| GENRE: | AEROSPACE ENVIRONMENTS |
| DIFFICULTY: | ANY LEVEL |
| BACKGROUND MUSIC: | ELEVATION – U2 |
“WHERE AN ETERNAL SILENCE REIGNS”
Perhaps considered one of the more benign methods of flight these days, it may be surprising to some that so much early aerospace medicine was investigated and discovered by balloon.
Whilst it may be hard to imagine it now, ballooning in the later 18th and early 19th Century was the equivalent of supersonic flight in the post-war era or space travel in the 1960s – the edge of the envelope for both technology and humans.
The tragic story of the Zénith balloon flight perhaps highlights this more than anything. The achievements of the brothers Montgolfier and Charles inspired yet another pair of skyward looking siblings to venture into the unknown. The Tissandiers – chemist Gaston (drawn to the skies due to an interest in meteorology) and architect, artist and faithful companion Albert – remained at the cutting edge of aviation throughout their lives.
Their record achievements were numerous. Some examples include inventing the first electric powered dirigible and perhaps the first example of a wartime evacuation by air, escaping a besieged Paris by postal balloon. Some time between these events, however, Gaston, achieved a far less enviable record – the only survivor of the first in-flight fatal incident.
The mission had been planned to ascend to high altitude for both scientific discovery and – befitting of the national rivalries of the time – to win back the altitude record from the British. At the time the highest recorded altitude reached by a crewed flight was 28,900ft by James Glaisher and Henry Coxwell. The flight almost ended disastrously as shortly after this reading Glaisher fell unconscious.
It was only the swift actions of a frost-bitten Coxwell tugging at the balloon’s rip chord with his teeth that allowed them to descent. After 7 minutes of unconsciousness Glaisher awoke and, ever the scientist, continued his recordings. Estimates suggest the apex of this flight was likely actually between 31,200ft to 35,800ft and their discoveries of decreasing moisture at altitude refined understandings of how clouds formed and rain developed. The pigeon they had brought with them did not fare so well…
The Tissandiers, perhaps learning from the experience of their rivals, reached out to none other than Paul Bert to prepare for the flight. In what was perhaps the first ever example of hypoxia training, the chosen crew – Gaston as scientist, pilot Théodore Sivel and engineer Joseph Crocé-Spinelli – journeyed to Paris to experience a simulated 20,000ft in Bert’s hypobaric chamber. Noting the rather “disagreeable effects” at this altitude they decided to bring bags of a gaseous oxygen-air mix to breathe on if symptoms occurred.
Despite these preparations, the crew of the Zénith were not as lucky as Glaisher and Coxwell. A record from Gaston’s journal of the fateful flight on 15 April 1875 can be found here. The balloon lifted to cries of “Lâchez tout” at 11:35 carrying crew, scientific instruments and Bert’s oxygen supply. Despite early symptoms at 23,000ft, pressure to beat the record meant that the team pushed on higher (a future tutorial will look at how such human factors still result in accidents to this day).
As they climbed, the symptoms worsened to the point that, despite being aware of their predicament, the crew seemed unable to take their prepared actions to resolve it. At one point they even threw ballast and equipment over board causing the balloon to rise further still. The recollection is unsurprisingly hazy after this point and it is unclear how and why the balloon descended. After reaching an estimated 28,000ft and some 4 and a half hours after lift off, the Zénith touched down 155 miles from Paris. Upon recovering Gaston attended to his friends to find they had not survived the flight.
Sivel and Crocé-Spinelli were the first aviators to perish in the air. France entered morning for its heroes. Memories of how the Tissandiers and others daringly escaped Paris to bring news to the rest of the country of the capital’s siege added to the sorrow of the lost aviators and adventurers.
A lavish funeral was held at which Paul Bert gave the euology:
“They ascend and death seizes them, without a struggle, without suffering, as a prey fallen to it on those icy regions where an eternal silence reigns. Yes, our unhappy friends have had this strange privilege, this fatal honour, of being the first to die in the heavens…”
The perceived immortality of these early adventurers had been shattered and the scientific community shaken by the hostile world above the clouds. No one would dare take a similar flight for two decades.
