Dave: Was the moon behind you or in front of you?

Sam: It was in front of me and I was going into the rain.

Dave: I think that is a straight moonbow. It's exactly the same as a rainbow caused by the sun so what happens is, if you shine light on a droplet of water the light will bounce around inside it and when it comes out different colours will come out in different directions. So if you look at different places you'll see a rainbow. The moon is a source of essentially white light so you'll get exactly the same effect. Because it's so dim your eyes won't be sensitive to the colour so you just see a bow of brightness rather than a colour. If you took a very long exposure photograph of it or you had an image intensifier which worked in colours then you would be able to see the colours and it would look just like a normal rainbow.

Sam: I was looking them up afterwards to see if I was imagining things. It seems they're quite rare.

Dave: Yes, proper rainbows are. Normally people just see a ring around the moon itself, maybe 20-30 degrees outside the moon which is normally created by little ice particles. It isn't a true rainbow, it's a similar sort of effect.

The reason for this is that milk isn't just a straightforward solution, it's not like dissolving salt in water.  Milk is what's called an emulsion.  It's a mixture of various things which are suspended in water.  Milk is about 5% fat and that fat is dispersed through the water as tiny globules.  They're cells and surrounded by little proteins.  They have a fat-loving part of the protein and a water-loving end.  The fat loving part of the protein sticks into the fat and the water-loving bit sticks into the water.  This helps to solubalise or keep suspended the fat molecules.  Also in the milk are these things called pepsinogens or proteins because milk contains a lot of protein.

The idea of milk is to nourish a growing animal and protein is what you need to grow more muscle and get bigger.  When you put your milk in the freezer all these things which are normally well-suspended and kept separate begin to become very close together.  The freezing process means that the water in the milk, 95% forms big chunky ice crystals and they don't want to have the proteins and the fat in them.  The proteins and the fat get squeezed out of the mixture.  They tend to form around this central core of ice.  So because you see all the fat in one place it looks yellow.  If you look where butter comes from, butter comes from milk and it's yellow.  That's where the yellow colour comes from.  It's all conglomerated in one place and you can actually, physically see it.

Chris:  In that sort of time frame I reckon the answer is roughly the same.  It does increase by a small amount.  I think the stated geological figure is about 1 inch every 20,000 years or so.  That's extrapolated over the lifetime of the Earth.  Most of the water we have comes in the form of comets hitting the Earth.  When the Earth first formed, of course, there was a disc of stellar debris which was basically the material left over from when the sun formed.  Out of that we condensed planets as planetesimals.  They slowly aggregated more material and formed bigger planets.  The material left over was comets and other asteroids and other bodies which were out there in orbit.

Comets are viewed as what are called, "dirty ice balls," basically lots of water with some other stuff chucked into them.  Occasionally one of them's going to cross the orbit of another planet, get drawn in by gravity and crash land.  Most of the water on Earth, we think, comes from comets, originally.  Given that they're not actually that common these days but over the millions of years time scale I'd say the amount of water on Earth hasn't changed a huge amount.  I would add that it probably is increasing very, very slightly.  What do you think, Dave?

Dave:  There is also the mechanism whereby the Earth loses water.  What happens is, in the upper atmosphere you've got a little bit of water vapour high up in the atmosphere and it gets hit by ultraviolet light from the sun.  That can split apart into hydrogen and oxygen.  This light hydrogen will tend to float up really high and then get blown away by the solar wind.  It's a very, very slow process but we are losing hydrogen from the water all the time.  The oxygen will stay on the Earth because it is much heavier.Chris:  The same thing happened to Mars, didn't it?  About 4 billion years ago when Mars was about 400 million years old it lost its magnetic field because the planet got too cold to have a liquid and then solid iron core.  Because that's how the planet generates its magnetic field it couldn't therefore have a magnetic field.  That meant it was vulnerable to the solar wind which was just plucking all of the gas, the atmosphere and the water from the planet and it dried out.Dave:  And also to Venus.  Venus is very similar to the Earth but has a much weaker magnetic field which tends to protect it from the solar wind.  We think that Venus at the moment is about 500 degrees centigrade on the surface and the difference between Venus and Earth is that the Earth has a magnetic field which stops it losing water.  Venus has lost all its water.

Spot on, Alan.  They are, as you say, the same chemical.  H₂O is water.

