One of the biggest and brightest rainbows I've ever seen.

Rainbows caused by sunlight only occur early or late in the day and directly opposite
the sun when it is shining brightly on one side of the sky and rain is falling on the
opposite side. The higher the sun is in the sky, the more shallow the arc of the rainbow.
If the sun is too high, the arc will be too shallow to be seen. Since this photo was taken
very near sunset, this is about as tall a rainbow arc as can be seen from ground level.

Photographed near Smiths Falls, Ontario, Canada.