INTO THIN AIR
Long duration flights at 39,000ft PA are a regular occurrence these days (at least when there is no global pandemic!) but getting to this point took many years of hard work and personal risk by aerospace medicine specialists. Before we look at why high altitude is such a hostile environment it is important to get some terminology clear.
The ATMOSPHERE
The atmosphere is the layer of gases that surround us due to Earth’s gravity. It is one of the key things that means life as we know it can exist here; it protects us from harmful cosmic radiation, it moderates the temperature, it allows water to exist in liquid form and much more.
To help us understand it better, the atmosphere is split into theoretical levels defined by a variety of characteristics:
| Name | Altitude(*) | Comments |
| Troposphere | 0 – 36,089ft | All known complex life naturally lives in this sphere. |
| Stratosphere | 36,089- 158,000ft | Ruppell’s griffon vulture can just about break into the stratosphere (37,000ft). Bacteria and fungi have been found here, perhaps from volcanic eruptions or updrafts. |
| Mesosphere | 158,000-290,000ft | The top of the mesosphere is the coldest part of the atmosphere (see below). This is sort of a ‘no-mans land’ of aerospace; too high for aircraft but too low for orbital spacecraft meaning we have very little knowledge of this layer. |
| Thermosphere | 290,000ft-1,968,500ft (600km) | The ISS orbits in this layer (between 408-410km AMSL). If the Kármán line is used as a definition of space, a decent proportion of the thermosphere is in ‘space’. |
| Exosphere | 1,968,500ft – ? | Molecules in the exosphere are still bound to the Earth by gravity but the density is so low they collide so rarely that they no longer behave like a gas. By some definitions this reaches as far as halfway to the Moon. |
Between each layer is an imaginary line called a ‘pause’. Each line is named for the layer below so the tropopause is at 36,089ft PA, the stratopause is at 158,000ft PA and so on.
*All altitudes quoted here can and do vary. The thickness of the troposphere varies with latitude as the warmer air at the equator is less dense. This brings us on nicely to our next point.
Flying High
It may seem a silly question but why do aircraft need to know how high they are flying? Obviously it’s good to know you’re not going to hit the ground but it’s equally important when operating over 3 dimensions to have a frame of reference to know you’re not going to temporarily try and share the same space with another aircraft.
Altitude, Elevation, Height…
Whilst colloquially we may use the terms altitude, height and elevation interchangeably, each of these terms has a very specific meaning in aviation.
Altitude is the vertical distance of a point in the air above a reference point (usually Mean Sea Level or MSL)
Elevation is the vertical distance of a point on the ground above MSL
Height is the vertical distance of a point above ground level (AGL)
It pretty easy to see how, if you’re travelling in anything but the slowest aircraft over the plains of Cambridgeshire, your height can change pretty rapidly as the ground rises and falls below you. If a pilot aimed to maintain a fixed height for their flight it would look something like this:
The above diagram might make this initially look sensible. Height above the ground is very important if you are taking off, landing or flying at low level. When you’re all the way up at 30,000ft, however, whether you are flying over a hill or not is all rather irrelevant.
Most aircraft are happiest (and most efficient) when flying straight and level – changing pitch all the time is tiring and inefficient, except in very specific scenarios. It’s clear, therefore, that vertical distance above a mean datum (e.g MSL) would be a better way to judge how high to fly. But this is not that easy to assess (particularly if you are flying over land) and MSL changes*
*Click here to learn why the sea is not flat
Mean Sea Level is not the same everywhere and actually looks more like this (This model shows the changes in a 10,000x scale to exaggerate the effect):
Geoid model with 10000x scale factor – Image under CC Attribution 4.0 International via Wikimedia Commons
This is called the geoid model – it is a method of assessing where MSL would be around the globe (including where land actually us) due to the effects of gravity and Earth’s rotation. Mass is not distributed evenly within and upon the Earth, resulting in areas of higher and lower gravity. This would affect the ‘thickness’ of the sea and hence it’s surface level. Whilst highly theoretical it is a useful way of assessing the elevation of various features such as mountains.
