Lecture 5b (extra): Extra help with determining locations of clouds and thermals
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- Опубликовано: 29 сен 2024
- Continuing with the handout I started in the lecture on cloud formation and convection. We use the dry adiabatic lapse rate (DALR) and the saturated adiabatic lapse rate to determine where any clouds would form. I have this handout posted on my website melstrong.org under the materials for Lecture 5.
This was originally part of the Blue Planet lecture series at the University of New Mexico. If you are interested in weather, this lecture is now part of my "weather short course" playlist, where there are another ~15 videos similar to this one.
Got my private pilot license recently. Although there’s quite good theory on the weather during the course your videos are soooo brilliant! Thanks for doing them!
Thanks for the feedback! Glad you find them useful!
The lapse rate in that cold air aloft is more than the dry adiabatic lapse rate. So any slight turbulence that makes a bit of that air get higher than the air around it would set off convection, which would keep transporting heat upward until it brings the lapse rate down to the dry adiabatic rate. Is the air in the example impossible, or is there something that can cool the lower part or warm the higher part fast enough that the convection can't keep up? Radiating heat to space cools the air, but I didn't think that could happen fast enough to get appreciably ahead of convection.
How much to subtract in case of celcius when dew point is equal to temperature when clouds forming?
It's the "don't " point that worries me!?
You are THE MAN! love your lectures. thank you! Q: Say you're standing on a tropical mountain & it's 0°C at 5600 m and -2°C at 6000 m, but -9°C at 6700 m. A very close cumulus congestus turret to the windward direction of the mountain is approaching & extends up past 6700 m altitude. Is this typical of deep moist convection? What kind of circumstances produce something like this? It seems so high & so cold up there that it's hard to imagine!
I've watched all of your lectures with interest. From a glider pilot's perspective it's just a gold mine of information. Thank you.
Thanks for your great lectures! My question is: Why would the air drop after reaching the top of the cloud or thermal? The air would have the same temperature as the surroundings so wouldn't it simply stay there? Why would is cool further?
In reality the air moves in both directions at once. As soon as air starts to rise upwards, there must be air from above moving downwards to replace it. To make the entire process easier to visualize for my students, I concentrate on one single parcel of air moving upwards. But, the process is reversible. So when the atmosphere is conducive to convection (unstable), then a parcel of air at any altitude will want to continue to rise if initially pushed upwards. But at the same time, an air parcel will continue to sink if pushed downwards. I don't do any examples of the latter in any of these lectures, but this is why you get both updrafts and downdrafts within thunderstorms for example. These two winds can be very strong and right next to each other within the same cloud. On a day with a lot of convection, the sky might be filled with convecting cumulus congestus. But in between the clouds are downward moving air columns that you can't see. If you were fly through it though, you would definitely feel the presence of both the updrafts and the downdrafts.
@@MelStrong Thank you very much for the explanation!
Enjoyed this supplemental video. Thanks.
If a cold front is moving too fast will this break up the front line as it moves into a warmer air mass?
I have a question: what happens if the amount of water in that air parcel is actually not enough to keep feeding the cloud until the air cools down enough to sink again? Is it possible that completely dried air continues rising (with dry adiabatic lapse rate) above a cloud?
Once a cloud has started, you keep making new cloud as long as the parcel is rising. The reason is that the air will continue to get colder and colder, and will keep removing more and more water vapor from the air. At the top of the cloud is growing upwards, both the temperature and the dew point are dropping together - and the relative humidity is remaining at 100%. For example at the very tops of cirrus clouds you might have temperatures of -100 degrees and dew points of -100 degrees. That is super super super dry air, but is still a cloud - a little waft of ice crystals.
A technical way to get air rising above the cloud is if the atmosphere is structured such that there are inversions. Air could rise from the ground, form a cloud up to a certain height, and then stop due to an inversion. But then higher up, the conditions might be conducive for convection, so a new plume of cloud-free air would start rising. So in that case, you would have air rising above a cloud, but not for the reason that you are thinking.
A second technical way to get air rising above the cloud stems from the fact that when I do these calculations, I am assuming that the air does not mix with its surroundings. Which is mostly true. But in fact sometimes you can get mixing of dry air into a vertically rising column of air. If you start mixing air horizontally, then that complicates everything as you could in fact mix in enough dry air to make your cloud disappear but the column of air is still rising.