The Sun must be lower than 42°s altitude to produce a rainbow. At the time indicated in the left-hand column, only a small arc - essentially the cut-off apex of a full rainbow would be seen - only maybe three to seven degrees high. As the Sun approaches sunset, the angular size of the rainbow arc grows to a maximum possible of 42°s high - just as the sun sets. The smallest visible rainbow will occur within the hour after the time listed in the left-hand column below. The largest practically possible rainbow can occur about an hour before sunset listed in the fifth column. Rainbows might be seen between those times and between the azimuth directions listed in third and tenth columns.

Two hands held outstretched with the fingers spread apart, measure about 36 angular degrees. Align your bottom finger on the horizon. If the Sun is lower than your highest finger, rainbows can form.

A large rainbow will cover 80 arc degrees and its apex will be between 10 and 40° above the ground. Its apparent position will be one to several kilometers distant.

A smallest visible rainbow appears a short distance away (one or two blocks), is about 12 degs wide, is almost directly touching the ground on its base, and appears as a truncated apex of a normal large rainbow. Such rainbows occur when the Sun is just below the altitude at which rainbows can first form.

For a visible rainbow to form, both a favorable solar angle and favorable weather conditions are needed. During the spring, look for rainbows when there rain sheets can be seen along the east bench but the western sky is either partly cloudy or clear.

Another special time to see rainbows in Salt Lake is around Christmas and the winter solistice between noon and 1:00pm. At the winter solistice near noon, the Sun is due south at an altitude of 23 1/2°. On winter days after storms where there is alot of residual mositure in the air, between the second week of December and the first week of January, good rainbows can form due north around noon. These "Christmas rainbows" are also listed on Figure 4.

In the 1600s, the great French naturalist and mathematician Descartes explained how rainbows form. Each raindrop in a sheet of falling water droplets is a small sphere. The light from the Sun passes over the observer's head and into the droplet-sphere. The light is refracted two twice and reflected twice off by the inside of the water droplet sphere. By these refractions and reflections, the light reverses direction. The sunlight exits out the front of the droplet at a characteristic angle relative to the observer and ground of about 42°.