These final weeks of summer are "rainbow time." The suddenly lower late afternoon sun, the periodic showers and the right type of cloud all conspire to manufacture rainbows - which are very rare in winter.

Why does cloud type matter? Because in winter, the rain commonly falls from overcast skies, while summer often boasts individual puffy rainclouds, offering far better chances for sunshowers - those reliable rainbow factories. For, whenever you're positioned between low sun and rain, there must be a rainbow.

Its invariable 42-degree radius, always centered on the shadow of your head, dictates its limitations. Rainbows can't be seen between 10:30 a.m. and 3:30 p.m. because the sun is then higher than 42 degrees and the anti-solar point lurks too far below the horizon for the rainbow's top to appear. It makes sense that rainbows display their largest arcs when the sun is lowest.

Colors? Blue is always on the inside, red on the outside, but the intensity and composition vary. The biggest raindrops make the brightest rainbows, but also rob them of blue. So if you see an extremely bright version, with brilliant green and red but weak blue, it's caused by a heavy thunderstorm with huge drops up to a twelfth of an inch wide.

Often, ten degrees outside the main rainbow, a second, weaker one appears, with reversed colors: Red is now on the inside. All rainbows are caused by reflections and refraction within water droplets, and this secondary bow is fainter because the light has undergone an extra internal reflection.

The sky between the two rainbows is darker than anywhere else. As a kid I was struck by how the name for this phenomenon, Alexander's Dark Band, is so oddly similar to the old song with "Ragtime" in the middle. In this case it honors Alexander of Aphrodisias, who in 200 AD first noted the phenomenon.

Another favorite rainbow word is supernumerary. It's hard not to love once you've said it a few times. Supernumerary arcs are those strange pinkish and green bands sometimes seen just inside the primary rainbow, caused by diffraction patterns. These prove that light is a wave and not a particle (or at least is acting that way at the moment).

A lawn sprinkler produces a rainbow that we can walk right up to, and raincloud rainbows can be miles away, yet they're all the same apparent size. It's also true that your rainbow is unique. Someone next to you is receiving light from an entirely different set of raindrops. They see a completely separate rainbow.

Here's something cool: You can never see a rainbow and its reflection. That's because rainbows are not real 3-D objects. Below a rainbow in a calm lake you'll either see no reflection at all, or else the reflection of a different rainbow whose brightness, width or color-bias may (or may not) appear quite dissimilar.

Seeing something that cannot cast its reflection? Sure that's weird. If you'd like to see a great photo of an apparent reflected rainbow and a diagram of what's really happening, go on the Web to www.atoptics.co.uk/rainbows/rflctd.htm.

Everything about rainbows is either strange or beautiful. But understanding the science behind them is a real pot of gold.