A rainbow is a meteorological phenomenon resulting from the interaction of sunlight with water droplets suspended in the atmosphere. This optical display is caused by reflection, refraction, and dispersion of white light within the droplets, separating it into a continuous spectrum of colors. The duration of a rainbow is highly variable, often lasting only a few seconds, yet in rare instances, it can persist for many hours. The visibility of this arc is governed by atmospheric conditions and the geometric relationship between the sun, the water source, and the observer.
The Role of Solar Geometry in Duration
The primary factor determining a rainbow’s short life is the constant motion of the sun across the sky. A rainbow is not a physical object; it is a cone of light rays scattered by water droplets that reaches the observer’s eye at a specific angle. This cone is always centered on the anti-solar point, the location in the sky directly opposite the sun.
The anti-solar point is where the shadow of the observer’s head would fall. For the primary rainbow, the arc is always seen at an angle of approximately 42 degrees from this point. Since the sun is continuously moving, the anti-solar point constantly shifts relative to the observer.
The rainbow arc must follow this shifting geometric center. This means the set of water droplets required to form the visible arc is constantly changing. Even a small movement by the observer causes a new set of droplets to align at the correct angle, making the previous rainbow vanish instantly. The brief duration of most rainbows is due to this dynamic and observer-specific geometry.
Atmospheric Conditions That End Visibility
The existence of a rainbow depends on two fundamental ingredients: direct sunlight and a continuous supply of airborne water droplets. The light source must be behind the observer, while the water droplets must be in front. The visibility of the arc ceases the moment either component is removed or altered.
The most common reason a rainbow disappears is the cessation of the rain or mist that provides the necessary water medium. The rainbow will also instantly fade if clouds obscure the sun, eliminating the direct light source required for refraction and reflection.
The sun’s position relative to the horizon also limits visibility. If the sun rises higher than about 42 degrees above the horizon, the anti-solar point drops below the ground. This means the center of the rainbow is below the observer, and the arc is no longer visible from the ground, regardless of the presence of rain.
Conditions for Extended Rainbow Sightings
The longest-lasting rainbows occur when the atmospheric conditions that create the arc remain exceptionally stable. This stability allows the necessary light and water geometry to persist for extended periods. Such events are often associated with fog bows or mist bows, where the water source is constant and localized.
A stationary mist, such as that produced by a powerful waterfall or a persistent lawn sprinkler, can create a bow that lasts as long as the sun illuminates the water vapor. The longest recorded rainbow sighting was a nine-hour event documented in Taipei, Taiwan, on November 30, 2017. This phenomenon, lasting 8 hours and 58 minutes, was attributed to a unique combination of moderate seasonal winds, hilly terrain, and stable moisture.
The university location on a mountainside helped trap the moist air, creating a continuous source of airborne water droplets that were slowly replaced. This constant replenishment, combined with the slow movement of the sun, allowed the specific geometry of the rainbow to remain visible for most of the daylight hours. These extreme events demonstrate that a rare alignment of stable atmospheric conditions can sustain the optical phenomenon for a long time.