A month ago I saw a rare cloud:
These two blobby clouds were quite a mystery to me at first, but I did manage to figure them out after consulting the weather reports for the day. It turns out that there had been a series of thunderstorms along the Oregon coast about 100 miles to my west. They were the answer.
Here’s a photograph of a thunderstorm:
Note the flat top, called the “anvil”. That’s the lower edge of the stratosphere, where the warm moist air can no longer rise. So it spreads out sideways, pushed by the prevailing winds at that altitude. Those winds in my area are generally from west to east. Hence, after the thunderstorm had ebbed, that anvil moved eastward towards me. By the time it reached me, the anvil had been rounded and smoothed by the winds. There are two anvils in the top photo, from two thunderstorms.