What if rainbows appeared at night?

We usually see rainbows on rainy, sunny days.

But what about at night? Could the Moon make a rainbow?

Most people see rainbows in the daytime, when the Sun shines while rain is falling. So it seems as if rainbows need sunshine. But do they really? What if rainbows appeared at night?

Here's a surprise: they sometimes do. This lab follows the clues: what a rainbow really needs, why moonlight can make one, and why some rainbows are almost white. Each card says whether it's a fact or an idea.

The World Meteorological Organization defines a rainbow as concentric colored arcs produced on a screen of water drops by light from the Sun or Moon. So the premise of this what-if is partly real: lunar rainbows exist.

The lab asks what conditions make them possible, why they are rare and pale, and how the same optics produce fog bows. Measured geometry and observations are facts; questions about how often you might see one are ideas.

A rainbow needs two things: light and drops of water.

The light must come from behind you. The drops must be in front of you.

A rainbow needs light and a curtain of water drops. Light goes into each drop, bends, bounces off the back of the drop and comes out again, spreading into colors. That's why the light source has to be behind you and the rain in front of you.

The rainbow is a circle centered on the spot directly opposite the Sun. Red is on the outside and violet on the inside. When the Sun is high in the sky, that circle sinks below the horizon, so rainbows are easiest to see in the morning or late afternoon.

WMO: a rainbow is mainly due to refraction and reflection of light in drops. The primary bow is centered on the antisolar point, the point directly opposite the light source, with violet at a radius of 40° and red at 42°.

The visible arc therefore depends on the source's elevation. Near sunrise or sunset over flat ground the bow can be almost a semicircle; with the Sun near 40° only its top shows, and above 42° the primary bow lies below the horizon. Thick curtains of drops give more vivid colors than thin ones.

At night, a bright full Moon can shine on falling water.

Then a faint rainbow can appear. It is called a moonbow!

Now switch the Sun for the Moon. Bright moonlight shining on rain or waterfall mist works just like sunlight, so it can make a rainbow too, called a moonbow or lunar rainbow.

In Yosemite National Park, the National Park Service says moonbows can appear in the mist of Yosemite Falls when the water is flowing especially strongly, generally in April and May, under a full moon. Clear skies and the angle of the Moon matter too.

Replace the Sun with the Moon and the geometry is unchanged: the bow is centered on the antilunar point, at the same 40–42° radii. The WMO notes that moonbow colors are much weaker than solar ones, or sometimes absent.

The NPS reports moonbows on Yosemite Falls' mist when flow is especially high, generally April and May, under the full moon, with visibility highly dependent on water flow, clear skies and the Moon's angle.

Inside each raindrop, light bends and bounces.

It doesn't matter if the light comes from the Sun or the Moon. Light plus drops can make a bow!

A raindrop doesn't care where light comes from. Any bright light that enters drops from behind you gets bent, bounced and spread into colors in the same way. So a bright moon can do the same job as the Sun, only much more faintly.

The size of the drops matters too. Big raindrops give bright colors. Very tiny droplets, like those in fog, blur the colors together into a pale, almost white bow called a fog bow.

Rainbow angles are set by refraction and internal reflection in spherical drops, so they depend on the observer's line from the source, not on which source it is. The WMO notes that drop size determines which colors appear and how wide each band is.

In very small droplets the bands broaden until they overlap, with diffraction playing a part: the fog bow is a white band, usually with a faint red fringe outside and a faint blue fringe inside, also called a cloudbow, mistbow or white rainbow.

You can look for a fog bow on a misty, sunny morning.

And you can make a little rainbow with a garden hose, with the Sun behind you!

Rainbows come in more kinds than you might think. Look for a fog bow on a misty morning when the Sun breaks through behind you: it looks like a ghostly white arch. Over the ocean, sea spray can make bows too.

You can also make your own rainbow. Stand with your back to the Sun and spray a fine mist from a garden hose in front of you. Never look straight at the Sun.

The WMO's International Cloud Atlas lists several bow types: primary, secondary and supernumerary bows, fog bows, sea-spray bows, and bows involving reflection from water surfaces. All share the antisolar geometry.

The moonbow is not a separate optical effect but the same primary bow lit by a dimmer source. That is why it needs a bright full moon, a clear sky and a thick screen of drops such as waterfall spray.