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When referencing the tropopause, pilots and weather types converse using the truncated version, trop. But it rhymes with rope, not top. I am unsure why trope wasn’t used.
The standard atmosphere
You are probably wondering why you are reading about the intricacies of the tropopause – where the first two layers of the atmosphere meet. After all, your single-engine piston aircraft struggles to climb to 10,000 feet, and Sporty’s Air Facts readership caters primarily to aircraft below the altimeter setting region of 18,000 feet. However, you may be training to fly aircraft that soar higher, or being an airline pilot is your destiny. Perhaps you are flying a business jet that pokes up to great heights (frequently above an airliner), or you want to impress your peers by estimating the height of a developing thunderstorm because you peeked at the tropopause height coded on high-altitude weather charts. Flight training and flying opportunities are transpiring at warp speed, so if cruising at flight levels is where you want to be, read on!
The suffix pause denotes a boundary among the four layers of the atmosphere. The first pause is the tropopause, dividing the troposphere and stratosphere. There, jet streams corkscrew the globe like atmospheric snakes (think meandering air currents with extraordinary velocity), and create turbulence. Airline pilots strive to bestow passengers with a smooth ride, so knowing where the tropopause is in relation to the aircraft’s flight level is a must.
The illusive height of the tropopause is higher near the equator than at the poles. As you near the equator, tropopause heights reach approximately 55,000 feet, so you won’t be flying over the equatorial tropopause unless you pilot the SR-71 spy plane or sat in Concorde before 2003. The tropopause is why thunderstorm tops flatten and spread out, resembling a blacksmith’s anvil, as their upward momentum abates due to temperatures aloft becoming steady (isothermal) or increasing. Yes, some thunderstorms poke through the tropopause, but they are rare, perhaps giving telltale signs of a tornado. If compelled to fly over the top of these meteorological monsters, give yourself several thousand feet; if there is a strong wind aloft, give yourself even more clearance. Contending with these beasts is like walking gingerly on a thinly frozen lake. One hiccup, and you’re going in.
But why do we want to fly at these contentious heights? Jet engines are most efficient at higher altitudes— coincidentally, near the tropopause. Pilots who fly just below, in, and above the tropopause, i.e., flirt with this boundary, always want to know its whereabouts. Most flight plan packages depict the tropopause height for each waypoint along the flight plan.
The ICAO (International Civil Aviation Organization) pegs the average tropopause height at 36,089 feet at 45 degrees latitude. Why so precise? As it happens, the average tropopause height defined by ICAO is metric; thus, 36,089 feet is an exact imperial conversion of the tropopause average of 11,000 meters. Tropopause height can vary from 25,000 feet at the poles to more than double that height — between 52,000 and 56,000 feet — at the equator. The tropopause height also varies by season. It rises in the summer and lowers in the winter, and each air mass brings its own inherent tropopause height. It also varies by day, known as diurnal variance, which is lower at night and higher during the day.
According to ICAO, the average tropopause temperature is −56.5° C. For me, I am always looking at temperatures aloft. If we are at flight level 380 and the outside temperature is
-50° C, it means we are above the trop. I frequently point out temperature trends to my flying partner, but I usually get blank stares, with most thinking what is the skipper talking about? I have seen temperatures quickly transition from frigid -60° C to a balmy -45° C within minutes. Most weather books that include atmospheric temperature profiles do not fine-tune this sometimes significant inversion. My two weather books are at fault as well. But with decades of flirting with the trop, this inversion certainly exists.
Pilots who fly just below, in, and above the tropopause always want to know its whereabouts.
There are four basic inversions: 1. frontal (from warm fronts). 2. subsidence, when air sinks and warms due to a high-pressure system. 3. nocturnal, when the overlying air cools at night, causing warm air aloft. 4. the tropopause.
Importance of the tropopause:
- It acts like a “lid,” capping most weather because the stable isothermal layer (or inversion) extends several thousand feet above the tropopause. However, thunderstorms can penetrate the tropopause depending on severity because the violent updrafts have enough energy to poke through the boundary.
- Turbulence frequently occurs at the tropopause and is not necessarily associated with strong winds. The winds may only be 20 to 30 knots, but you are shaking about in moderate turbulence.
- Winds typically increase with height up to the tropopause. They decrease above it because temperatures are relatively constant. This can be seen on wind read-outs in the flight deck.
- The tropopause marks the tops of high clouds, which may produce turbulence and affect visibility.