Imagine you’re flying at 10,000ft above MSL and another aircraft is coming towards you. You call up on the radio and check their altitude. They let you know that they are flying at 11,500ft above MSL you might think you’re not going to hit. But if the difference in MSL is 500ft when you have that conversation…*
(*granted this could only happen if one of you was in Iceland and the other in Southern India but the point is valid!)
So, vertical distance above the sea fails both of our requirements – it doesn’t tell you how far away the ground is, nor does it give you a constant frame of reference with other aviators. Luckily for aviation, a certain feature of the atmosphere gives us a useful alternative…
PRESSURE & DENSITY
As we discussed in a previous tutorial, in any column of fluid there is a hydrostatic gradient with pressure at any point calculated by the equation P = ρgh (where ρ = density of the fluid, g = acceleration generally due to gravity and h = height of the column above that point).
The atmosphere is a column of fluid and hence has it’s own hydrostatic gradient. The bottom of this column sits on the ground (the top is slight harder to define as the atmosphere sort of merges into space in the exosphere). This means that as you climb higher the small the column above you (i.e ‘h’ decreases). It’s like Molecule Paul moving up the tower the images below.
Unlike the simplistic models we talked about in our previous tutorials, things get a bit more complicated when it comes to the atmosphere. In our swimming pool example we assumed that density stays the same, hence pressure increases linearly with depth (1atm per 10m water). That’s (almost) true for liquids, but is certainly not the case for gases which like to compress.
Due to this, the density of the air is greater at ground level, where the molecules are all squished together. This means that as we change altitude we are changing two different variables in our equation, ‘h’ and ‘ρ’.*
*Gravity also decreases with altitude, but it’s a small change and something for another day
Therefore, when we climb to higher altitudes the pressure falls, not linearly, but exponentially:
The greatest change in pressure is over the first 1,000ft, with the change over the same distance reducing the higher you go. In fact, by 18,000ft PA, the barometric pressure is half that at ground level, despite still having 94.5-99.9% of the atmosphere still above you (depending on your definition of where it ends).
The fact that pressure and density change with altitude leads can be really useful:
- It means we can measure the pressure outside an aircraft to work out its ‘pressure altitude’ (PA)*
- Lower density air higher up means less drag and hence higher fuel efficiency
*Pressure Altitudes (PA)
Saying that something is at 9000ft doesn’t really mean much unless we define how we are measuring it. It could be 9000ft above level (AGL) or 9000ft above mean sea level (AMSL).
When we measure altitude in aircraft (unless using a radar altimeter) we are actually generally measuring the barometric pressure outside the aircraft. If we know what the pressure should be at any given altitude, we can then work out how high we are. This is what pressure altimeters do.
When we give a height calculated from pressure it is called (no prizes here) a Pressure Altitude and is signified by the suffix ‘PA’. Most of the time on this website we are talking about pressure altitude and hence you will see those two letters a lot.
For some context, the peak of Mt Everest is 29,029ft AMSL but is only 27,278ft PA (on an average day…see the section below about Standard Atmospheres). Don’t tell this to climbers…they get very upset!
Whilst this makes this much easier for aviators, it creates all sorts of problems for aerospace medicine specialists:
If you look back how we worked out the partial pressure of the various components of air using Dalton’s Law in our last tutorial, you’ll remember that we multiplied the fraction of that gas in the air (oxygen = 21% of air hence a fraction of 0.21) by the barometric pressure (760mmHg at ground level).
As we ascend and the fraction of oxygen stays the same but the barometric pressure falls. This means that the partial pressure of inspired oxygen also falls. This is why we get hypoxic at altitude. It’s not that there is less oxygen, it’s that it is at a lower partial pressure. This is what, at least partially, was responsible for the deaths of Sivel, Crocé-Spinelli and many more aviators since.
