Several
atmospheric effects are formed by backward-scattering refraction of light while looking directly away from the sun. The particular optics that form these effects ensure that they are always centered around the head of the observer's cast shadow.
They are the heiligenschein, the dewbow, the glory, and the opposition effect.
Seeing a dewbow is rare. Sometimes you can see them on grasses, in beautiful arrays along the intersections of a spider's web, or possibly along the canopy of a forest from the vantage point of a plane. They are the bottom part of the 42° rainbow cone and so share many aspects with rainbows that you see in the sky and in sprinkler-spray.
Despite their name, they are not often caused by dew, strictly speaking. Dew is formed by the condensation of water vapor and can occur anywhere along the blade of a leaf. Guttation forms as a single drop at the tip of the blade. It is a result of water extruded from leaves as a result of root pressure. Being at the tip of the blade is more conducive to visible diffraction, and so is the more common source. The name guttationbow, though more accurate, is too ugly to seriously consider.
Because both dew and guttation occur in the morning, this is the time you are most likely to see a dewbow.
Dewbows appear as upturned full-spectrum parabolas as you face away from the sun. As with a rainbow, red is on the outside of the bow and blue on the inside.
In the geometric sense, dewbows on the ground are the section of the rainbow cone as intersected by the plane of the earth. As the sun rises and the section becomes more fully perpendicular to the sun, the bow will become fully elliptical and, if by some miracle the guttation or dew persists until the sun is directly overhead, looking down you would see it as a perfect circle under your feet.
Sources
- http://www.meteoros.de/tau/taue.htm
- http://amsglossary.allenpress.com/glossary/search?p=1&query=dewbow
- http://amsglossary.allenpress.com/glossary/search?id=guttation1