Clues to locate the trop from your flight deck:
- Where temperature stops decreasing or rises.
- Height of the thunderstorm anvil.
- Height of the cirrus clouds gives a good indicator of where the trop exists and when turbulence may end.
- Winds are typically strongest just below the tropopause.
- Most flight plans will depict tropopause height for every waypoint along the route.
- Flirting with the tropopause - October 16, 2023
- Fate was on my side – a lesson in scud running - March 22, 2023
Very interesting and informative article, thank you for sharing
Hector. Thanks for the kind words. Flying in and near the tropopause is part of doing business for many pilots. Happy flying! Captain Doug
Captain Morris,
Thank you for the great article.
I was taught years ago that if you watch the Trop Altitude climb along the flight plan then it drops significantly, there is a good chance for clear air turbulence just past the drop in Altitude. Kind of like flying thru an lenticular cloud.
Any truth to that?
Ed
Hi Ed. Absolutely, regarding your comment. Clear air turbulence exists in the “fold” or “break” of the tropopause, i.e., where two airmasses collide. It’s akin to a fold-in a sheet of paper. Years ago, we were “coasting out” from Ireland en route back to Canada, where we encountered continuous moderate turbulence. Once we cut through the “trop,” the nasty bumps stopped within seconds. It can be that abrupt.
And yes, likening it to a lenticular cloud is valid. The lens shape exists because the rising air hits the tropopause that acts like a lid.
I have a dedicated chapter in my book, Pilot Weather: From Solo to the Airlines, regarding the tropopause. (I hope I am allowed to hype up my book). :)
Thank you for the comment!
Captain Doug
Still a bit confused. Maybe I missed it, but is the intention to cruise in the tropopause, or above it? Is that why I’ve been on flights where the captain announces an altitude change, looking for smooth air? Thanks
Hi Robert. I hope my article didn’t confuse you regarding the tenacious tropopause. Overall, you want to avoid the tropopause because it is a boundary, a lid, a transition, a fold, a break, and an area where bumps lurk. You want to be below or above it, but not in it. Flirting with the “trop” is a bouncy proposition. So yes, changing altitude to find smooth air is what pilots prefer.
Thank you!
Captain Doug
Cap. Doug Morris
Behind being an excellent meteorologist, You’re, sir, a very good teacher, what is attested by the answers You’ve gave to the preceding comms.
José. Those are incredibly kind words. Gracious! Captain D
I have been flying 49 years mostly blow the 18,000 mark until the last 5 years , great explanation of all. Thanks
Hi Ken. It sounds like you are now flying corporate. 49 years? Wow! Welcome to the flight levels and the illusive tropopause. Thanks for your comment. Captain D
Hi Captain Morris.
Great writing! I’m also nearing retirement and fly a lowly King Air 250, now. Less nights away from home. Previously, I flew long range MedEVac Lear 36’s. We flew many flights to Asia, usually via Alaska. One early morning flight many years ago, I clearly recall an atmospheric phenomenon that I haven’t seen before or since. Cirrus clouds were shaped in a long helical pattern and extended for a very long distance. I can’t remember our altitude, but I’m sure it was above the tropopause as we were coming back from Japan somewhere over the gulf of Alaska. Lear 36’s are certified to FL450 and we typically flew there for maximum economy. Maybe you’ve seen this, too?
Hi Terry. Flying a King Air is not so bad. I’m sure it wasn’t cirrus clouds you witnessed above the tropopause. Maybe they were nacreous clouds, also known as “mother of pearl” clouds. Or perhaps noctilucent clouds?
Both of these clouds exist well above the tropopause.
FL 450? I can only soar to FL 410 in my Dreamliner. On that note, I am out the door for London, Heathrow. I might get a peek at the northern lights. Keep your eye on the sky, and happy flying! Captain D
Yes, most likely not cirrus clouds. When I look at internet images, I see corkscrew clouds that seem to be associated with contrails. My recollection is that the clouds I saw were below me and there is a lot of heavy jet freighter traffic across the Gulf of Alaska in the 30’s so that may be what I saw.
When reflecting upon why I would not have seen more of this, I realize that many of my flights were in the dark. However, that was a great way to see spectacular Aurora’s and one of my copilots introduced me to Star Chart and Sky Guide apps, so we passed the time cataloging the cosmos.
Terry. That’s another phenomenon, for sure, contrails. I heard about the Apps. Thanks for reminding me. Years ago, a captain I flew with to Europe pointed out the stars and planets, and he knew where they would be in relation to us while we trekked eastward through the night.
