>>Chapter 9: How to make a great discovery>> <<Chapter 7: Everything is interesting, even the weather<<
The wind is tricky to explain so let’s talk about a movie instead
Hello, my alien friends, today I wanted to carry on telling you about the wind, but it’s a bit complicated, so first I want to tell you about the last scene of a great movie I watched recently, Gravity, from 2013. I know I’m not supposed to spoil movies for you, but it’ll make talking about the weather more interesting.
In the final scene of this movie, Dr Ryan Stone, an astronaut who is the only survivor of a disastrous space mission, has managed to get back to earth: her space capsule – the part of the ship that protects astronauts when they are blasting through the earth’s atmosphere into space, and when they’re returning to Earth – has floated down on its parachute and landed in water, luckily, or she wouldn’t have survived the landing.
Do you remember landing on Earth? Not really? That’s a pity. It looks exciting.
Why do you need a rocket to get to space?
Do you know why you have to blast through the Earth’s atmosphere to get to space and to get back to Earth from space? You would like me to remind you? Fine.
The reason is gravity. Have you ever been on a roundabout in a playground, that round thing you can sit on and push round and round? Do you remember how, the faster it goes round, the harder it is for you to stay on, and if you don’t hold on tight you’ll go flying off? Well, the Earth is spinning fast, about 1600 kilometres an hour (km/h), and gravity is the reason we don’t all go flying off into space.
How much does air weigh?
Gravity is a force, pushing down on us. When we jump or throw things into the air, we have to push against gravity. Gravity makes us heavy, and everything on this earth has a weight, even feathers and dust, which are lighter than air, because air is pretty heavy stuff. Did you know that each of us is carrying 1,000 kilos – a ton – of air above our heads? That’s the weight of a small car. The reason we don’t all collapse from all this pressure is that we have air pressing against us from all directions, balancing out the pressure from above. And we’ve got air inside of us, in our lungs, stomachs and ears, otherwise we’d look like a crushed water bottle.
Gravity is different on different planets. Do you know how much you weigh? Why don’t you ask the internet how much you would weigh on the planets Mars, Neptune or on the moon?
How do hot air balloons work?
By the way, hot air weighs less than cold air, so hot air rises and cold air sinks, and all this air moving in different directions is what creates wind. You can experience this directly in a hot-air balloon. The first time we humans flew up into the air was 235 years ago, when the Montgolfier brothers attached a big basket to an enormous balloon-shaped bag made of silk, and sent hot air into the opening at the bottom of the bag by burning straw and wool under it; when the balloon was full of hot air, it rose into the air with its basket of passengers and sailed for 9 kilometers over Paris. Hot-air balloon technology hasn’t changed much since then, although I’m pretty sure we’ve found a safer way to heat up the air in the balloon.
Why can’t we just take a plane to the moon?
So, why can’t we just fly airplanes into space and straight to the moon? Because airplanes need air to fly, and the higher up you go, the less air there is, and there’s no air at all in space. So, you need a rocket to break through Earth’s gravity, which is very strong and is what keeps the moon, nearly 400,000 kiliometres away, orbiting around the earth, once every 27 days.
However, you can fly more quickly where there is less air, and the higher up you go, the less air there is. There isn’t even enough air for us to breathe properly at the top of Mount Everest, which is nearly 9 kilometres high. Airplanes fly at about 10 kilometres above the earth, at a speed of around 900 km/h, above where most weather is happening, and where the air doesn’t slow them down so much. But it still takes about 20 hours to fly from Europe to Australia. But we’ll soon have “rocket-planes” which will fly at nearly 300 kilometres above the earth, at about 9,000 km/h, where there is so little air that the journey will take just two hours.
The International Space Station flies even higher, at 400 km, at 28,800 km/h. It only takes 90 minutes for the Space Station to go all the way around the earth. Astronauts working and living on the Station see 16 sunrises and sunsets each day.
Why Dr Ryan Stone is lucky to be alive, and has good chances of survival
So, back to the last scene of the movie, Gravity. Dr Ryan Stone has escaped from her space capsule and is swimming in a lake. She – and we – have NO IDEA where she is or what part of the world she’s landed in. We think she’s landed in a lake and not the sea because when she’s underwater, escaping from the capsule and from her heavy spacesuit, we see a frog, and frogs don’t live in salt water. When she swims to the shore, we notice that although there’s no sign of people or buildings around, there are green hills nearby, and there’s a mosquito flying above her head. There’s no wind.
This is very good news for her. Earth is about 70% water, so she had a good chance of landing in water. However, she wouldn’t have survived if she’d landed in the middle of an ocean, or on land, or on the side of the earth where it was winter in some climates, or anywhere on earth with a cold climate. We know she’s somewhere warm. Because of the mosquito.
The film ends with Dr Ryan Stone, dressed only in a vest and pants, without even a watch, walking barefoot towards the nearest hill. Does anyone know where she is? How long will it take her to reach somewhere with people? Will they speak a different language? What will they think of meeting a woman who’s walking around in her underpants?
Where in the world would Dr Ryan Stone not have survived?
We don’t know, but we can eliminate a few possibilities, and one way is by guessing what climate she’s landed in. The world is a great big ball, which takes 365 days to orbit around the sun. The world is also spinning around its axis, an imaginary line between the North and South Poles, once every 24 hours. If you imagine drawing a line around the world cutting it in half with the North Pole at the top and the South Pole at the bottom, you have the imaginary line we call the equator, separating the northern and southern halves, called hemispheres. The equator is the part of the world that spins closest to the sun, and is hottest, whereas the poles are the coldest parts of the world because they are furthest away from the sun. So, we can eliminate the North and South Poles.
How would she know if she was in the northern or southern hemisphere? That is a very good question and the short answer is: with great difficulty. Because the northern and southern hemispheres have symmetrical – mirror-image – climates.
Look at how rainforests and deserts are arranged on our planet
If you look at Earth from space you will see that on land there is a big line of green around the equator, and these are rain forests; around two other imaginary lines parallel to the equator (called parallels) north and south of the equator, you can see grey and brown and these are deserts; then on the parallels closer to the poles, around Europe in the northern hemisphere, and Southern Chile in the southern hemisphere, it gets green again, and then it fades at both ends to icy white at the poles.
So, we can eliminate the desert areas, because she’s landed in water. And she doesn’t seem to be in a rain forest, so we can eliminate the equator. That still leaves us with a lot of the planet that she could have landed on.
Why are there deserts there? I know, it’s strange, isn’t it? The short answer is the wind. I’ll try and tell you the longer answer next time.