Lecture 7: Air pressure and wind
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- เผยแพร่เมื่อ 15 ก.ย. 2024
- UPDATE: I'm making materials available for all of my lectures on my website (melstrong.org) for either those bored folks stuck at home due to the virus or teachers looking for online content. For this lecture I have the maps I am writing on in the lecture and an assignment using such maps with the answer key.
Introduction to air pressure and wind. We go over pressure units, sea level pressure, causes of high and low pressure, Coriolis effect, and wind direction.
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.
Actually after each your lessons I realize how lucky am I to have found your channel on TH-cam! It’s the best meteorology class I’ve ever had! Congratulations for this great work 👌👏🍀😊🙌
Thank you for this comment and your others as well. I appreciate knowing that someone finds these useful!
Same +1
@@MelStrongJust wanted to say how much we miss your lectures! I hope you are doing great right now.
Mel, thank you for making these public. I’m trying to prevent myself from binge watching so I can spend some time thinking about each lecture! You’re an excellent educator.
I also appreciate these videos. I am taking online classes, relying solely on course textbooks. You have helped me understand these concepts much clearer with your explanations. Thank you!
Thanks for the feedback...glad to know you are getting something out of them!
You are a great teacher! Thanks for posting and sharing with everyone.
So happy I came across your content. Amazing lectures organized and presented fantastically. Thank you!!!
I have learned so much from this channel. Hard to put in words - but have no words to describe the pleasure and gratitude. You are really an amazing teacher - and i thank God (TH-cam Algo). If i can be of any assistance - please reach out.
Old barometers have markings to predict weather. I used that idea with my modern digital barometers to predict weather. It's been pretty accurate but took trial and error to figure out the actual numbers.
< 964 Storm
< 983 Rain
< 986 50/50
< 989 cloudy
> 1000 Sunny
I also found a rapid drop of 4 mbar usually indicates bad weather coming soon.
I'm now trying to implement dewpoint to better aid my predictions.
Very eye opening concepts about pressure and air patterns also explained excellently. I am absolutely astonished that your channel hasn't exploded in growth yet. This channel is 100% a hidden gem that too many people will skip over because they don't actually like to use their brain themselves in todays world. Keep up this great work!
Americans struggling with SI units is such an amusing thing to watch!
Thank you again for such a good lecture. This course has been amazing.
You are a great teacher Mel. Clear explanation and too the point conclusion.
Absolutely brilliant videos Mel !! Thank you. Weather is such a complex topic. You have achieved what many text books didn't get back in the day !! Merry Christmas.
Studying for my ATPL license... this is gold. thank you such much!
These are outstanding lectures
You the man, thank you and I think you have done a great job!!
Thanks for your comment - hope it was helpful
Blessed are those aviators who never fly across the equator!
Thanks for these beautiful lectures . Hope you're well and happy 😊
Mel, any chance you could do a lecture combining a lot of /your lecture materials as they apply to soundings, skew-t log p?
Excellent lectures Mel
When I started as a weather observer in the USAF in 1980, we had a mercurial barometer in a wood case attached to the wall to protect the long glass tube (about 2 feet high). We used a desktop aneroid barometer for pressure readings, but the mercurial was used to verify once a week that the android was working correctly by comparing the readings. The mercury was sealed in the glass tube, which was probably safe, though I did play with a ball of mercury from a broken thermometer (before we knew how dangerous mercury was).
No brane dammige notid yaet.
These mercurial barometers were removed entirely from AF inventory around 1984. Sometime later, the aneroid barometer was replaced with red digital LED readouts, similar to the versions used everywhere today.
you explain things so well. thank you!
Thank you so much for your videos!
Amazing lectures
very easy to understand, lecture. thanks.
Wow, excellent lecture!!! Thank You!!
Thanks for your comment!