I am now on my way to the airport. I see flight dispatch has us above the tropopause most of the flight. We have to poke through it in the climb, but it should be smooth, and we are 15 minutes ahead of schedule. After that, I know a beer or two awaits me in London. Thanks again. Captain D
When Captain Morris referred to his inflight temperature of -50C being below the tropopause, I think that he was comparing it to a standard day in the theoretical world of the International Standard Atmosphere where the tropopause is marked by a temperature of -56.5C. But in the real world as I understand it, the tropopause may either be warmer or colder than the nominal -56.5C of the average atmosphere. The captain knows this better than I do. For example, today’s aviation forecast from the US National Weather Service shows that the tropopause temperature is forecast to vary from -85.5C to -28.1C around the globe with an average of -65.9C. The captain’s comments about the variation in the height of the tropopause are confirmed by this same forecast that shows a low of 17633 feet to a high of 59035 feet with an average of 40558 feet. These are worldwide numbers that consider the conditions at the poles and around the equator. But they are quite remarkable differences from the standard atmosphere’s theoretical tropopause values of -56.5C and 36089 feet respectively. It would seem to me that it would be almost impossible for a flight crew to determine their position relative to the real tropopause in flight with variations such as these.
Neil. You undoubtedly did your homework. Well done! I agree. My temperature rule of thumb has caveats. It doesn’t work everywhere, particularly near the poles or at the equator. Yes, the average equatorial tropopause temperature is anywhere from -70C to -80C, but no one except an SR-71 driver will ever fly near it. I’ve crossed the equator many times, contending with the ITCZ (Intertropical Convergence Zone), and the tropopause was 15,000 to 20,000 feet above us. I used to transit the North Pole (it hasn’t happened lately due to Russian airspace closure), and the tropopause was thousands of feet below. My temperature rule of thumb works for the USA, much of Canada, the North Atlantic, Europe, the North Pacific, and most of Asia.
FYI, I mentioned encountering -50C ABOVE the tropopause, not BELOW, as you noted. This sometimes confuses pilots because -50C would also be encountered as one ascends to the tropopause from below. In other words, there are two -50C values, one below and one above the trop.
I am now in London after a 7.5-hour flight. We flew at FL370, and the temperatures soared to plus 13C above average, i.e., -43C. We were above the trop for most of the flight with smooth conditions.
Thank you for taking the time. I am impressed with your data. I’ve been flying jet aircraft for over 30 years and flirted with the “trop” thousands of times. Yes, many rules of thumb have caveats. Mother nature is consistent with inconsistencies.
Captain D is in London, England, where the trop sits at 36,600 feet this morning.
Hi Capt. D:
At your weight and ISA+13 was 370 was your ceiling or were the tracks so full you were stuck there? The Lear 36XR’s that I flew had an endurance to dry tanks of about 6+45. Going across the Pacific tracks to Hawaii we always tried real hard to get above 410 before “Coast Out”. In the track non-radar environment it’s was very hard to get a climb clearance from Arinc. Once above 410 we routinely could step climb to 440 since all the airliner traffic was below us. In the 30’s with winter winds, making Hawaii with reserves became dicey.
Hi Terry. We were on a random track north of the eastbound tracks (about 100 miles south of Greenland). The Dreamliner usually gets up to altitude pronto, but last night, it was happy at FL370. We did climb to FL390 before making landfall. I’m glad you acknowledged winter winds are stronger. I am surprised how some pilots don’t know that. I appreciate your curiosity.
Hi Terry. We were on a random track north of the eastbound tracks (about 100 miles south of Greenland). The Dreamliner usually gets up to altitude pronto, but last night, it was happy at FL370. We did climb to FL390 before making landfall. I’m glad you acknowledged winter winds are stronger. I am surprised how some pilots don’t know that.
Indeed it may be problematic to use temperature to determine one’s altitude in relation to the tropopause. However, as Captain Morris wrote, wind is a very good indicator. As I mentioned in a previous note, I used to fly Lears. Several models are certified to FL510 and I was at that altitude during various flights. One in particular sticks in my mind. It was before GPS but the FMS calculated position and winds from what we referred to as Rho-Rho which became Rho-Rho-Rho when DME off more than two stations was used for triangulation. It was very accurate. As we climbed thru the 30’s we were seeing wind readouts in the 130 knot range. Not at all unusual in the winter. As we climbed thru about FL470 the wind readout was 2 knots.
