Roll Clouds
These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form.
Image and text from “Astronomy Picture of the Day.”
looks insecure.
I used to see those from time to time, when I lived in the Midwest. They’re frequently found at the leading edge of intense thunderstorms. Sometimes the whole cloud rotates along a horizontal axis, in a rather unsettling way.