Despite knowing CW and CCW wind circulation around highs and lows in the northern hemisphere I had thought that winds flow across the gradient (perpendicular to contour lines). Sort of high pressure to low pressure. But due to Coriolis Effect high to low are deflected to the right. Thanks for clearing this up. I worked in electronic navigation in the day and we had to keep a platform locally level with gyros so the accelerometers would reflect only the boats velocity and no effect of gravity due to tilting. Two effects that had to be corrected were 'earth rate of 15.0159 degs/hr' and Coriolis, for movement north-south. My instructors 4 decades ago used a 33 RPM record player to demonstrate Coriolis. =)
Winds would cross the gradient if there was no planetary rotation. But if that happened, then the pressure difference would balance out and the gradient would almost immediately disappear. The Coriolis force is what actually keeps the winds going. The math behind this reveals that the pressure gradient force is always perfectly balanced by the Coriolis force in the absence of friction. So in the mid and upper atmosphere, we assume that the winds are going parallel to the pressure gradient contours. However, near the surface there are frictional effects as the wind encounters the surface. So there the winds do start to cross the pressure gradient contours. In a low pressure, the winds cross the contours spiraling inwards towards the center of the Low. In a high pressure, the winds cross the contours as the wind moves away from the center of the High.
@@MelStrong Thanks for clarifying wind directions for H and L near the surface. This is very helpful.
@@MelStrong Just let me say "Wow!" for the lessons! I started with lesson #1 yesterday and have been binging ever since!
Question about the wind characteristics - do high/low pressure systems have any discernible effect on lift/descent in a more 3D perspective to wind direction? I imagine the winds for the northern hemisphere in a low pressure system lifting & turning counter-clockwise in a corkscrew fashion vs just laterally circling at any given altitude (exempting the influences of terrain closer to the Earth's surface), and inversely for high's - twisting downward in a clockwise direction.
Given the grand scale of the systems, is it fair to say any such 'twist' would be barely discernible, even at higher altitudes?
Thanks!
@@bwjbrown While there is technically a vertical component to the wind, the scales are such that if you were out moving about in a low pressure system or a high pressure system, you wouldn't really notice it. The scales of low pressure systems and high pressure systems are so large that any point the wind would seem to be straight and level with the surface. In textbooks the wind in a northern hemispheric Low is shown as a counterclockwise ascending corkscrew. But they are always draw exaggerated.
@@MelStrong I'm a pilot working on his flight instructor rating and I've learned more from you than everyone I've ever met about weather. I can actually read a darn WX chart now with some clarity. Why they cross the contour at the surface never made sense to me. Thanks so much for your time and your effort. Perhaps I'll learn as I watch more but at what altitude roughly does the friction start to no longer "counter" the rotation of the Earth?
Excellent
With what you told about using the SLP-map to tell wind direction and speed, is it then so that when there is a hurricane, the contours (those pressure lines) are extremely close to each other?
Mel, one question! how does these websites like null school manage to find this pressure, currents, wind information from all over the earths surface ? even on oceans where its impossible to install the equipments to measure ?
Awesome, as all of your other videos.
🎯 Key Takeaways for quick navigation:
00:06 *🌬️ Introduction to Air Pressure and Its Effects*
- Introduction to the concept of air pressure and how it's created by air molecules colliding with surfaces.
- Explanation of how air pressure varies with altitude, being higher at the surface and decreasing with height.
- Introduction of psi (pounds per square inch) as a unit to measure air pressure and its application in daily life, such as in tires.
02:22 *🌊 Understanding Air Pressure through Water Analogies*
- Discussion on how air pressure is felt from all directions, similar to water pressure, and why we don't typically notice air pressure.
- Comparison of air and water as fluids and their behavior under pressure.
- Introduction to the concept of vacuum and air pressure's role in liquid movement through a straw.
04:31 *📏 Historical Attempts to Measure Air Pressure*
- Early experiments to measure air pressure, including attempts to pump water using vacuums and the discovery that water cannot be sucked up more than 34 feet.
- Introduction of mercury in experiments to measure air pressure, leading to the development of the barometer by Torricelli.
- Observation of mercury's height changes with weather, leading to early weather prediction methods.
