About Sandhya

Freelance science writer

How to make a rainbow

Sunlight is made of different colours, and water in the atmosphere splits it up.

To know about the rainbow, we need to start with the nature of sunlight.

The light from the sun is actually made of different colours – violet, indigo, blue, green, yellow, orange and red (VIBGYOR). These colours form the spectrum, a range of colours that our eyes can see. On the long journey that the sun’s rays make from the sun to the earth, the different kinds of light in the spectrum mix together and form the whitish sunlight that we see.

Each of these colours have different energies, and each of them move through the atmosphere at slightly different speeds. When they hit a raindrop, each of the colours bend, instead of coming toward the earth in a straight line. On one side of each raindrop, the white light is complete; on the other side, the white light is split into VIBGYOR, forming a rainbow. This is because each of the colours are bent by the rainbow, to different levels. Glass also does the same with light, and a triangular piece of glass that can split light is called a prism. In our atmosphere, raindrops are acting as prisms.

Rainbow_CC_Rod Trevaskus

A rainbow across the sky. © Rod Trevaskus

You can make your rainbow! Ask your parents for a compact disk (CD). If you hold it against sunlight, with the label side facing down, you will be able to see a rainbow. This is because the smooth surface of the CD has many many prisms on it, all of which are making rainbows.

Here is a web-page showing SIX different ways to make rainbows.

Thunderstorms

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Thunderstorms are caused in hot places, where hot air moves up and cold air comes down very suddenly.

On very hot days, the Earth’s surface gets heated by the sun. The air that is in touch with the Earth also heats up.

Think of a hot air balloon. Because it is full of warm air, it starts floating up into the sky. Similarly, the air heated by the Earth expands, and immediately rises up. Now there cannot be gaps in our atmosphere – so as soon as hot air rises, cool air rushes down to take its place. This causes strong winds to blow – leaves are shaken from the trees, your hair whips around your face, and your clothes billow out.

In some places where there are lots of water bodies like lakes, rivers and the oceans, the hot air that goes up carries with it lots of water vapour. Once the hot air moves up, it cools more and more and the water vapour forms clouds.

Clouds forming just before a thunderstorm. © Alan Cressler

Clouds forming just before a thunderstorm. © Alan Cressler

The wind blows clouds that form in different places together. Now the cloud is full of water, and bits of ice, that bump into each other, and get charged up. The whole cloud fills with electric charges, like the power socket you have at home. But, your power sockets are insulated to protect you from getting a shock. There is no such insulation in the clouds – so when they bump into each other, and they give out electricity, making lightning flash across the sky.

A flash of lightning. © Simon Colferai

A flash of lightning. © Simon Colferai 

Lightning is followed by thunder. When lightning strikes, it literally slices across the air, like knife going through butter. After the strike, air rolls back to fill the gap left by the slice, making the sound we call thunder.

Why do you think you first see lightning, before hearing thunder? This is because light travels much, much faster than sound.

When does it rain?

When there is enough water vapour in the air, and when it’s cool enough.

If you take a glass of water, what do you see? You see a liquid, which is colourless and odourless. It looks like it’s standing still, but actually, water is made of molecules. A molecule is too small for you to see, but it’s there in everything.

In ice, the molecules are very tightly packed. When you warm ice, the molecules get restless, start moving around, and break free from each other to form water. When you warm water still more, the molecules move further apart, and form water vapour.

This can still happen when it is cold, if the water is left open for long enough. This is why water disappears when you leave a glass uncovered. The water molecules at the surface, which are in touch with the air above, try to break free from the liquid water below. This is called evaporation.

All around the world, water evaporates from all the water bodies – lakes, ponds, rivers, the seas and oceans. This releases water vapour into the atmosphere. The amount of water vapour in the atmosphere is called humidity.

When the humidity increases, and the temperature drops, water molecules in the water vapour in the atmosphere start getting together to form liquid drops of water. This is called condensation. Once there enough water drops fuse together, they get too heavy to be floating around in the air, and they fall down, usually as rain. This is called precipitation.

Evaporation, condensation and precipitation together form the water cycle. This is how water on the earth is moved around from place to place, and we get enough to drink and use.

A simple figure showing the water cycle

A simple figure showing the water cycle

Whether precipitation occurs as rain, snow, sleet or hail depends on the temperature of the place. Sometimes, changes in the weather can be very sudden, causing thunderstorms. At other times, the sun can shine and it can rain at the same time, causing a rainbow.

Fish in winter

What happens to fish when the lake or river freezes up?

An ice sheet forms on the water body, leaving below it a slightly warmer world, though with an icy ceiling.

When water freezes to form ice, only the uppermost layer freezes. Ice is lighter than water, and floats on it, like a toy floating in a tub of water. Ice is also an excellent “insulator” – it prevents heat from escaping out. So under the ice sheet, is a warmer world, at around 1 to 4 degrees Celsius. This is why Eskimos from Iceland and Greenland build igloos to live in. Even though it is built of ice, the inside of an igloo is warmer than outside.

