What clouds appear long, flat, and spread across the sky?
1 Answer
This is dependent on altitude.
Explanation:
You are describing stratiform clouds but we need more info to give a more complete answer.
If the cloud is uniform and low to the ground it is stratus. If the cloud is composed of elements (darker and lighter areas) and is relatively low to the ground it is stratocumulous.
If the cloud is uniform and a mid level cloud it is altostratus. If it is mid level and composed of elements it is altocumulous.
If the cloud is high level and is uniform it is cirrostratus.
Since your question says spread across the sky it cannot be cirrocumulous since it does not cover the whole sky.
Since your question says long, it seems to imply elements, so I would guess it is either stratocumulous or altocumulous.
It would be helpful to know if the cloud was translucent or not.
