A fall of snow and the afterglow.

[venta]

Humph. mejoff promised me snow yesterday, and it never turned up. The sky in Reading is toying with me, alternating between a grey, bodey snow-promise colour and a cheerful blue with little fluffy clouds. To rub it in, the traffic reports this morning suggest that my home town of Darlington has lots of snow.

The BBC forecast for Reading says it's snowing now. It lies :(
(Can it be a forecast if the period covered has already started ? It's clearly not a report. An aftercast ? Nowcast ? Guess ? Outright lie ?)

Update: Oh, right, so even London has snow now. That's just adding insult to snowlessness.

And I also have a request for any scientific brains out there:

On the way back from Leyland last weekend, Jane, who's about 15, was looking out of the car window at a rainbow. Without warning, she suddenly asked "Why are rainbows curved?"

Now, I've got an A-level in physics, I reckon I should know the answers to questions like that. But I don't.

My mother, who has had way more practice at fielding random trivia requests from teenagers, calmly said "Because the surface of the earth's curved." Which didn't sound all that plausible to me, because the curvature of the earth and of an average rainbow are very different.

So. Why are rainbows curved?

The best I've managed so far is that the layer of air close to the earth has bands of different temperatures, which curve with the earth, and that air at different temperatures causes different amounts of refraction of light. Which is a bit vague, and I'm not very happy about it. And don't feel it really qualifies as an explanation.

Can anyone provide a coherent and convincing explanation ? (Of their own - I'll try googling in a bit.)

Grr. Very stiff hands this morning. Too much rapper at the weekend - we're practising for the national competition in a fortnight.

Comments

[wimble.livejournal.com]

Because every point in the rainbow is the same distance from the observer? (or within a certain tolerance?)

[zotz]

Because the light is reflected back at an angle from the raindrops, you only see colours from raindrops at that angle from the sun.

Think of a triangle, with one obtuse angle (you) and two acute ones - one at the sun, and one at the raindrop. Hold you and the sun steady, and sweep the triangle round in a circle - that makes a ring where you would see colours reflected from raindrops, if there are any there. When you're on the ground, there normally won't be, but you can see complete circles from planes sometimes, or dimly reflected off raindrops on grass occasionally.

[venta.livejournal.com]

Are you calling me obtuse ?

[zotz]

I think you've just demonstrated that you can be when it suits you.

[wimble.livejournal.com]

It doesn't have to suit her.

It's just a habit.

[zotz]

Marxism.

[lanfykins.livejournal.com]

A rainbow is actually a circle, half of which is hidden by the earth.

And I'd like to know why double rainbows happen.

[waistcoatmark.livejournal.com]

Second order refraction.

[stegzy.livejournal.com]

Ah...http://van.hep.uiuc.edu/van/qa/section/Underwater_and_in_the_Air/Sky_and_Air/20010430083033.htm

Thanks to crusty trusty Uni Profs.....

[waistcoatmark.livejournal.com]

The rainbow is a circle. The centre of the rainbow, your head and the sun (in that order) should be a straight line. The rest of the explanation is just a simple matter of reflections and refractions left for an exercise fo the reader.

[lanfykins.livejournal.com]

Incidentally, my boss's wife, who lives in Reading, phoned him to tell him it was snowing.

I'm beginning to entertain suspicions about this snow.

[venta.livejournal.com]

It did start snowing, very briefly. And at the time the sky had that set-for-the-day look. I got quite excited about it. But it stopped very shortly afterwards.

[bateleur.livejournal.com]

Footnote to the (correct) rainbow explanations above: At the right time of day on a sunny day you can make rainbows with a garden hose. Experimenting with this will give you a pretty good practical feel for why rainbows are round (since you can successfully make anything from a full circle to an almost-straight line with sufficient messing around).

[davefish.livejournal.com]

And in addition you can soak housemates in the name of science.

[secondhand-rick.livejournal.com]

Why are rainbows curved?

Because pots of gold can't float, and so each end of a rainbow has to be tethered.

[broadmeadow.livejournal.com]

From an aircraft I once saw an entire (circular) rainbow above the clouds.

Do I win £5?

[drdoug.livejournal.com]

Rainbows are curved because violet light (on the inside) has a shorter wavelength than red light (on the outside). With the curve in, the violet band is shorter than the red band. If you made a straight one you'd either get rumples in the red band or tear the violet one.

[onebyone.livejournal.com]

Best so far.

[mejoff.livejournal.com]

nope, look down

[fluffymark]

Why are rainbows curved?

As if they were stright they'd never go anywhere near a Gay Pride event.

[mejoff.livejournal.com]

this is the best so far!

[narenek.livejournal.com]

Given the journey back from Reading last time it snowed, how can anyone who has to drive those roads to get home want it to snow?

[venta.livejournal.com]

It's a trade-off, innit ? Improved prettiness and throwable meterology in return for travel delays.

I'm prepared to put up with quite a lot of inconvenience for snow. I like snow.

[lanfykins.livejournal.com]

Ditto.

And I regard driving in snow as a good thing, as the commute suddenly becomes lots less boring.

Mental note: put good book in car for when traffic stops completely

Indeed

[ringbark.livejournal.com]

New snow is fun and fluffy.
Week-old snow is a pain in the posterior.

[chrisvenus]

http://science.howstuffworks.com/rainbow1.htm is the best explanation I've come across in a remarkably brief search. It also adds interesting facts like in a double rainbow the colours get reversed in the secondary rainbow.

Also that implies what I suspected that you don't usually have the sun in the middle of the rainbow but that it is usually a reflection before it gets to you. This means its not as simple as triangles. But in effect it is just as other people described. Circular arcs being described by having to have given fixed angles and all that. I'll say things like polar co-ordinates and one degree of freedom too in the hope that somebody might think I know what I'm talking about. :)

[(Anonymous)]

Moonbows are incredibly beautiful - and also curved

[(Anonymous)]

Moonbows are incredibly beautiful = and also curved.

[neilh.livejournal.com]

Given these explanations, and a little thought, I'm guessing that all rainbows are the same size, too. And at the same optical distance from the observer. I wonder how you'd go about demonstrating that in a scientifically rigorous way?

[wimble.livejournal.com]

What determines the size of a rainbow? [Scientific American]

[neilh.livejournal.com]

That only really talks about apparent size, rather than actual size (but then, I'm not sure they really have an actual size, being an optical effect)

[onebyone.livejournal.com]

And at the same optical distance from the observer

No, because if there's enough water in the air then the rainbow can always appear "in front" of stuff that you might think ought to obscure it on account of the way it usually appears to be anchored at or around the horizon.

So if you only count rainbows caused by rain, not those caused by waterfalls or hoses, the closest I've ever seen the end of a rainbow is about 20 yards away or so. I didn't notice any gold, leprechauns or the like.

[neilh.livejournal.com]

Doh, you let them get away?