Atmospheric Spectaculars

  Morning Rainbows by Les Cowley

The skies of Britain are often not very kind to astronomers. Daytime weather frequently includes a miscellaneous collection of rain, cloud, mist, haze and turbulent winds, and these regularly persist into the night. But there is a silver lining, in that these atmospheric conditions can lead to some of the most stunning and colourful daytime displays above our heads.

To show us what to look for, when to look, and to explain all about the "Optics of the Near Sky", we were fortunate to have an experienced observer, one Les Cowley. A retired chemist by profession, but an enthusiastic recorder of the many wonderful effects that can be spotted just with the naked eye. He showed us many pictures that appear on his Atmospheric Optics website where he has not only an impressive gallery of atmospheric phenomena but software that can be downloaded to simulate some of the particular natural displays.

Take the rainbow, for example. It is quite a rare phenomenon as a number of conditions have to be met before one can manifest in all its glory. On average there are less than ten a year in any one place.

For a rainbow to form there needs to be a mixture of sun and heavy showers and the raindrops have to be about 5 millimetres in diameter, which makes them roughly spherical. A rainy summer's morning or afternoon is a good time to look out for these multi-coloured arches, as you need the Sun to be at your back and to look into the rain. The lower that the Sun is in the sky, the higher the rainbow's arch will appear. The arch will appear highest at sunrise or sunset.

In fact, the angle between the top of the arch and the point directly opposite the Sun (through the observer's eyes) will always be 42 degrees. Sometimes a second rainbow will form outside the first and this appears to be at a larger angle (52 degrees). The first arch is known as the primary and the outer arch the secondary rainbow, with the secondary's colours being reversed and spread out more.

We see the primary rainbow due to light rays from the Sun entering a raindrop and bouncing back off its rear surface. The secondary bow is formed by light bouncing twice inside the raindrop before leaving it for our eyes. The secondary is fainter than the first as at each 'bounce' some light is lost. But there is not only reflection at work but refraction, and so the raindrops act as tiny prisms splitting the light up into different frequencies from red through to purple.

If the rainbow is rare, then much rarer is a "moonbow". You need to watch out for a rising or setting Moon in a dark sky where there is falling rain opposite the Moon. As moonlight is reflected sunlight it is faint and you may not see any colours in the moonbow. This is because the light is not intense enough to activate the cone colour receptors in our eyes. But if you are able to capture the moonbow on film or with a digital camera then some of the colours should show up.

These free lightshows are not restricted to Earth, and the Mars Polar Lander was going to check for atmospheric halos Sun before it had its fateful crash. So, we will have to wait a little longer to see Martian halos brought about by high-altitude carbon dioxide crystals.