: Lenticular cloud over the Licancabur Volcano (5916m) |
Even in the dry Atacama Desert, we sometimes have heavy rains and thunderstorms during the summer season. The period from January through February is called “Altiplanic Winter” which brings unsettled weather and occasional snow due to the moist air coming from east (where Bolivia is located). In this season, the famous Atacama “almost black, dark blue sky” turns into winter scenery with snow-capped Licancabur volcano. This is why we call this season “winter” despite the middle of summer. As we had many heavy rains accompanied by thunder this summer, usually-parched streets of San Pedro de Atacama were also muddy and it was difficult to walk.
