A rainbow is caused by refraction and reflection of sunlight in raindrops. The rainbow phenomenon, however, is not only restricted to rain, but can also be observed in several other accumulations of water drops, as for example spray (also of ships), waterfalls - or even in the drops of dew on a meadow.
Dew forms especially in spring and autumn, when during clear nights the ground cools down considerably and so vapour condensates.
One can say that a dew bow is the lower continuation of a rainbow projected upon a horizontal plane. It should therefore be possible to observe a half of a circle, the top of which is at the end of the observer's shadow and which opens towards the outside.
In reality there often is just a colourful glittering visible at the sides of the rainbow. The reason can probably be found in backward diffraction of light and the faint contrast to the green grass. A dew bow can be seen better when the dew drops are on spider webs which especially in autumn can be largely spread over the meadows, especially near forest fringes.
Unfortunately, a dew bow cannot be observed very often. When the sun is very low in the sky, shadows are too long to see the dew bow. When the sun climbs higher, the dew evaporizes rather fast and when the end of the shadow finally is within sight, there is in most cases not enough dew left to generate a "rainbow". Best conditions are thus to be found in clearings in a forest or on meadows which lie in the shadow for a comparatively long tine in the mornings.