Two of our other eponymous laws are also important here. Boyle’s law shows that the volume of a gas increases as pressure falls which is a big problem for any trapped pockets of gas (such as in your gut!). Henry’s law tells us that as the pressure falls, gases are more likely to come out of solution.
These two laws together can be used to explain another group of issues we run into at altitude; decompression illness (including decompression sickness or evolved gas disease, arterial gas emboli and ebullism). Given some aspects of Gaston’s eyewitness account there is some consideration that Sivel and Crocé-Spinelli may have run into some of these issues too.
The medical aspects of altitude exposure will be covered in much more detail in a future tutorial.
TEMPERATURE
Temperature varies with altitude. This is not surprising if you think about Gay-Lussac’s law. However, it’s not quite as simple as temperature falling as the barometric pressure falls. There are all sorts of other factors that affect how the temperature is affected with altitude such as weather, convection currents from the ground, local greenhouse effects, radiation absorption etc.
The rate of change of temperature (lapse rate) varies throughout the atmosphere and is one of the key defining features the individual layers:
As we climb through the troposphere things are nice and simple. Temperature falls linearly by 1.98°C per 1000ft. This continues to be true until the temperature reaches -56.5°C at 36,089ft PA, marking the edge of this layer of the atmosphere.
Ascending through the lower end of the stratosphere the temperature remains constant at a chilly -56.5°C until 90,000ft PA. This is called the isothermal layer. After this things start to warm up again due to the rapid photolysis and reformation of ozone, reaching a balmy -3°C at the stratopause.
Climbing into the mesosphere ozone is no longer able to form and less solar radiation is absorbed due to the falling density, with the higher energy particles already absorbed or transformed in the higher layers. The temperature plummets, with the coldest area of the atmosphere being found at the mesopause (approximately 600km) at a shiver-inducing -101°C.
Despite the common misconception that space is freezing cold, the thermosphere, is actually the hottest part of our atmosphere. Here large volumes of high energy cosmic radiation is absorbed or transformed into different particles, producing vast quantities of heat. Temperature generally increases with altitude but varies considerably based on time of day and solar activity (between 500°C and 2,500°C in the upper thermosphere). Why we think of space as cold is that, despite all this thermal energy, the density of the atmosphere is too low for molecules to conduct heat.
Like the thermosphere, temperature in the exosphere varies with the amount of solar cosmic radiation, ranging from 0 to 1700°C.
Cold may have been another reason for the fateful Zénith flight. Using the temperature lapse rate of -1.98°C per 1000ft PA and a standard ground level temperature of 15°C, the temperature would have fallen to freezing at around 7,500ft PA. By the time they reached 28,900ft PA and noticed symptoms it would have been around -42.2°C. At the peak of the climb they would have almost hit the isothermal layer with temperatures as low as -55.9°C.
Clearly hypothermia could have affected their flight, but there’s more…Expired air is saturated with water vapour. Without adequate precautions this could have frozen in their simple oxygen system. Even if they had remembered to use their equipment through the haze of hypoxia, it may well have failed them when they needed it most.
WEATHER
Whilst to us on the ground it looks like the clouds go up forever, most of our weather is found in the troposphere. This is another reason why pilots like to fly as high as possible (and not just for great views like this):
Weather is one of the main causes of turbulence due to sudden changes in wind speed or rising hot air amongst other things. Flying above the cloud level leads to more comfortable, accurate and economic flight. The low stratosphere is therefore a good place to be – the cool layer in the isothermal region, with a warmer layer above makes for comparatively stable flight.
Low partial pressure of oxygen beyond 40,000ft PA not only threaten the crew and passengers in an emergency but also reduce the efficiency of air-breathing engines. The sweet spot is hence around 36-40,000ft PA. Check the in-flight information next time you hop on a long haul flight and you can be pretty certain you’ll be around those altitudes.