09:28 *🌡️ Modern Measurements of Air Pressure*
- Transition from mercury barometers to modern instruments measuring air pressure in inches of mercury, millibars, and hectopascals.
- Discussion on the dangers of mercury and the evolution of barometer technology.
- Explanation of air pressure measurement units used today, including millibars and the equivalence of millibars to hectopascals.
13:04 *🗺️ Air Pressure Variation with Elevation and Weather*
- How air pressure changes with elevation, demonstrating lower pressure at higher altitudes.
- Introduction to the concept of sea level pressure as a standard measure to compare air pressures at different elevations.
- Explanation of how air pressure varies slightly with weather conditions and the significance of these changes.
16:07 *☁️ Impact of Air Pressure on Weather Patterns*
- Description of how converging and diverging winds aloft affect air pressure and weather conditions at the surface.
- Explanation of high and low-pressure systems, their representation on weather maps, and their implications for cloud formation and weather predictions.
- Introduction to thermal lows and cold highs as mechanisms for creating low and high-pressure systems.
21:00 *🔍 Analyzing Air Pressure on Global and Local Scales*
- Examination of global air pressure patterns using real-time data and their correlation with weather phenomena.
- Discussion of thermal lows along the equator and cold core highs in regions like Siberia.
- Explanation of how air pressure data informs weather predictions and the significance of variations in air pressure across different regions.
25:03 *📊 Correcting Air Pressure Readings for Elevation*
- Challenges in comparing air pressure readings due to elevation differences and the solution through mathematical correction.
- Detailed explanation of reporting air pressure as if at sea level, allowing for accurate weather prediction and comparison across locations.
- Importance of sea level pressure in meteorology and weather forecasting.
34:59 *🌎 Identification of High and Low Pressure Areas*
- Detailed analysis of pressure gradients on weather maps, highlighting high and low-pressure zones.
- Introduction to the concept of troughs and ridges in meteorology, and how they indicate areas of low and high pressure, respectively.
- Explanation of how to interpret U-shaped contours as troughs, indicating areas of rising air and potential cloud formation.
38:25 *🌀 Wind Patterns around Highs and Lows*
- Observation of wind directions around high and low-pressure areas using real-time data.
- Explanation of clockwise (high pressure) and counterclockwise (low pressure) wind patterns in the Northern Hemisphere, and their reversal in the Southern Hemisphere.
- Introduction to the Coriolis effect and its role in affecting wind direction around pressure systems.
44:06 *🌬️ The Coriolis Effect Explained*
- Detailed explanation of the Coriolis effect and its impact on wind direction at different latitudes.
- Demonstrations of the Coriolis effect using practical examples and animations to show how it causes wind to deflect.
- Discussion on common misconceptions related to the Coriolis effect, emphasizing its relevance only at large scales.
47:00 *🗺️ Predicting Wind Direction and Speed*
- Techniques for predicting wind direction based on high and low-pressure areas on a weather map.
- Explanation of how wind direction is always described in terms of where it's coming from and how to use contour lines to determine wind direction.
- Discussion on pressure gradients and their relation to wind speed, highlighting areas of faster winds where pressure contours are closely spaced.
Made with HARPA AI
Is there an estimate as to how much of the Earth's atmosphere is lost into space as the Earth rotates around the sun?
Sir you are awsome ❤
That is highly doubtful but thanks for your comment!
I wish Mel had been one of my school teachers
Great snd simplicity
Mel is that you rippin a quick drum solo at the beginning?
I wouldn't exactly call it 'rippin' but yes
This guy is just awesome
Thanks, i personally grasp want to lived in... cool placed.😊
I don't understand how sinking air can be stable. The air at every height is at the temperature it warmed to by adiabatic heating as it sank from near the tropopause. So if sunlight absorbed by the ground warms the air at the surface even a little, the dry adiabatic lapse rate should have it stay the warmer than the surrounding air by the same amount, all the way up. It should be able to convect freely, i.e. the air ought to be unstable, with thermals extending very high.
