In a few minutes the cloud withers to a mesh of dim filaments and disappears, leaving the sky perfectly clear and bright, every dust-particle wiped and washed out of it.  Everything is refreshed and invigorated, a steam of fragrance rises, and the storm is finished—­one cloud, one lightning-stroke, and one dash of rain.  This is the Sierra mid-summer thunder-storm reduced to its lowest terms.  But some of them attain much larger proportions, and assume a grandeur and energy of expression hardly surpassed by those bred in the depths of winter, producing those sudden floods called “cloud-bursts,” which are local, and to a considerable extent periodical, for they appear nearly every day about the same time for weeks, usually about eleven o’clock, and lasting from five minutes to an hour or two.  One soon becomes so accustomed to see them that the noon sky seems empty and abandoned without them, as if Nature were forgetting something.  When the glorious pearl and alabaster clouds of these noonday storms are being built I never give attention to anything else.  No mountain or mountain-range, however divinely clothed with light, has a more enduring charm than those fleeting mountains of the sky—­floating fountains bearing water for every well, the angels of the streams and lakes; brooding in the deep azure, or sweeping softly along the ground over ridge and dome, over meadow, over forest, over garden and grove; lingering with cooling shadows, refreshing every flower, and soothing rugged rock-brows with a gentleness of touch and gesture wholly divine.

The most beautiful and imposing of the summer storms rise just above the upper edge of the Silver Fir zone, and all are so beautiful that it is not easy to choose any one for particular description.  The one that I remember best fell on the mountains near Yosemite Valley, July 19, 1869, while I was encamped in the Silver Fir woods.  A range of bossy cumuli took possession of the sky, huge domes and peaks rising one beyond another with deep canons between them, bending this way and that in long curves and reaches, interrupted here and there with white upboiling masses that looked like the spray of waterfalls.  Zigzag lances of lightning followed each other in quick succession, and the thunder was so gloriously loud and massive it seemed as if surely an entire mountain was being shattered at every stroke.  Only the trees were touched, however, so far as I could see,—­a few firs 200 feet high, perhaps, and five to six feet in diameter, were split into long rails and slivers from top to bottom and scattered to all points of the compass.  Then came the rain in a hearty flood, covering the ground and making it shine with a continuous sheet of water that, like a transparent film or skin, fitted closely down over all the rugged anatomy of the landscape.