Welcome to the last 'Did You Know' for this week. 'Did You Know' is a daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks.
Tonight's question comes from our subscriber social network at https://community.ozcyclonechasers.com.au
"our world is spinning in a clockwise directing (sic) from our view, so how can our atmosphere and weather move across our earth faster than the earth is spinning in a west to east direction.You would think it should lag behind the earth seeing as it is a gas and not attached to the earth." Why does this happen?
There's two parts to this question/answer 1 - How does air move relative to the earth? 2 - How does weather move faster relative to the earth? interested? well read on
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HOW DOES AIR MOVE RELATIVE TO THE EARTH?
Air on its own without an external force acting on it will move at roughly the same speed as the earth in the lower atmosphere. This is due to friction between the earth's surface and the air in contact with it and the viscosity of air. Think of viscosity as the friction between air molecules, they all move in the same direction and encourage each other to move in that direction too. Consider Newton's First Law Of Motion "...An object will remain in motion unless it is acted upon by an external force" The force of the rotation of the earth affects everything and everyone on it, make no mistake, the wind is moving on average at about 1500km/hr but so are you. People and wind at the equator move faster than folks and winds closer to polar areas but it's all relative so you don't feel it. For this "Did You Know" I am assuming a blanket rotational velocity figure of 1500km/hr. If air somehow found a way to detach itself from the frictional effect of the surface and the rotational effect of the earth's spin we would immediately ALMOST all die as the air suddenly went from nothing to a 1500km/hr super wind (as we continued to move at 1500km/hr but it didn't) destroying everything and ripping us to pieces from limb to limb (cmon Sci Fi industry surely there's a wicked movie in that somewhere). The only folks capable of surviving this are our brave men and women living within a few kilometres of the South and North Pole because its rotational speed is negligible (see the image below). Now we will revisit this concept of different rates of earth spin in a later "Did You Know" when we discuss why Tropical Cyclones can't cross or form on the Equator - but I won't spoil that surprise.
HOW DOES WEATHER MOVE FASTER RELATIVE TO THE EARTH?
The key concept here is to remember that Newton's First Law Of Motion I mentioned at the start. "...An object will remain in motion unless it is acted upon by an external force" Now you need to understand this - one of the most fundamental rules of Meteorology. If you know nothing else about the weather just know this: 1 - Temperature differences drive Pressure differences 2 - Pressure differences drive Winds 3 - Winds drive Weather
The earth is in a constant battle to keep itself stable, however the earth's rotation, tilt, and surface type consistently upset this balance. The sun warms up surfaces irregularly both because of the type of surface and how that surface is positioned relative to it. So those areas that heat up fastest result in rising air, that rising air then exerts less pressure on the earth's surface, and the earth's response to that is to send in more air (what we call wind) to plug the gap and recreate that balance it seeks. As wind tries to rush in to 'plug the gap' it has to do so at a speed that is faster than the rotational speed of the earth itself because if it too continued to move at 1500km/hr like the rest of the earth, one particular surface or region would warm up and continue warming indefinitely sending our surface temperatures into hundreds of degrees celsius making parts of the planet uninhabitable. The opposite would occur to places not facing the sun resulting in temperatures on half the planet likely to drop to more than 100 degrees celsius below zero at night.
The most efficient way to plug the gap is by sending cold air to the region that is warming up and the way that must happen is to accelerate the airflow across the surface (troposphere) so it gets there faster than the earth is spinning. The accelerated air flows from west to east (in both hemispheres) because it is the most efficient way the earth can get air from point A to point B. The earth works with its spin not against it, just like a cricketer who hits with the spin has to exert less force on the ball than a cricketer who hits against the spin of the ball.
This transport of air from cold to warm locations results in the jet stream, a key component in the earth's fight to neutralise the temperature differences. It's a band of air about 7-10km high that moves wind at very high velocities from west to east and when it senses a significant temperature difference it tries to deliver colder air to warmer regions so for us in Australia it kinks to the north. As you can imagine colder air and warmer air don't like each other, they have different densities so you try and force cold air over a warm location and you can guarantee they are gonna have an epic fight especially if the warm air has some moisture in it. The jet stream keeps wind moving from west to east at a faster rate than the earth can rotate. Thus our weather systems are able to move from west to east relative to our location.
It's a little different in the deep tropics as you may notice lots of Tropical Cyclones move from east to west as the jet stream does not impact the deep tropics as much and other localised (but still strong)forces are acting on weather systems. But you will also notice that as Tropical Cyclones move away from the deep tropics they have a real tendency to want to move south and then east in line with the rest of the planetary weather. We will have a chat about cyclones and their apparent disregard for this west to east motion in the Deep Tropics in a different "Did You Know"
This ends our first week of "Did You Knows" Hope you've enjoyed them and most of all I hope you have learnt something. Now go watch some weather movies and pick them apart :D :D :D
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