Hi Mel, I just watched your video and have a question about what's going on with the wind direction in my local area. I live in Duluth, MN. There's a high pressure system to the east and a low pressure system to the west/south-west. The wind is doing what I would expect it to, blowing from the south east but then it hits lake superior and nose dives towards the south west. What's causing this sudden change in the direction of the wind? The temp of the lake is cold, about 35 degrees, does it have something to do with that? Thanks!
I took screenshots from the windy app but unfortunately don't think I'm able to post them here.
Just guessing at which time you are looking at, I think you are in a region of high wind shear. There are a series of closely spaced low pressure and high pressure systems and you are in between them as they pass through. Being so close to these alternating systems, there is a lot of wind shear (changing wind direction with height). If your windy app allows you, compare the wind direction at surface, 1000mb, 850mb, 700mb, and 500mb. You will see that the wind direction radically changes. This is probably causing the wiggle in direction you are noticing. For example, on 6-12 17:00UTC you are under a high with lows on either side of you - look at how the wind changes with height.
In general, when predicting the wind direction from the position of Highs and Lows, we are thinking about the wind 700mb and above. Anything lower than that is subject to wind friction so you get strange artifacts.
Wow, that is crazy! The wind is almost blowing in every direction! That makes sense, it looks like my app is showing me wind pretty close to the surface. Higher up it's very different. So while the wind was blowing from the northeast at the surface, as the temps warmed today we started to get dense fog moving north/northwest off of the lake. Could the fog moving in that direction be the wind up at the higher altitude picking up the evaporation off the lake?
@@MelStrong Thank you for your reply!
I was watching another weather video and he mentioned that say in a low the air on the surface is counterclockwise however when it rises it diverges at the top in a clockwise direction. That makes sense but I never thought about it. As a pilot we often fly near the tropopause which I believe would be the top of where a low would spill out and turn opposite the surface winds? True?
Well it is a bit more complicated than that. Circulation around the low near the surface appears orderly and counterclockwise in the northern hemisphere. However, as you start moving farther up into the atmosphere, things become strange. For example, take a large low-pressure system that is part of a mid-latitude cyclone. The reason that this low pressure exists in the first place is because of divergence in the upper atmosphere along a section of a large wave along the polar front. So that means that if you were to follow the circulation from the surface to the upper atmosphere, you would see that there is a transition from the nice and orderly counterclockwise rotation shown in weather books to a much more laminar-appearing flow above. On top of that, the column of circulation is tilted with height, so the 'top' isn't even directly over the 'bottom'. So in your question, if you are visualizing counterclockwise wind spiraling into the low at the surface and the spiraling clockwise out of the low above, then no that doesn't really happen like that.
You can do a little experiment to see this for yourself. In many of these lectures I use earth.nullschool.net. If you go there, you can find a nice big midlatitude cyclone somewhere in the world and observe the circulation around the surface low. Now change the pressure level (1000mb-->850mb-->700mb-->500mb etc) and you will see how the circulation changes with height.
@@MelStrong thank you.
Hidden message at the five minute mark?
I understood the section on why high pressures turn clockwise. I watched it several times and I’m unclear as to why the low pressure turns counterclockwise it didn’t seem to be addressed in your video. What am I missing?
Does it make sense to you that anything moving in the northern hemisphere must turn to the right?
Coriolis Force/Effect - look it up. You can do a thought experiment - suppose you at equator and you kick a ball straight north...
7:35
I believe your blood pressure is also measured in mm Hg.
great!
I like your cat.
The Cat!!
Yes!! it probably knows more about weather systems than we do :)
So far have enjoyed your presentations… until the repugnant political toned picture… come on - I agree with freedom of speech but as an educator would have expected a more disciplined behavior. I would suggest you start another channel if you need to insult a former president - disappointed is all I can say.
That was not cool. I caught that. I was enjoying your lectures , but I'll unsubscribe
You aren't that important. You sound insane. Go away.
many thanks