There are (so far as I know) three entirely separate mechanisms contributing to the effect we saw today. When all three are working at once, the difference in brightness between the “halo” and an area in the sunlight a few degrees away can be as much as 10:1. Also there can be some pretty color effects. Here are the mechanisms, from smallest to largest contribution (in terms of halo brightness):
- Grasses, like most plants, have a mechanism inside them that causes their leaves to tend to aim directly at the sun. The wind still blows them, and many leaves are oriented such that they can't bend far enough, so this effect is slightly subtle. Certain kinds of trees (not grass, though) are particularly good at this – especially trees in the north, where sunlight is scarce. Those grass leaves that manage to point directly at the sun are also pointing directly at your head, where your head's shadow is (as in the photo). Those leaves will reflect a little more light back toward you than the surrounding leave.
- Grass leaves are covered with tiny hairs and (sometimes) equally tiny dust particles. Since we've just had rain, the grass in our case probably didn't have much dust on it. But those hairs are certainly there. The ends of those hairs act like little tiny retroreflectors, though not particularly good ones. This contributes to the halo effect in grass, and even more on plants with hairier leaves (such as Dusty Miller).
- The grass this morning was covered with raindrops and dewdrops – so much that the dogs were soaked within moments of their first passage through it. Each of the tiny drops acts as a combination of retroreflector and prism, bouncing the sun's light straight back toward the sun – and breaking it up into colors. This effect is by far the strongest when there as many droplets as we saw this morning. It really was quite beautiful...