An Inuit village in Baffin village, Canada. © photographed from a book by C.F.Hall.

An Inuit village in Baffin village, Canada.
© photographed from a book by C.F.Hall.

Under the icy ceiling, fish can live quite happily, because they are ‘cold blooded’ – they can adjust their body temperature to the temperature of the surrounding water. We are warm blooded – we have to maintain our body temperatures in all situations. where life carries on as usual, even in peak winter. They also do the ‘usual tricks’ of storing fat in their bodies, like squirrels and birds.

Still, they do have to eat once in a while! Big fish eat smaller fish, so they don’t have much of a problem. Smaller fish eat insect larvae and small insects found in the muck at the bottom of ponds and rivers, and can get by until warm weather.

What do other animals do during winter?

They store food, they eat a lot and store fat in their bodies, or they pack their bags and leave.

Come winter, we pull out our heavy jackets, gloves and scarves. We even put sweaters on our dogs! What do the birds, squirrels and fish do?

A plump, fluffed out robin on the ice.

A plump, fluffed out robin on the ice.
© creative commons

Next time you see a bird out in the cold, notice how plump it looks. Some birds, like the robin, fluff out their feathers to trap a layer of hot air near their bodies. They also eat a LOT before winter, so they have a layer of fat under their skin for extra protection from the cold. Fat is also important because the bird can use up the fat from its own body when there is not much food around.

Other birds migrate from colder areas to warmer areas during winter. Birds that eat usually migrate, because it is hard to find insects in cold countries. For example, swallows from Europe head south to Africa for winter.

An arctic tern in flight

An arctic tern in flight
© Andreas Trepte

The most spectacular migration is by the Arctic tern, a sea bird – meaning it lives by the sea and mostly eats fish. The tern lives around the north and south poles, which are literally the upper and lower ends of the earth. When summer ends in one pole, the birds fly about 20,000 kilometres to the other pole, every year!

An arctic tern chick, which was ringed in the UK, turned up in Melbourne, Australia. It flew 22,000 km in 3 months. Airplanes themselves take almost 24 hours for this journey.

A grey squirrel busy feeding before winter© David Dixon

A grey squirrel busy feeding before winter
© David Dixon

Squirrels do something different. They do as little as possible, so that they don’t have to eat too much. This is why you don’t see them around too much in winter. They make nests on trees, and in tree holes. They line the nest with soft material like dropped bird feathers – their version of blankets and duvets! They also huddle together with their families to keep cosy.

Even before winter – that is during autumn – squirrels do two things. They start eating a lot, and become fat, like the birds. They also start putting away food stores. They collect nuts and other food stuff – the red squirrel collects pine cones – and bury them somewhere. But the thing is, they tend to forget about these food stores! They end up acting as gardeners, as these seeds grow up to be new trees.

Birds and squirrels live on land, so they are relatively better off.

Gritting roads

It does not allow water to freeze, and makes ice melt faster.

First of all, what is gritting?

A gritting truck at work (Stephen Craven)

A gritting truck at work (Stephen Craven)

Those of you in cold countries where there is snow and ice must have seen it. Huge ‘gritting trucks’ go out in large numbers at night, to add something on the road. You will probably see the ‘grit’ on the roads the next day.

Although it is called gritting, there is actually no grit involved. Grit is small pieces of stone, pebbles and sand. But, gritting roads uses rock salt. Rock salt is similar to cooking salt, but it’s too rough and has too many other things in it, so we don’t use it for cooking.

How does it work?

For water to freeze, enough of its ‘molecules’ have to meet together and hold hands. (A molecule is too small for you to see, but it’s there is everything – it is the building block of things, same as bricks are used to make a building.)

When rock salt is added, water molecules keep meeting up with salt molecules instead of other water molecules. They cannot hold hands anymore, so water is unable to freeze and form ice.

Rock salt also interferes with ice. Ice is just water molecules, packed tightly together. Rock salt manages to get inbetween the molecules in ice, and breaks them apart, making ice melt.

So rock salt does two things: it prevents water from forming ice, and makes ice melt and form water again. Now you can walk safely on the roads, and people with cars can drive without skidding off the roads.

Why do we slip on ice?

Because the uppermost layer of ice has a thin layer of water on it. 

An icy road

An icy road (Erin McKittrick)

Ice is like an extremely thick stack of newspaper. The top most layer of the ice we move on is ‘loose’ compared to the lower layers. Lower layers have each other to stick to. The upper layer is in contact with the air above. It is warmer than the lower layers, and so tries to melt and become water.

When we move on ice, we are actually helping it become water. I’ll tell you how. Our shoes (and skates) rub on ice, causing friction. Friction warms the ice further, making it more water than ice. This makes ice slippery.

So it’s actually the water on the ice that makes us fall – not the ice itself.

How does gritting help? Go here to find out.