Certain large storms can cross into the stratosphere and there is reported to be some cloud formation in the upper reaches of the mesosphere – perhaps due to meteorite breakup. Unfortunately, unlike Gaston, I am no meteorologist so that’s all I have to offer on that one!
OZONE
When we talked about temperature in the stratosphere we mentioned ozone as one of the reasons that the temperature climbs beyond 90,000ft PA. But why?
Ozone is the tri-atomic form of Oxygen (i.e O3 not O2). Unlike oxygen which we think of as odourless and colourless, it is pungent and has a slight blue tinge. It is also naturally highly unstable and can only form in significant quantities in certain circumstances. The ozonosphere (or ozone layer) lies between 40,000 and 140,000ft PA. There are very low levels of ozone at the bottom, climbing to a peak at around 100,000ft PA and falling to almost zero by 158,000ft PA.
Ozone forms when cosmic radiation causes O2 molecules to dissociate to monatomic oxygen. These can then reform as either O2 or O3. This process releases heat as a byproduct, contributing (along with the radiation itself) to the increased temperatures above the isothermal layer.
Ozone formation is very altitude specific – too little radiation and the O2 molecules won’t dissociate, too much and the monatomic oxygen won’t be able to reform.
Whilst we cannot live without oxygen (although too much isn’t great for you either!), ozone is one of the strongest oxidizing agents known. Unsurprisingly it is not very good for our health. Whilst concentrations required for a clinical affect differ with exertion and health, levels 20 times that found at ground level can reduce vital capacity and FEV, whilst those found at 100,000ft PA can cause fatal pulmonary oedema.
Luckily, the unstable nature of ozone means very little, if any, makes it through oxygen systems or cabin pressurisation systems. Health and safety regulations exist that limit exposures to safe levels.
RADIATION
We’ve mentioned radiation a couple of times already. This is a huge topic and is worth its own tutorial in time. What it is worth being aware of at this point is that cosmic radiation is constantly bathing our planet from both solar and galactic sources. Our atmosphere is one of our main barriers against this. Incoming radiation collides with molecules in the atmosphere, imparting some of its energy and breaking down into smaller, less harmful molecules. Ionization of certain molecules in the inonosphere (predominately in the thermosphere) results in the emission of coloured light which we see as Aurorae.
As the number of molecules between yourself and space reduce as you climb through the atmosphere, the protection from cosmic radiation decreases (by approximately 15% per 2000ft). A transatlantic flight will expose you to roughly 0.08mSv in addition to your normal background levels. For context, a chest X-Ray is approx 0.014 mSv, a CT head is about 1.4mSv and your usual background dose is approximately 2.2-6.2mSv depending on where you live.
The effects of low dose repeated exposure is not particularly well understood and is unlikely to affect irregular passengers. Regulations are in place to limit the amount of radiation aircrew receive however to prevent possible long term effects. The higher you fly, the more likely these limits will be reached which is why Concorde (which flew up to 60,000ft PA) had an active dosimeter on board.
Outside of the atmosphere’s protection, radiation can be significantly more harmful. Even in low earth orbit with some protection from the magnetosphere, doses received during peaks of solar weather could cause acute radiation sickness or death without sufficient shielding.
OTHER CHANGES WITH ALTITUdE
If you want to build a car to break the sound barrier on an average day (temperature = 15°C) you would need to go faster than 761mph. However if you want your aircraft to reach the speed of sound at 40,000ft PA you only needs to get to 660mph. So the speed of sound decreases with altitude?
Yes, but primarily due to the change in temperature. Easier than making your car fly if you want to go supersonic is waiting for a colder day. Just a 10°C fall in temperature means your target speed is now 13mph slower!
Similarly, if you are like John Mayer and gravity is bringing you down, you just need to gain some altitude. We’ll go into gravity in a lot more detail soon but it is worth knowing that, whilst we say that acceleration due to gravity is 9.81m.sec-2 this is only completely true at a certain point on Earth’s surface. Gravity falls with distance from the centre of the Earth. Climb a high mountain or ascend in an aircraft and gravity will reduce.
