Why is the sky broken?

That is the question my cats spent much of Monday asking me. Why is the sky broken? What is wrong with it? The eldest of the cats sat as close as he could to the middle of our house and growled at it – and shouted when I tried to take him near a window to explain! Yesterday, of course, they were all off out exploring again, and checking the world was pretty much as they left it.

We’re not particularly used to things like Hurricane/Storm Ophelia in the UK. Our weather is really rather average, and we don’t often see extreme weather. So where did Ophelia come from, and why did the sky go all weird?

There is a little bit of science you need to understand to grasp the basics of how hurricanes form. (The full science is really quite complex!) It’s all to do with particles and how they respond to heat.

Air is made of particles of lots of different elements and compounds. These particles are distributed at random, and not touching each other. It’s important to remember that they are also constantly moving, and as they move they will bump into each other from time to time (like children in a playground – all spread out and random, but occasionally touching briefly as they pass each other).

particles gas heating cooling

It’s not easy to show movement in a single picture, but the arrows can help. Cooler gas particles, on the left are moving slower and occupy a smaller volume than the warmer gas particles, on the right.

If you warm the particles up, you give them more energy, in the form of heat. This makes them move around more, so they bump into each other more and push each other further apart. Because they are further apart, they take up more volume for the same amount of mass (if you still have the same particles, just more spread out, the mass hasn’t changed but the volume has increased), and so the density decreases. Less dense things float on top of more dense things, so hotter less dense air will rise above colder more dense air. (This is exactly how hot air balloons work.)

The hotter, more energetic particles will also have a higher pressure, because they are colliding with their surroundings more often and at higher speeds. (You can investigate this at home – blow up a balloon and put it in the freezer for a few hours – you’ll be able to feel the change in pressure! It is also why balloons sometimes explode if they get too near lights – the heat from the light causes enough of a pressure increase in the balloon to make it explode.)

balloon pressure particles collision heating cooling

Pressure in balloons comes from the air particles inside the balloon colliding with the sides of the balloon. Heat the balloon up, and you increase the pressure, so eventually the balloon explodes. Cool it, and the balloon will become soft.

So, armed with this knowledge of particles, we can start to explain how hurricanes form.

You need two basic things for a hurricane to start forming: warm moist air, which is found above oceans near the equator; and wind which is a similar speed and direction across all the levels of air above the water.

The warm and wet air close to the oceans rises above the cooler denser air above it, and cool air flows in to replace the warm wet air as it rises. As the warm air rises, it gradually cools, and the water condenses into lots of tiny droplets, forming clouds. (The condensation of water releases heat into the air too.) This starts of a cycle, with towers of clouds growing taller and larger; it’s also why the winds are important – they stop this column of cloud being broken up by different wind speeds and directions. As the top cools down (as it gets higher), the colder and more dense air sinks, some of it to the outside and some of it to the inside of the column. The whole column of cloud is still getting warmer though, as more water is condensing, increasing the pressure and pushing winds outwards. The constant evaporation (from the surface of the ocean) and condensation drives the column faster, and causing the wind to circulate rather like water down a drain.

hurricane explained heating cooling evaporation condensation

A forming hurricane.

You can probably spot that this cycle drives itself on and on, making the storm bigger and faster and more powerful. When the wind speeds reach 39mph, it becomes a Tropical Storm and gets a name; at 74mph the storm becomes a hurricane. Once they move over land, the supply of warm water dries up, and they gradually lose their power again – but not until they’ve caused quite a lot of destruction and flooding first.

The formation of hurricanes is tracked from above by drones and satellites, so people can be given plenty of warning. There are some excellent videos on YouTube showing their formation – some of the aerial footage showing Hurricane Katrina (2005) is really good to watch, because you can easily see the clouds getting bigger and faster as it moves in over the ocean.

Because of the need for very warm oceans, hurricanes generally start forming near the equator. For the Atlantic Ocean, this puts them in the path of the trade winds, which blow east to west from Africa across towards the Gulf of Mexico, the Caribbean, and the south-eastern part of America. With sea waters everywhere getting warmer though, hurricanes can start forming further away from the equator, putting them in the path of other global wind currents. This can lead to them ending up in rather unexpected places, like the UK, and this is what happened with Ophelia.

But why the yellow sky and red sun?

The general consensus is that as the winds moved towards us, they picked up sand and dust, mainly from the Sahara, which can travel a long way, especially when it is high up in the atmosphere, and this affected the light reaching us at ground level. Please note, the sky doesn’t appear yellow because sand is yellow (seriously, there are people saying that on the internet, so let’s be clear!).

Light as we think of it is made up of the seven colours of the rainbow; all mixed together, they make white light. Red light at one end has the longest wavelength, while violet light at the other has the shortest wavelength. When light from the sun enters Earth’s atmosphere, it bounces off the really tiny particles in the Earth’s atmosphere, scattering the light. The shortest wavelengths scatter the most, and so the sky normally appears blue (we can talk about why the sky is blue not purple some other time). As the sun rises and sets, the light is travelling further to us through more of the Earth’s atmosphere, and so we see more of the reds and yellows, because the blue light is too scattered by the time the light reaches us.

Sand and dust particles are much bigger than the atoms and molecules that normally make up our atmosphere. They caused longer wavelengths of light to scatter more effectively, and so the sky appeared yellow, orange, or red, rather than blue, as the remnants of Storm Ophelia passed near us.

You can test this scattering at home using a glass of water with a couple of drops of milk in it. Shine a torch (or the flashlight on a phone) through it and you should see a yellower colour if you look directly through the glass at the torch, but a bluer colour viewed side on.

 

In the classroom:

Hurricanes have so many links to chemistry and physics, at all levels. Be it particle theory, pressure, or light, hurricane science can really easily fit into a lesson!

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