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DID YOU KNOW?

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.



Tonight's question is an interesting one and comes from images of a Hollywood movie from 2004 - The Day After Tomorrow. The movie deals with Global Warming melting the polar ice and introducing fresh water into the oceans that destroys the North Atlantic heat transport current. The destruction of this heat current creates an ice age. The plausibility of that scenario isn't the thing that was questioned, it's the awesome scene above that this question relates to.

"Nitso in the "Day After Tomorrow" movie a bunch of cyclones formed and if you got caught in their eye, the air from the top of the cold atmosphere rushed down and snap froze everything. Could this ever really happen?

The answer - Absolutely not Want to know why? then read on


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As air rises it expands, cools and condenses (if it rises far enough it freezes) however as it comes back towards earth it compresses and warms.




Now if you notice in the video at the top of the page the air at the top of the eye was dry so that means that the air would warm at the dry adiabatic lapse rate of 10 degrees per kilometre. The tops of those "cold core" cyclones are likely to be between 12 and 20 kilometres high therefore if we were to take a parcel of air from the top of that eye (isolated it in from all the surrounding air) and forcefully push it all the way to the ground, it would warm by between 120 and 200 degrees celsius (depending where exactly we dropped it from). So seeing people heating up on the ground, sweating and possibly cooking if exposed long enough while in the middle of a rapidly melting snow field would probably be a more accurate representation of what should have happened. Let's say we take the temperature of New York (where a lot of the storyline revolves around) about 15000m above the surface of the earth today and drop an air parcel from there (it would take a lot of compressive force to drop it all the way to the ground), we assume the air parcel has no moisture/very limited moisture because the eye is obviously a dry place (because we see no condensation/clouds in it). Today, the temperature in New York 15000m up is a negative 61 degrees by the time we slammed that air parcel onto the ground the temperature of that little air parcel would be a whopping 150 degrees warmer and would be cooking our insides at a positive 89 degrees celsius (almost as hot as the boiling point of water). With the eye moving slowly at about 20km/hr and assuming we saw that the eye in the movie is a hundred kilometres across, very few people would be able to survive but it would be the heat, not the cold that they couldn't survive.


Now in reality if air was to descend it only descends a portion of the way to the ground before it mixes with air above/below it. As soon as the descending air becomes warmer than its surrounding air parcels it would begin to rise again. That's why it would be impossible to see air from the top of the troposphere descend directly to ground without vertically mixing unless an almighty compression force was applied to it and the air was insulated from the surrounding atmosphere. Still you gotta admit it makes for a freakin cool (no pun intended) freezing scene. Check this one out too.




But Nitso, in the eye of a Tropical Cyclone, the air descends, why doesn't that burn you up? Absolutely it does and it too undergoes a lot of compression warming at the top of a Tropical Cyclone the air is about -70/-80 degrees (notice how the blue in the image below turns red in the centre of the eye, that is the compression warming process happening). In our 'on the ground' experience we can definitely feel the hotter air inside the eye compared to the cooler (much stronger)air outside the eye. But that descending stable air in the eye is well mixed with the air above it, below it and on the edges of the eyewall. And while the air is warmer it also remains near saturated. We have not been in an eye yet where the relative humidity of the air has dropped below 90% even with that small rise in temperature there is a small rise in the dew point as well. Therefore the descending air that undergoes compression warming does so at a much slower rate because it is moist and well mixed. So while it definitely is warmer in the eye of a Tropical Cyclone at least that's what we feel on the ground, the temperature jumps by just a couple of degrees and doesn't melt us and certainly doesn't freeze us.



So it would be fair to say the temperature difference at the ground in the eye of those cold cored cyclones in "The Day After Tomorrow" should be warmer than the surrounding air not colder, but it would be unrealistic to assume that they would warm up without being mixed as they descend. So while my 'melting humans' scenario is slightly more plausible than the Hollywood frozen humans version, in reality because air doesn't exist in a vacuum and air parcels interact with the atmosphere around them both extremes would never happen in the real world. But by all means check out the movie for all its cool scenes, just don't trust all its scientific bits. I'm not a person you wanna watch a weather movie with, trust me on that....

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