From AMS Glossary
A rainbow formed in the small drops often found on grass in early morning.
While the name implies that those drops are dew, as a practical matter, that is probably rarely the case. Rather, the drops are usually the result of guttation rather than dew. When seen stretched across a meadow, the dewbow may appear in the shape of a hyperbola, but that is merely the intersection of the cone of light that forms the bow and the surface of the meadow. Because they are seen against a surface, dewbows sometimes are perceived as hyperbolas even though, like ordinary rainbows, they are arcs with a constant angular radius.