Terry. Thanks for your recount. Certified up to 51,000 feet? I would be nervous at that height. LOL
Your climb depicts how winds vary with height in relation to the tropopause – another component to the equation of “trop” whereabouts.
It’s great fun to recount my hi altitude time in Lears now that I seldom get above 300 in the KA. I also worked at Lear in KICT so was involved in certification. One of the questions that comes up is why aren’t airliners certified higher than ~410? My understanding which I’m sure you can confirm or refute is physiological. Without a pressure suit, blood will boil somewhere above FL410 even with pressure breathing.
Lears that were certified above 410 have an emergency pressurization system that triggers on when the cabin altitude exceeds 12,000’. This system bypasses all the normal conditioning components of the pressurization system and ducts bleed air straight off the engines into the cabin. In flight test we had to demonstrate that it would hold the cabin at or below FL200 even with the equivalent of a blown window. The test setup included a fast acting ball valve connected to a plug in the emergency exit hatch which opened to the outside. My recollection is that the exit area of the valve had a diameter of 2 3/8”. How that was equivalent to a blown window, I don’t recall. However, the flight test procedure was to snap open this valve at 510 and initiate an emergency descent all the while recording cabin altitude. Somehow we passed but I wasn’t the flight test engineer on this flight.
Soon after that, though, I did the hot fuel test to 510. That was a great demo of jet pump physics as jet pumps in the wing provide the positive head pressure to prevent cavitation in the the high pressure engine pumps. Same as the 787 or have they gone to electric pumps since my recollection is that it was designed to be an all electric airplane?
We had the test plane in the paint hangar overnight so the fuel temp was +120F. We pulled out and quickly taxied and took off. We coordinated with Center so we had an unrestricted climb to 510. I was monitoring the flight test panel with the jet pump outlet pressures. IIRC, those started out above 30 psi but they were decreasing toward 20 due to the hot fuel and decreasing fuel tank pressure as we climbed thru the 30’s. 20 psi was the min allowable boost. However, as we climbed through about 400 the pressure started increasing again as the fuel temperature cooled.
Reflecting on my reply, I may have this wrong. Pressure breathing is required above 410 which would not be available to airliner cabin passengers. Blood boils above 510 which is how the limit for Lears was set.
Terry. You can recount stories with great accuracy. And you have done things in an airplane I would not want to venture with. I like my flights “straight and level.” I am hearing rumors they want to increase the altitude to where masks must be worn. The Dreamliner is capable of FL430, but we are limited to FL410 due to the oxygen mask criteria.
Captain D:
Thanks for the article and knowledgeable replies. My accounts of flights probably make them sound riskier than they actually were. Learjet was a pretty small company until the buyout by Bombardier (when I left) and as such management was smart about not making big changes that would be financially risky (like from a crash). Our flight test cards were well vetted and thoroughly discussed such that, really, little unanticipated or unplanned occurred. My old flight test boss had a sign in his office that I think the Company took to heart. I certainly have tried to over my 47 year career:
“Superior pilots use their superior judgement to stay out of situations where they’d have to use their superior skill”.
Hi Terry. We were on a random track north of the eastbound tracks (about 100 miles south of Greenland). The Dreamliner usually gets up to altitude pronto, but last night, it was happy at FL370. We did climb to FL390 before making landfall. I’m glad you acknowledged winter winds are stronger. I am surprised how some pilots don’t know that. I appreciate your curiosity.
Thanks for, “ Hyping up”, your book. I just ordered it.
Terry. I truly appreciate you buying my book. Captain. D
Terry. You nailed it with that line from your flight test boss. I am still in LHR and just found out I am now deadheading home. They subbed my flight to an Airbus 330. I get to critique the flight from a seat in the back. Pilots can be the worst passengers. LOL
In the 737, we would regularly look forward to climbing above the trop hoping for a smooth ride, only to find that as we did, the temperature increased, causing the airplane to exceed its altitude limit and forcing us to descend back down to cooler air. Be careful climbing through the trop!
Hi Jim. That must have been with an older model 737. My Dreamliner GENX engines don’t mind the warmer air. Plus, my airplane frequently suggests the “higher, the better.” But I agree, warmer temperatures above the trop can be a hindrance. Iconic Robert Buck insinuated the same. I am out the door for another LAX venture and will be flirting with the trop along the way. Thanks for the comment.