The water cycle on Earth is why we have fresh water.  What happens is the energy from the sun hits the Earth's surface and the oceans' surface.  Every square metre of the Earth gets about one kilowatt of energy coming from the sun.  That's like having a one bar electric fire shining on it.  That heat evaporates the water from the surface of the ocean.  Water can leave quite easily but the heavier elements and ions which are dissolved in the water find it much more difficult so they stay behind. 

What evaporates is essentially fresh water.  This goes up and forms clouds.  These then drift across the ocean's surface until they're forced to rise over say, a mountain range or something.  As the clouds are forced to rise they find it much harder to hold on to water which is condensing inside them, clouds are just massive bodies of tiny droplets of water.  Water rains down on Earth and falls on the ground as fresh water.  As it percolates through rocks and rivers it absorbs small amounts of salts on the way through becoming slightly salty in the process but not perceptibly salty.  As it slowly drains into the ocean it carries those salts with it to the sea, picks up those salts.

What leaves the sea is fresh water again, so the sea is continuously becoming slightly more salty.  Why is the sea not becoming more and more salty?  The answer is it's become about as salty as it's ever going to because it's now reached sort of equilibrium position where if you add more salt to the sea chemical reactions kick in and take it out again.

If the wind isn't blowing and the rain's falling straight down vertically, as you travel through the rain then your front will always hit the same amount of rain no matter how fast you run.  If you go slowly you'll get more that falls on top of your head, so running is slightly better than walking.

But don't fall over because if you fall over you'll get very wet indeed!

We've done a couple of rough calculations on this.  If you've got a drip, say, 1 tenth of a millilitre and it drips every ten seconds, it's going to lose you about 36 millilitres an hour.  If you multiply it out for the whole year it's about 315 litres a year, one or two baths full, depending on how big your bath and your drip is!

So if you have a dripping tap it's really worth fixing because one drip every ten seconds is really quite slow for a tap but if you're talking several a second then you're losing quite a lot of water.

There's two things to think about here.  Number one, when it's raining the air's pretty moist.  Lots of water in the air even if you're sitting inside the car the airs going to have a lot of water vapour in it.  It's going to condense onto the inside of your windscreen.  The second point to think about is that when it's raining outside you've got cold water hitting the outside of your windscreen.  Previously you'd just have some air going over the windscreen.  The cold water's just going to take the heat away much better than the air does.  That's really encouraging any water on the inside of the car to condense on the inside of the screen.  It'll mist up your windscreen very quickly.

Well, it's all about energetics.  In other words, when you bring the temperature back up you've got molecules of water bumping into each other and the consequence of that is that the particles of milk reform into these little cells which then spread out evenly through the milk.   That's the most energetically stable way for them to organise themselves.

Chris:  I'm afraid actually, it is Nicholas, it's you and all the other people who have lived and visited your house.  The stats really stack up and they are really quite scary.  The average human loses about 30 or 40 dead skin cells every single second.  If you were to add them all up and put them in a giant bag over the course of a lifetime that would weigh about 1-and-a-half stone in dead skin alone.  Most of that debris you see lying around your house is dead bits of you floating around.  You're breathing that in, you're breathing in bits of your partner, your family, your visitors, your friends.  It's just bits of yourself.

Nicholas:  Well, I'll just have to accept that but even so 1.5 stone over the period of a lifetime doesn't seem like that much.

Chris:  Yes, but a skin cell is very, very tiny.  The weight of a cell is measured with about 9 zeros in front of it (10^-9kgs).  It's tiny!

Nicholas:  But a particle of dust is not one skin cell, is it?

Chris:  No, these things are clusters.  They get stuck together and other fats and materials that are present stick them together.  As a result you end up with something that forms a blob of dust.

The CO₂ does escape from the water, but you shouldn't worry about it because of the source of the CO₂.  It comes from the waste streams of power plants, so it's essentially a by-product that would have made it's way into the atmosphere anyway.  But, any consumer product takes manufacturing, shipping etc which does use up fossil fuels and release CO₂ into the atmosphere.  So giving up fizzy drinks could help reduce climate change, but not inthe way you thought!

Dave:  I don't know about an airborne version, but we do already have microbes which do this - algae and plants.  In the presence of sunlight, they feed on CO₂ and give out oxygen.  At the moment, they're not consuming the CO₂ as quickly as we add it to the environment.

Chris:  Those algae are largely in the sea, and the sea accounts for over a third of the absorbtion of the CO₂ that we put into the atmosphere.

Dave:  But even if more algae grows, something will come along and eat the algae, and then metabolise the sugars and carbohydrates and produce even more CO₂.