A rainbow appears only when direct sunlight reaches water droplets in front of you while the Sun is behind you. Our hourly forecast cannot predict the arc itself, but it can help you spot hours with showers and brighter intervals.
What Creates a Rainbow?
A rainbow appears when sunlight reaches water droplets at the right angle for some of the light to return to an observer as a spread of colours. The droplets may be falling in a shower, hanging in mist or moving through spray from a waterfall. Sunlight, water and the observer have to line up. Rain alone is not enough.
The bow is an optical effect rather than an object sitting over a particular field or road. Its position depends on where you stand and where the Sun is behind you. Move, and a different set of droplets sends light to your eyes.
Rainbows are closely tied to changeable weather because they often need rain in one part of the sky and direct sunlight in another. Understanding precipitation and cloud cover makes it easier to recognise that pattern.
Refraction, Dispersion and Internal Reflection
Three processes inside each droplet produce the familiar bow.
First, sunlight refracts as it passes from air into water. Refraction means that light changes direction when it enters a material in which it travels at a different speed. The curved surface of the droplet bends the incoming ray.
At the same time, the light undergoes dispersion. White sunlight contains a continuous range of visible wavelengths, and water refracts those wavelengths by slightly different amounts. Shorter visible wavelengths near violet generally bend more than longer wavelengths near red.
Some of the light then reflects from the back inside surface of the droplet. It travels towards the front again and refracts a second time as it leaves the water. Many rays enter a droplet, but light becomes concentrated near particular outgoing angles. Millions of droplets contribute light from those angles, building the arc you see.
A simple ray diagram often follows one droplet, but no single droplet produces the whole visible bow. Droplets near the top of the arc send the right light from above you, while other droplets supply its sides and lower portions.
Why the Sun Has to Be Behind You
The centre of a rainbow lies at the antisolar point, the point directly opposite the Sun from the observer. If the Sun is behind your right shoulder, the bow is centred in the opposite direction, slightly to your left. The shadow of your head points towards the antisolar point when the ground is level enough for the shadow to be visible.
For a primary rainbow, the brightest red light usually reaches the observer at an angular distance of about 42 degrees from that point. Violet arrives a little closer to the centre. These are viewing angles, not distances to the rain.
The Sun’s height controls how much of the bow can appear above a level horizon. With a low Sun, the top of the bow can stand high in the sky. As the Sun climbs, the antisolar point moves farther below the horizon and the visible arc becomes lower. From an aircraft or a high viewpoint, droplets below the observer can reveal more of the circle.
You do not need to turn and look at the Sun to check the geometry. Face away from it and use the direction of shadows. Looking directly at the Sun can permanently damage your sight.
Why a Rainbow Is a Circle
Light that reaches your eyes from the primary bow comes from droplets positioned at nearly the same angle around the antisolar direction. In three dimensions, those useful rays form a cone with your eye at its tip. Where that cone meets a field of droplets, you see a circle.
The horizon and landscape usually hide the lower part, leaving an arc. The ground does not make the rainbow semicircular. It blocks the droplets that would complete the view below the observer. A high vantage point, with illuminated rain or spray beneath it, can sometimes reveal a complete circle.
This geometry also explains why chasing the end of a rainbow cannot work. There is no fixed endpoint to reach. As you move, the antisolar direction moves with you and other droplets form the bow.
The Colours Are a Continuous Spectrum
The outer edge of a primary rainbow is red, followed by a gradual transition through orange, yellow, green and blue towards violet on the inner edge. The familiar list of seven colours is a useful naming convention, not seven hard-edged stripes built into the sky.
The spectrum is continuous. Boundaries depend on human colour perception, brightness and the words a language uses for different hues. Bands may merge when the light is weak, the droplets vary greatly in size or cloud and haze reduce contrast. A camera can also record colours differently from the eye because exposure, processing and screen settings affect the result.
The sky often looks brighter inside the primary bow than outside it. Droplets direct more scattered light towards the observer inside the bow’s outer edge, adding a pale background behind the colours.
Primary and Secondary Bows
The bright inner arc is the primary rainbow. Its light has reflected once inside each contributing droplet. Red lies on the outside of the arc and violet on the inside.
