THE RULE
Low sun, rain opposite
You need the sun behind you and 42° or lower, with rain falling in front. Mornings and late afternoons are best.
Check the sun height- Sun behind you
- Rain in front
- Sun below 42°
- Then look up and out
00A RAINBOW SPOTTER'S GUIDE
Sun at your back.
Rain in front.
Rainbows are not luck. They follow simple geometry.
A rainbow only appears in one place: opposite the sun, about 42 degrees from the point your shadow's head points to, and only when the sun is low enough. Know the rule and you will spot far more of them, including the faint second bow most people miss.
Plain English, with the sources linked.
Check the sun height01Principles
02THE SHORT ANSWER
Turn your back on the sun.
Every rainbow is centred on the point
directly opposite the sun.
THE RARE ONE
White rainbows in fog
Tiny fog droplets make a broad, almost colourless bow, the fogbow. Look for it in thin mist with the sun behind you.
Why it is white
THE HOME-MADE ONE
Make one in the garden
A fine spray from a hose works like a rain shower. Same rule, same 42°, just a smaller-looking bow.
How to try it03A QUICK SUN CHECK
How high is
the sun right now?
Pick the sun's height above the horizon. A primary rainbow forms a circle of about 42° around the point opposite the sun, so the higher the sun, the lower the top of the bow.
Figures are rounded, for a flat horizon and a primary bow. Hills, buildings and where the rain is all change what you actually see.
04TWO NUMBERS WORTH KNOWING
Look a little higher,
stay a little longer.
The second bow is easy to miss,
and some rainbows outstay the shower.
42° vs about 50°
Angle from the point opposite the sun
Light that reflects twice inside the drops leaves at around 50°, so a fainter second bow sits outside the main one, with its colours reversed. The sky between the two is darker: Alexander's dark band.
THE SECOND BOW
~1 hour vs 9 hours
Typical rainbow vs a reported record
The Royal Meteorological Society says rainbows usually last about an hour. A display over Taipei was reported to have lasted 9 hours, as the sun and showers lined up again and again.
HOW LONG IT LASTS
Both depend on you staying in the right place: the bow moves with you, because each observer sees light from a different set of drops.
05NEXT TIME IT SHOWERS
Four habits
of rainbow spotters.
Watch the low-sun showers
Sunshine and showers in the morning or late afternoon are the classic recipe. When rain clears with the sun out and low, go and look.
Follow your shadow
Your shadow points towards the centre of the bow. The bow sits about 42° out from that point, all the way round.
Look outside the main bow
Once you find a rainbow, look further out for the fainter second bow with reversed colours, and the darker band of sky between them.
Make your own on a sunny day
With the sun behind you, set a hose to a fine mist and spray it in front of you. The same 42° geometry produces a small rainbow in the spray.
06RAINBOW FACTS
Five things the
sky is doing.
The sun must be 42° or lower
The US National Weather Service and Australia's Bureau of Meteorology both put the limit at 42°: stand with the sun behind you, the rain in front, and the sun no higher than that.
Each colour has its own angle
The Bureau of Meteorology gives red at 42°, green at 41° and violet at 40°. That is why red sits on the outside of the main bow and violet on the inside.
The second bow is back to front
A secondary rainbow sits outside the first, is fainter and shows its colours in reverse order. MeteoSwiss notes the darker sky between them, known as Alexander's band.
Fog makes white rainbows
Fog droplets are usually under 0.1 mm across, 10 to 1,000 times smaller than raindrops, says the Met Office. The resulting fogbow looks almost colourless.
It is really a full circle
From the ground you usually see only the top, because there are fewer raindrops towards the ground. From the air, as NASA's Astronomy Picture of the Day shows, the whole circle can appear.
07QUICK QUESTIONS
Rainbows, answered.
Short answers to the questions people ask most.
Why do I rarely see rainbows in the middle of a summer day?
The sun has to be about 42 degrees or lower in the sky. Around midday in summer it is usually higher than that, so the bow would sit below the horizon. Mornings and late afternoons give the best chance.
Which way should I look for a rainbow?
Away from the sun. Stand with the sun behind you and the rain or spray in front of you. The rainbow is centred on the point directly opposite the sun, so it always appears on the opposite side of the sky.
What order are the colours in?
In the main bow, red is on the outside and blue and violet are on the inside. In a second, fainter bow outside the first, the order is reversed, with red on the inside.
Can you see a rainbow at night?
Yes, occasionally. Moonlight can make a rainbow, often called a moonbow, when the moon is bright and close to the horizon and there is rain in the opposite part of the sky. It usually looks pale to the eye.
How long does a rainbow last?
Usually not long: the Royal Meteorological Society says rainbows typically last about an hour. One seen over Taipei was reported to have lasted 9 hours, an unusually long display.
08Sources
Where the facts come from
- Rainbows: the 42° angle, the sun's height and the secondary bowUS National Weather Service
- Ask BoM: how do rainbows form?Bureau of Meteorology (Australia)
- What is a fogbow?Met Office
- Rainbow, and Alexander's dark bandMeteoSwiss
- A full circle rainbow over Norway (2022)NASA Astronomy Picture of the Day
- How are rainbows formed?Royal Meteorological Society