NASA 天文图片日历

Is that a cloud or an alien spaceship? It's an unusual and sometimes dangerous type of thunderstorm cloud called a supercell. Supercells may spawn damaging tornados, hail, downbursts of air, or drenching rain. Or they may just look impressive. A supercell harbors a mesocyclone -- a rising column of air surrounded by drafts of falling air. Supercells could occur over many places on Earth but are particularly common in Tornado Alley of the USA. Featured here are four time-lapse sequences of a supercell in 2013 rotating above and moving across Booker, Texas. Captured in the video are new clouds forming near the storm center, dust swirling on the ground, lightning flashing in the upper clouds, all while the impressively sculptured complex rotates ominously. Finally, after a few hours, as shown in the final sequence, dense rain falls as the storm begins to die out. Notable images submitted to APOD: Last week's solar eclipse