A secondary rainbow can appear outside the primary when some light reflects twice inside the droplets before leaving. Only part of the light continues along the path at each reflection, so the secondary bow is normally fainter and broader. It forms at a larger angle from the antisolar point.
The extra reflection reverses the colour order. On the secondary bow, red is on the inner edge and violet is on the outer edge. The area between the two arcs often looks darker than the sky inside the primary. This gap is called Alexander’s band and results from the directions in which droplets do, and do not, send concentrated light towards the observer.
The strength of either bow depends on droplet size, rainfall, background brightness and cloud. A secondary bow may be clear along one part of the arc and invisible elsewhere.
Supernumerary Bows, Fogbows and Spray Bows
Closely spaced pastel arcs sometimes appear just inside the primary rainbow. These supernumerary bows cannot be explained fully by tracing light as separate rays. They arise from interference between light waves that follow slightly different paths through droplets and emerge in the same direction. Fairly small droplets with a narrow range of sizes can make the bands easier to distinguish. They often appear pinkish and greenish rather than repeating the entire main spectrum.
A fogbow forms in much smaller droplets. Diffraction spreads and overlaps the colours, producing a broad, pale or nearly white bow, sometimes with weak red and blue edges. It follows the same basic observer-centred geometry as a rainbow.
Spray from waterfalls, waves and sprinklers can produce compact rainbows when direct sunlight reaches the droplets. These are useful demonstrations because you can change the viewing geometry by moving. The bow shifts with you rather than staying attached to the water source.
Why Every Observer Sees a Different Rainbow
Two people standing beside each other may describe the same arc, but each receives light from a slightly different collection of droplets. Their antisolar points also differ because each bow is centred on the line through that observer and the Sun.
The effect becomes obvious when the observers are far apart or at different heights. A rainbow seen from a hill may include light from droplets below the viewpoint, while someone in the valley sees a different portion of the shower. Even each of your eyes receives light along a slightly different path, though the separation is too small to change the broad appearance.
This does not mean the observation is imaginary. The droplets and sunlight are physical, and the optical rules are repeatable. The image is observer-dependent in the same sense that a reflection on water depends on where you view it from.
When and Where to Look
Look for direct sunlight behind you and rain, mist or spray ahead. A low Sun gives more of the primary circle room above the horizon, so mornings and later afternoons often provide favourable geometry. The exact opportunity depends on latitude, season, terrain and the position of the shower. Airpult’s guide to sunrise and sunset helps you judge the Sun’s direction and height.
Broken or retreating showers are promising because they can place a sunlit gap on one side of you and rain on the other. An open view towards the rain helps, especially when a dark cloud provides contrast behind the bow. A bright sun shower still does not guarantee a rainbow. The droplets may be in the wrong direction, the Sun may be too high for an above-horizon arc, or intervening cloud may weaken the direct light.
Rainbows can change within seconds as a shower moves and the sunlight brightens or fades. Move to a clearer view only if conditions are safe, and do not stop on a road. If you hear thunder or see lightning, stop looking and move into a substantial building or hard-topped vehicle. No outdoor viewing spot is safe during a thunderstorm.
Combining Sun Position With a Forecast
A forecast cannot promise a rainbow at a precise spot. It can identify the ingredients and help you choose where to look.
Check the hourly forecast for a mix of showers and brighter intervals. Use the weather forecasting guide to interpret timing and uncertainty, then compare precipitation with cloud cover. Radar can show where showers are moving, while satellite imagery can reveal clearer air behind them. The useful arrangement is direct sunlight at your location with illuminated droplets in the opposite part of the sky.
A rainbow is not a reliable sign of what the weather will do next. Its position may show which part of a nearby shower is lit, but local winds and storm movement determine whether rain is approaching or leaving.
Use Airpult Explore to choose a location, then open its forecast. The hourly row shows precipitation chances and weather-condition icons, while the daylight card shows local sunrise and sunset. Airpult does not forecast whether a rainbow will appear or show the exact direction of a shower. When sunlight and a passing shower overlap, face away from the Sun and scan the sky around the antisolar direction.