The effect is small, however. At 40,000ft PA gravity is 99.6% of that at the surface. Even by the altitude of the ISS, gravity is still 88.3% as strong as it is on the surface of the Earth. The microgravity that astronauts experience is not due to their distance from the Earth, it is because they are constantly falling…
High Standards
You may have noticed that throughout this tutorial we have both stated some pretty specific numbers, for example mean sea level barometric pressure (760mmHg) and temperature (15°C) or acceleration due to gravity (9.81m.sec-2).
Clearly these values vary. Temperature at ground level and in the troposphere varies significantly with weather. Barometric pressure is also affected by weather – the lowest recorded barometric pressure in the Northern Hemisphere was 687mmHg whilst the highest was 819mmHg. Elevation of the ground station where pressure is measured will also affect results (including the two just quoted). Even the height of the tropopause will change with local weather and temperature effects.
Given that we want to use pressure to compare altitudes of aircraft to prevent collisions, varying figures could cause significant issues. Therefore, various international standards have been used to allow direct comparison. The most recent of this is the ICAO Standard Atmosphere. This makes the following statements:
- Temperature at MSL = 15°C
- Temperature lapse rate = 1.98°C/1000ft
- Barometric pressure at MSL = 760mmHg
- Acceleration due to gravity = 9.80655m.sec-2
- Atmospheric density at MSL = 1.225kg.m-3
- Relative molecular mass of air at MSL = 28.96
- Height of the tropopause = 36,089ft PA
- Temperature at isothermal layer = -56.5°C
To get these figures, the following assumptions were made:
- The location is set as 45° North
- Air is dry with no dust
- Air is of a stated composition (78.09% N2, 20.95% O2, 0.93% Ar, 0.03% CO2, and tiny amounts of Ne, He, Kr, H2 and Xe)
So How High Are We Really?
By using aspects of the physical nature of our atmosphere, aligned with a standardised atmosphere we can get a pretty good measure of our vertical position in the sky compared to other aircraft. Clearly this isn’t particularly accurate at ground level.
Luckily altimeters can adjust for this:
The little dial of numbers on the right (between the large ‘2’ and ‘3’) on this altimeter show the selected pressure setting which can be adjusted by the knob on the bottom left (in this case the units is inHb not mmHg).
Above a fixed altitude called the ‘transition level’ (generally 3000ft in the UK except in certain locations), the pressure setting will be set to 760mmHg (or 1013.25hPa). This allows all aircraft above this level to deconflict. Above this, aircraft will refer to ‘Flight Level’ rather than altitude to show that they are referencing a pressure altitude, not vertical distance from MSL. Flight Level is pressure altitude in ft divided by 100 (so 25,000ft PA becomes FL250).
Below the transition level different pressure settings called QNH and QFE are used.
- QNH is current pressure set at MSL – if entered at ground level the altimeter should give the vertical elevation of the centre point of the runway at that specific aerodrome.
- QFE is the current local pressure at ground level – if you entered this into the altimeter whilst sat on the ground, it should read 0ft.
There is also a Regional Pressure Setting that can be used at low altitudes which gives the lowest forecasted value for QNH in that region to ensure safe separation with the ground.
Clearly all pressure settings below the Transition Level will be affected by local weather and terrain and hence need to be updated by communication with Air Traffic Control. Receiving or entering in the wrong setting can cause aircrew to misjudge altitude and has led to numerous accidents.
Hopefully we have learnt a little bit about how the atmosphere sets up our altitude environment and what changes to expect as we climb higher. We’ve touched on some of the benefits these changes provide to aviation. Our next tutorials will look in more detail at the other side of this coin, the aeromedical implications of these changes.
Until then…
Yours,
JB
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[…] Very soon they started to run into many of the issues of altitude that we discussed in our previous tutorial. By the start of WW2 aviators were regularly flying as high as modern short haul aircraft. But […]