The 42-Degree Rule: Why Rainbows Vary by Season

Fall rainbows reach higher into the sky than summer arcs because the sun stays lower than 42 degrees for most of the day, governed by a fixed optical angle documented in the International Cloud Atlas of the World Meteorological Organization. When the sun exceeds a height of 42 degrees, the primary rainbow passes entirely below the horizon and becomes invisible to observers on flat ground.

The 42-Degree Geometry of the Primary Arc

The primary rainbow is separated from the exact opposite point of the sun by an angle of 42 degrees, with the red color appearing on the outside and violet on the inside at 40 degrees, according to Météo France and the WMO. This maximum optical angle does not change regardless of whether rain falls as a fine mist or a heavy downpour, meaning neither droplet size nor precipitation volume shifts the arc’s position.

To visualize this measurement without instruments, observers can stack four closed fists at arm’s length against the sky. The center of the circular arc is located on the antisolar point—an imaginary line connecting the sun through the observer’s head—which sits at the exact same altitude as the sun but directly below the horizon.

Calculating Rainbow Height Across Seasons

The height of a rainbow’s summit is calculated by subtracting the sun’s elevation from the fixed 42-degree threshold. A sun positioned 10 degrees above the horizon places the rainbow summit 32 degrees high, whereas a sun at 40 degrees allows only the topmost edge of the arc to skim the horizon.

October skies frequently showcase full afternoon arcs because the sun remains below the 42-degree threshold throughout much of the day. Summer conditions restrict full arcs to early morning or late evening hours, as midday sun heights push the entire optical circle beneath the horizon.

Canadian weather service meteo.org notes that morning rainbows typically signal precipitation approaching from the west, while afternoon displays indicate showers moving eastward. Observers facing away from the sun toward a curtain of rain can occasionally spot a secondary, fainter arc forming between 50 and 53 degrees, featuring inverted colors with red on the inside, as detailed by Futura Sciences. The 42-degree rule applies universally to any intense light source, including lunar rainbows.