A snow-laden sapling
An arched back
A cat stretching
Fingers tented or laced together
Melon rind wedge
Row of teeth
The Arc de Triomphe
some toilet seats
The letter N
Stained glass windows of a church
curve, semi-circle, bow, arc, dome, vault, bowed, archway
Describing a shape is best done in as few words as possible. Think of the shape as a camera snap shot--you want to capture the gist of what you mean as soon as possible so you can get on with other related (and more important) details, and the action happening in the scene
A weak example:
I gaped at the intricate shards of stained glass depicting The Last Supper in St. Bernardine's Church. I followed the facets of colors to the peaked top of the window that rose up like an angel's wing, a symbol of love and protection for God's only son, and a reminder of his sacrifice.
What's wrong with this example?
Look at all the detail used to describe the arched shape of the stained glass window! Surely the artistic rendering of The Last Supper is more worthy of description than the way the window aches at the top, right? Too, the windows of a church is something most (if not all) readers will be already familiar with, so the shape of the top doesn't require intricate detail to render it properly.
A strong example:
I bent over in the canoe, the wet paddle across my knees, soaking my jeans. I couldn't see anything in the dark--no trees outlined against the sky, no sparkles on the water, nothing. A splash sounded ahead. I gripped the paddle harder, wincing against the splinters. An arch loomed not a foot in front of the boat. I fell over backward and watched the curved underside of the bridge pass just overhead.
Why does this work?
Sometimes it's better not to get too fancy. A bridge is a fairly common sight; many are arched underneath where the posts enter the water. With such an easily-recognized landmark, a simple description is the best way to describe it so the readers understand without bogging them down in wordy detail.