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Welcome to 'Did You Know'. A daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks. Over the next few weeks we will be going through the 5 necessary conditions for cyclone formation and why they are important. Today we look at why a Tropical Cyclone spins


Did You Know:

Why does a Tropical Cyclone spin?


Ooo I like easy ones like this, it's due to the 'Coriolis Effect' (note I said Effect, not Force).

Want to know more? well read on




Firstly I'd like you to watch the clip above - it takes 4 minutes and well worth your while.

Basically the Coriolis Effect (purposefully misnamed a 'force' sometimes to make it easier to understand) is due to the difference in rotational speeds of our spherical planet. The equator which bulges outwards has to travel at an astonishing 1674km/hr but at the North and South Pole the earth travels an agonisingly slow 0.000008km/hr. The earth rotates from west to east in a clockwise direction as seen by someone peeking at us from below the South Pole in space and anticlockwise from above the North Pole in space.

So hypothetically, in the Southern hemisphere if I was to shoot a missile from Townsville (QLD, Australia) to Port Moresby (PNG) (which are almost directly north of each other and about 1000kms apart - check the map below), I shoot this missile at about 1000km/hr (it's a really slow missile ok) the missile would miss Port Moresby by about 75kms to the left. That's because in the hour it took for the missile to go from Townsville to Port Moresby. Port Moresby has moved 1653kms further east of where it was when I shot the missile, but in Townsville I have only moved 1578kms further east in that time. So the missile was fired straight to the north but will end up 75kms west (left) of its intended target. Now Port Moresby notices I fired a missile at them so they quickly fire one straight back at me - straight back to the south at the same speed I fired mine at them. Their missile too will deflect to the left (east) by that same 75km because relative to us they are moving faster than us. So both of our rockets will harmlessly hit the water left of our intended targets because we didn't correct the trajectory of our missiles to account for the different rotational speeds of the earth and the amount of time it would take for our missiles to reach their targets. The applications of this Coriolis Effect are extremely important in air travel and the armed forces. But meteorologically these same forces also act on air. They deflect large scale airflow to the left of where the airflow originated from (they deflect it to the right in the Northern hemisphere).

I keep firing my missiles at Port Moresby from Townsville but they keep missing to the left was I given dud missiles? or am I doing something wrong?

So as we know air wants to flow from HIGH pressure towards LOW pressure, BUT the difference in the earth's spinning velocity at different latitudes pulls those straight line winds towards the left from wherever they originated from. This left pull is strongest for winds travelling the longest distances and for winds that are strong. This deflection left of the wind results in our trade wind south-easterlies in Queensland, those dreaded strong winds that blow for large parts of the year that keep small boats off the water and make a lot of recreational weekend warrior fisherman upset. Those winds flow outwards from the HIGH pressure systems in Southern Australia to the warmer less dense air located on the 'monsoon trough' but as much as they would love to travel from south straight to the north (although they think they are travelling directly to the north), they kink to the left on their way there, so instead of southerlies we get south easterlies and the further north we go the more easterly they become. Now what happens at the Equator?

The Earth's Global Atmospheric Circulation System showing deflection due to Coriolis Effect.

At the Equator - the Coriolis Force is zero, the rapid speed of earth's rotation at the equator is what's exerting the pull on the rest of the earth, we're all constantly trying to catch up so at the equator itself the pull either left or right is zero. Therefore no deflection of the air that takes place due to it. That doesn't mean air can't move across the equator, it isn't a physical shield, it just means there will be no deflection of that air across the equator due to the earth's rotation. Because there is no Coriolis Effect here there can be no synoptic scale spin. We can certainly still see air being uplifted and small scale rotation taking place inside thunderstorms and convective air currents or near mountain ranges and in fact we see widespread thunderstorms along the equator all year round, but no large scale spin can take place on the equator. Even the few degrees closest to the equator see relatively small effects of the earth's rotation on them because they are moving at almost the same speed. Rotational speed differences increase per unit of distance as we head further away from the equator but this increased difference is not linear (check out the image at the very top of this page for a better idea of how fast different parts of the earth are spinning). For a cyclone, to survive a trip across the equator, it would actually have to stop spinning and then start spinning the other way after it crossed the equator and one thing we will learn in a different 'Did You Know' is that spin creates LOW pressure, if you lose the spin you lose the LOW pressure. So unless someone conducts an experiment proving that we can take a liquid spinning in one direction, stop the spin for a period of time and then spin it in the other direction without increasing the pressure of the liquid somewhere in the middle of the experiment it will remain a physical impossibility on this planet. Therefore given what we know right now, a Tropical Cyclone will NEVER be able to cross the equator without dying. But then again thousands of years ago we all just knew the sun rotated around the earth and look how that turned out for us...

In tomorrow's 'Did You Know' we go to our subscribers for one of their questions on La Nina. Back to more cyclone stuff next week. You can become a subscriber and ask your own 'Did you know' question at Becoming a subscriber helps us document Tropical Cyclones and gives you the most comprehensive cyclone intel out there during the season.

By the way if you know your latitude, this cool calculator will tell you just how fast you are spinning right now :) and if you're a little maths whiz you can use that formula to figure it out yourself.



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